Direct Force Measurements on the Colloidal Scale: From Modified Electrodes to Particle Manipulation

Direct Force Measurements on the Colloidal Scale: From Modified Electrodes to Particle Manipulation DISSERTATION zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) im Fach Chemie der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth vorgelegt von Volodymyr Kuznetsov Geboren in Stachanov Bayreuth, 2013 II III Die vorliegende Arbeit wurde an der Universität Bayreuth und der Universität Genf (Schweiz) angefertigt. Von Dezember 2008 bis April 2010 arbeitete ich am Abteilung für anorganische, analytische, und angewandte Chemie in der Universität Genf unter der Betreuung von Prof. Dr. Michal Borkovec und co-Betreuung von Prof. Dr. Georg Papastavrou; und von April 2010 bis Dezember 2012 am Lehrstuhl für Physikalische Chemie II in der Universität Bayreuth unter Betreuung von Prof. Dr. Georg Papastavrou. Vollständiger Abdruck der von der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth genehmigten Dissertation zur Erlangung des akademischen Grades Doktor der Naturwissenschaften (Dr. rer. Nat.). Amtierender Dekan: Prof. Dr. Beate Lohnert Tag des Einreichens der Dissertation: . Jan 2013 Tag des wissenschaftlichen Kolloquiums: . Feb 2013 Prüfungsausschuss: Prof. Dr. Georg Papastavrou (Erstgutachter) Prof. Dr. Andreas Fery (Zweitgutachter) IV V The larger the island of knowledge, the longer the shoreline of wonder. Ralph W. Sockman VI VII To my family VIII IX Table of Contents Summary ........................................................................................................................... XI Zusammenfassung........................................................................................................... XIII List of abbreviations and symbols ................................................................................ XVII 1. Introduction .................................................................................................................. 1 2. Theory / Status of the field ........................................................................................... 7 2.1. AFM and direct force measurements ..................................................................... 7 2.2. Data acquisition and interpretation in direct force measurements ......................... 8 2.3. Data analysis in direct force measurements ........................................................... 9 2.4. Application of direct force measurements ........................................................... 12 2.5. Colloidal probe technique and data interpretation ............................................... 13 2.6. Adhesion and short-range forces ......................................................................... 20 2.7. Adhesive properties of organic surfaces by AFM & Electrochemistry ............... 23 2.8. Diffuse layer properties of modified electrodes by AFM .................................... 27 2.9. Mechanical properties of ultrathin organic films ................................................. 30 3. Overview of the Thesis ............................................................................................... 45 3.1. A novel preparation method of mechanically stable colloidal probes by hightemperature sintering ..................................................................................................... 45 3.2. Adhesion control at organic interface by electrochemistry ................................. 46 3.3. Ion adsorption probed by direct force measurements .......................................... 49 3.4. Mechanical properties of ultrathin films by nanoindentation .............................. 50 3.5. Individual Contributions to Joint Publications .................................................... 52 4. Mechanically and chemically stable colloidal probes from silica particles for atomic force microscopy ................................................................................................................ 55 5. Adhesion of Colloidal Particles on Modified Electrodes ........................................... 65 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control .................................................................... 103 7. Tuning of the elastic modulus of polyelectrolyte multilayer films built up from polyanions mixture........................................................................................................... 135 List of Publications .......................................................................................................... 153 Acknowledgements .......................................................................................................... 155 Erklärung.......................................................................................................................... 157 XVII List of abbreviations and symbols A Hamaker constant AFM Atomic force microscopy C Capacitance CV Cyclic voltammetry or cyclic voltammogram D Distance DL Diffuse layer DLVO Derjaguin, Landau, Verwey, Overbeek (theory, forces) DMT Derjaguin, Müller, Toporov (theory, equation) F Force FIB Focused ion beam I ionic strength iHP Inner Helmholz plane JKR Johnson, Kendall, Roberts (theory, equation) k Normal cantilever constant kT Thermal energy LbL Layer-by-layer MASIF Measurement and analysis of surface interaction forces MEMS Microelectromechanical system(s) oHP Outer Helmholz plane PAH Poly(allylamine hydrochloride) PB Poisson-Boltzmann (distribution, equation) PEM Polyelectrolyte multilayer PGA Poly(glutamic acid) PSS Poly(styrenesulfonate) pzc, ϕ PZC Potential of zero charge pzf Potential of zero force R Radius R eff Effective radius of interaction given by Derjaguin approximation SAM Self-assembled monolayer SCE Saturated calomel electrode SECM Scanning electrochemical microscopy XVIII SEM Scanning electron microscopy SFA Suface force apparatus SFM Scanning force microscopy STM Scanning tunneling microscopy TEM Transmission electron microscopy TIRM Total internal reflection microscopy vdW van der Waals (forces) W Interaction energy γ Interfacial tension or surface free energy ζ Zeta potential κ - 1 Debye screening length σ a Surface charge density on an SAM surface σ D Diffuse layer charge density σ e Surface charge density on an electrode ϕ Externally applied potential, electronic potential ψ a Surface potential on an SAM surface ψ D Diffuse layer potential 1. Introduction 1 1. Introduction Direct force measurements have been essential in recent years for our understanding of interfacial phenomena. 1-3 They contributed in the fields of polymer research, biology, physical chemistry, physics to name just a few. Quantification of such phenomena as adhesion, friction, or interfacial charge accumulation became only possible by probing the processes on the nanometer-scale. The quantitative description of complex biological, polymer, and inorganic systems advanced profoundly due to revealing interactions at the nanoscale. 4,5 Historically, the first device allowing the interaction profiles determination with subnanometer resolution was the surface force apparatus (SFA). 6 Although in the recent 35 years other techniques have emerged, the SFA is still being used in many laboratories. 7 The two other important techniques are the MASIF technique (i.e. measurement and analysis of surface interaction forces) and the colloidal probe technique based on atomic force microscopy (AFM). All three techniques have defined interaction geometry. While the SFA and the MASIF use rather large probes in the order of a few millimeters, and employ force-determining sensors with relatively high spring constant, their force resolution is limited to some hundreds of piconewtons. 8 By contrast, the colloidal probe technique, utilizing micrometer-sized probes, allows determining forces down to a few piconewtons. Additionally, the range of sensed forces is controlled by the stiffness of an AFM cantilever. Thus, for probing interactions within the wide force range of the colloid domain the colloidal probe technique has the highest force resolution. 9 Currently, the colloidal probe technique is the most widely used method to determine the interactions between colloidal objects and surfaces. 8 The great advantage of such a probe is its versatility in terms of geometry, surface chemistry, and possibility to attach practically all types of objects on the colloidal scale to the AFM cantilever. For instance, sphere-sphere, sphere-plane, and crossed cylinders geometries are readily accessible. 10 Among the materials for probe reported so far are silica, glass, latex, and cellulose 11 , and even gasor air-bubbles. 12 Because of the large number of different applications the colloidal probe force spectroscopy contributed in the past to various branches of colloid science. 8 A selection of typical applications is presented in Figure 1. They include measurements of adhesion force (A, cf. section 5 of this thesis) 13 and forces due to diffuse layer overlap (B, cf. sections 5, 7 of this thesis). 14 Using this approach the particle-particle interaction as a 1. Introduction 2 function of separation can be obtained (C, cf. sections 5, 7 of this thesis) 15 , while performing the nanoindentation experiment with defined geometry of the indenter (D, cf. section 8 of this thesis) allows mechanical properties determination of nanoobjects of different nature such as films, capsules, or cells. 16 Therefore, force spectroscopy with a colloidal probe represents a complimentary approach to universal surface tester in the cases (A) and (D), to electrochemical techniques for studying diffuse layer properties (B), and, finally, complements the spectroscopic and electrokinetic measurements to determine properties of colloidal suspensions (C). 7,17 Thus, the colloidal probe technique allows addressing a number of phenomena on the nanoscale. Figure 1: Applications of direct force measurements with colloidal probe: (A) measurement of adhesion; (B) quantification of diffuse layer properties; (C) probing interparticle interaction; (D) characterization of laterally resolved mechanical properties. By employing colloidal probe technique it is possible to measure the interacting forces and mechanical properties in various media, such as air and electrolyte solutions. Together with adhesion and elasticity measurements the technique is widely used for measuring DLVO (Derjaguin, Landau, Verwey and Overbeek) and chemical forces, friction and steric forces, etc. Therefore, interfacial phenomena and forces causing them could be quantified. For example, by varying the ionic strength of solution the electrostatic diffuse double layer forces could be separated from the Van der Waals forces. 18 1. Introduction 3 In this thesis new approaches to apply the colloidal probe technique have been pursued. One important application is the combination of the direct force measurements with potentiostatic control of modified electrodes in electrolyte solution to identify different contributions in adhesion force between colloidal particles and modification layer. By utilizing cantilevers with high spring constant it became possible to correlate forces upon approach and adhesion. In particular the separation of electrostatic diffuse double layer, van der Waals, and solvent exclusion forces in total adhesive force has been addressed. Additionally the role of electrocapillarity effects for organic layers on electrodes could be assessed. Thus, in difference to previous studies 19,20 , the influence of longand short-ranged interaction forces on the adhesion could be identified in unambiguous manner. The control of adhesion force provides means to pursue microand nanomanipulation of colloidal particles. The same micrometer-sized silica particles used as colloidal probes for studying adhesion could be employed for micromanipulation to provide a proof of principle. Therefore, studying adhesive properties of modified electrodes became a prerequisite for micromanipulation application. Ion adsorption on non-ionizable organic interfaces has been also studied by colloidal probe technique. While the electrochemical control of modified electrodes allows variation of their diffuse layer properties, using direct force measurements those properties could be determined. In particular the variation of diffuse layer potential as a function of applied potential has been of major interest, since such data could be compared to the theoretical predictions. A novel approach to prepare colloidal probes by a sintering technique has been developed. For surface force measurements and nanoindentation experiments probes with mechanical stability are important to ensure the defined interaction geometry at high pressure. 21,22 Due to their smoothness and predictable surface chemistry silica particles are potentially well-suited as nanoindenter. 23 However, the fixation of such particles on an AFM cantilever without polymeric glue has not been possible so far. Here, a preparation by sintering with fixation “neck” made of Ludox particles allowed to prepare probes suitable for any solvent. The high-temperature treatment during the preparation could solve also the problem of organic contaminants and reproducible surface chemistry 17 . The preparation of the thin polyelectrolyte multilayer (PEM) films by layer-by-layer technique has numerous applications in medical, cosmetic, and sensor industries to name 1. Introduction 4 just a few. 24-26 Probes prepared by this sintering technique were applied to characterize mechanical properties of ultrathin organic films. Namely the elastic modulus of polyelectrolyte multilayer films susceptible to surrounding humidity was addressed by nanoindentation measurements. Primarily the change of the stiffness upon alteration of humidity conditions and the reversibility of this process were studied. References: 1. Senden, T. J. Force Microscopy and Surface Interactions. Current Opinion in Colloid & Interface Science 6, 95-101 (2001). 2. Butt, H. J. Analyzing Electric Double Layers with the Atomic Force Microscope. Encyclopedia of electrochemistry 225-252 (2003). 3. Leckband, D. & Israelachvili, J. Intermolecular Forces in Biology. Q. Rev. Biophys. 34, 105-267 (2001). 4. Stuart, M. A. C. et al. Emerging Applications of Stimuli-Responsive Polymer Materials. Nature materials 9, 101-113 (2010). 5. Butt, H.-J., Berger, R., Bonaccurso, E., Chen, Y. & Wang, J. Impact of Atomic Force Microscopy on Interface and Colloid Science. Adv. Colloid Interface Sci. 133, 91-104 (2007). 6. Israelachvili, J. N. & Adams, G. E. Direct Measurement of Long Range Forces between Two Mica Surfaces in Aqueous KNO 3 Solutions. Nature 262, 774-776 (1976). 7. Claesson, P. M., Ederth, T., Bergeron, V. & Rutland, M. W. Techniques for Measuring Surface Forces. Adv. Colloid Interface Sci. 67, 119-183 (1996). 8. Butt, H.-J., Cappella, B. & Kappl, M. Force Measurements with the Atomic Force Microscope: Technique, Interpretation and Applications. Surface Science Reports 59, 1-152 (2005). 9. Holmberg, K., Shah, D. O. & Schwuger, M. J. Handbook of Applied Surface and Colloid Chemistry (John Wiley & Sons Inc, 2002), p. 388. 10. Israelachvili, J. N. Intermolecular and Surface Forces (Academic Press, 2011). 11. Giesbers, M., Kleijn, J. M. & Cohen Stuart, M. A. Interactions between Acidand Base-Functionalized Surfaces. J. Colloid Interface Sci. 252, 138-148 (2002). 12. Chan, D. Y. C., Klaseboer, E. & Manica, R. Theory of Non-equilibrium Force Measurements Involving Deformable Drops and Bubbles. Adv. Colloid Interface Sci. 165, 70-90 (2011). 1. Introduction 5 13. Pericet-Camara, R., Papastavrou, G., Behrens, S. H., Helm, C. A. & Borkovec, M. Interaction Forces and Molecular Adhesion between Pre-adsorbed Poly(ethylene imine) Layers. J. Colloid Interface Sci. 296, 496-506 (2006). 14. Toikka, G. & Hayes, R. A. Direct Measurement of Colloidal Forces between Mica and Silica in Aqueous Electrolyte. J. Colloid Interface Sci. 191, 102-109 (1997). 15. Popa, I., Trulsson, M., Papastavrou, G., Borkovec, M. & Jönsson, B. Long-Ranged Attractive Forces Induced by Adsorbed Dendrimers: Direct Force Measurements and Computer Simulations. Langmuir 25, 12435-12438 (2009). 16. Richert, L., Engler, A. J., Discher, D. E. & Picart, C. Elasticity of Native and CrossLinked Polyelectrolyte Multilayer Films. Biomacromolecules 5, 1908-1916 (2004). 17. Kobayashi, M., Skarba, M., Galletto, P., Cakara, D. & Borkovec, M. Effects of Heat Treatment on the Aggregation and Charging of Stöber-type Silica. J. Colloid Interface Sci. 292, 139-147 (2005). 18. Dishon, M., Zohar, O. & Sivan, U. From Repulsion to Attraction and Back to Repulsion: The Effect of NaCl, KCl, and CsCl on the Force between Silica Surfaces in Aqueous Solution. Langmuir 25, 2831-2836 (2009). 19. Rentsch, S., Siegenthaler, H. & Papastavrou, G. Diffuse Layer Properties of ThiolModified Gold Electrodes Probed by Direct Force Measurements. Langmuir 23, 9083-9091 (2007). 20. Rentsch, S. Direct Force Measurements Between Surfaces Under Potentiostatic Control. PhD thesis, University of Geneva, Geneva, Switzerland (2008). 21. Dimitriadis, E. K., Horkay, F., Maresca, J., Kachar, B. & Chadwick, R. S. Determination of Elastic Moduli of Thin Layers of Soft Material Using the Atomic Force Microscope. Biophys Journal 82, 2798-2810 (2002). 22. Gouldstone, A. et al. Indentation Across Size Scales and Disciplines: Recent Developments in Experimentation and Modeling. Acta Materialia 55, 4015-4039 (2007). 23. Matijevic, E. Preparation and Properties of Uniform Size Colloids. Chem. Mater. 5, 412-426 (1993). 24. Ariga, K., Hill, J. P. & Ji, Q. Layer-by-Layer Assembly as a Versatile Bottom-up Nanofabrication Technique for Exploratory Research and Realistic Application. Physical Chemistry Chemical Physics 9, 2319-2340 (2007). 1. Introduction 6 25. Tang, Z., Wang, Y., Podsiadlo, P. & Kotov, N. A. Biomedical Applications of Layer-by-Layer Assembly: From Biomimetics to Tissue Engineering. Advanced Materials 18, 3203-3224 (2006). 26. Bertrand, P., Jonas, A. & Laschewsky, A. Ultrathin Polymer Coatings by Complexation of Polyelectrolytes at Interfaces: Suitable Materials, Structure and Properties. Macromolecular Rapid Communications 21, 319-348 (2000). 2. Theory / Status of the field 7 2. Theory / Status of the field A milestone in surface science was the development of scanning tunneling microscopy (STM) by Rohrer and Binnig in 1982. 1 It was followed by the atomic force microscopy (AFM) in 1986. 2 The latter has allowed imaging topography and measuring surface forces on the nanoscale with sub-nanometer resolution. Both methods use the micrometer-sized cantilever or a wire with a nanometer-sharp tip to scan over the sample surface with simultaneous detection of tip-sample current (STM) or interaction (AFM). In contrast to STM the AFM does not require conductive samples, nor it requires semitransparent samples like the surface force apparatus (SFA) 3 , where the distance between two surfaces is controlled interferometrically. Therefore probing interaction forces on the nanoscale as a function of the tip-sample distance became possible. 2.1. AFM and direct force measurements Figure 2 schematically illustrates the working principle of atomic force microscope for imaging in so-called contact mode. The cantilever with a sharp tip presses on the sample surface, while the deflection of the cantilever is acquired by a laser reflected from it to a position-sensitive photodetector, connected to the controller. The latter receives a signal from the photodetector and is connected to a translational stage, which consists of X-, Y-, Zpiezos. When the tip scans a sample surface laterally (Xand Ypiezos are used), the deflection of the cantilever is kept constant by movement of the translational stage in Z-direction. Hence, the topography of the surface can be reconstructed. The controller is operated from a computer, where the resulting topographical data are displayed. Besides the deflection of cantilever other signals can be monitored during scanning. 4 For example, if the cantilever is set to oscillate during scanning then the change of the amplitude or frequency shift can be monitored. That gives the origin to different working modes of AFM. In various working modes not only topographical data can be obtained, but also one can determine the adhesive properties of the surface or laterally resolve the mechanical properties. Measurements of surface properties by AFM are based on determination of interaction forces between the tip or a probe and a sample. 5 Such forces are determined as a function of tip-sample separation. An overview on the different imaging modes is given elsewhere. 6 2. Theory / Status of the field 14 order of AFM cantilever width. Among colloidal probes made of natural materials cellulose beads, hair and skin pieces, even bacteria cells should be mentioned. 29-31,34 The natural material probes are object specific and may have large variation in their properties. By contrast, synthetic probes often possess stable chemical and physical properties. Examples of those include latex, glass, silica, alumina and zirconia colloidal beads. 32 Special place in this row belongs to silica particles, which have very high Young’s modulus and their surface chemistry can be controlled by preparation method. 33 Various methods have been described to prepare colloidal probes. To attach probes to the cantilevers a polymer-based glue is commonly used. 9 Therefore independently from the glue-curation method the possibility of contamination is present. Moreover, another aspect concerning cleanliness is the removal of suspension stabilizing surfactants, which are usually employed to protect colloid suspensions from coagulation. One way to overcome the problems associated with polymerand surfactant-contamination is to use a sintering preparation method. For the first time it was demonstrated by Vinogradova et al. 35 for polystyrene spheres and soon afterwards by Bonaccurso et al. 36 for glass beads. In the latter experiment the sintering temperature was close to 800 0 C at ambient atmosphere, that insured the burning out of any organic contaminations. However, in this method the contact area between a colloidal particle and a cantilever remains small, hence questions about mechanical stability could be raised. The CPFS can be applied towards a number of practical and fundamental problems. For instance, the interparticle interaction potential can be accessed directly by spheresphere measurement (cf. Figure 6a) instead of indirect light scattering or rheological measurements. 37 In Figure 6b the interaction force between two silica particles as a function of separation distance in electrolyte solution is presented. Since particles from the same material have been used, they acquire same charge in electrolyte solution. Therefore due to the overlap of diffuse layers upon approach, the long-range forces are repulsive. The interaction force can be further converted into interaction energy per unit area of infinite planes W by the approximation initially published by Derjaguin in 1937: , (5) where the value of W is equal to normalized force (cf. Figure 4d) divided by 2π. 38 Thereby the interparticle interaction energy versus separation distance is deduced as shown in the insert of Figure 6b. In semi-logarithmic representation this curve appears linear, because the interaction between weekly charged colloids could be described by the W = F/(2 π ⋅ R eff ) 2. Theory / Status of the field 15 Gouy-Chapman-Stern theory, and thus the ionic distribution in diffuse layers obeys the Poisson-Boltzmann distribution. This allows for the fitting of the data by full PoissonBoltzmann equation. 39 Figure 6: (a) Scheme for the interaction measurement between bodies of different geometries. (b) Interaction profile between two silica beads of 6.8 µm in diameter in electrolyte solution (I = 10 -4 M, pH 4.7). Derjaguin approximation allows converting interaction forces into interaction energy by normalization on the effective interaction radius (b: insert). The interaction energy profile in the insert is fitted according to full Poisson-Boltzmann equation including constant charge (CC), constant potential (CP), and constant regulation approximation (CR) (reproduced from Rentsch et al. 40 ). The Gouy-Chapman-Stern theory originates from Gouy-Chapman model, which was developed in the beginning of XX century to describe ion distribution over charged surface in electrolyte solution. 41 It involves the Boltzmann’s law, describing the distribution of charged species over the charged wall; and the Poisson equation, that relates a charge distribution with an apparent surface potential. Therefore its analytical expression is called Poisson-Boltzmann (PB) distribution. It is commonly expressed as 42 d 2 ψ (x) dx 2 =e εε 0 n i 0 z i i ∑ exp z i e ψ kT       , (6) 2. Theory / Status of the field 16 where εε 0 is the total permittivity of solvent, kT is the thermal energy at given absolute temperature, e is the elementary charge, n i and z i – volume concentration and charge of ionic species of type i. For a one-dimentional diffuse layer the surface diffuse layer potential ( ψ 0 ) decay could be described by integration of eq. (6) with boundary conditions. Then in the direction perpendicular to a flat charged surface in z:z electrolyte the potential ( ψ ) is given by eq. (7) 43 tanh(ze ψ / 4kT) tanh(ze ψ 0 / 4kT)      =exp − κ x ( ) , (7) where κ is a pre-factor, which characterizes the decay of the surface potential ψ 0 with increasing distance x from the surface. The reciprocal value of the pre-factor is known as Debye length 39 , (8) where N A is Avogadro’s number, and I is the solution’s ionic strength. The PB equation (6) can be used as a starting point for the determination of electrostatic potential distribution ψ (x) between two charged surfaces, yielding d 2 ψ (x) dx 2 = κ 2 kT esinh e ψ kT       . (9) As soon as such potential profile is known for surfaces in 1:1 electrolyte solution the disjoining pressure ∏(x) between them can be found from 44 Π(x)=2ni 0kT[cosh(e ψ /kT )−1]− εε 0 2 d2 ψ (x) dx2      2 . (10) This analytical solution relates the disjoining pressure between surfaces with their diffuse layer potentials under the assumption that one surface is situated at zero-distance of xaxis and the other is at a distance x. 45 Integration of the pressure over a given separation distance D gives the interaction energy per unit area W(D) 39 W(D)= Π(x)dx x=0 D ∫ . (11) The interaction energy W(D) can be directly inferred from the experimental interaction force and the Derjaguin approximation (cf. eq. (5)). Thus, the model could be directly fit to the experimental force data. κ −1 = εε 0 kT 2N A e 2 I 2. Theory / Status of the field 17 It should be noted that in Gouy-Chapman model the surface potential ( ψ 0 ) is considered identical to diffuse layer potential ( ψ D ). The relation between ψ D and diffuse layer charge density σ D is commonly given by eq. (12), which is also known as Grahame equation 44 σ D = εε 0 ∂ ψ ∂x      x=0 =(8kT εε 0 IN A ) 12 sinh ze ψ D 2kT       . (12) The diffuse layer capacitance, which is considered to be identical to the total capacitance within the model 42 , is given by C GC total =C D =∂ σ D ∂ ψ D     =2z 2 e 2 εε 0 IN A kT      12 cosh ze ψ D 2kT       . (13) In the model of Louis Gouy and David Chapman the diffuse double layer starts directly at the solid-liquid interface and consists of infinitely small ions. 46 Otto Stern extended this model by adding an adsorbed layer of ions of finite size. 42 As a result a total capacitance has to be described by two capacitances in series. Namely by internal layer capacitance C I representing the rigid compact layer of adsorbed ions and the diffuse layer capacitance C D : 1 C total =1 C I +1 C D . (14) As a consequence two different surface potentials have to be distinguished. The potential ψ 0 at the interface or inner Helmholz plane (iHP) and the diffuse layer potential ψ D originating at the outer Helmholz plane (oHP). In order to solve the Poisson-Boltzmann equation for two approaching surfaces, boundary conditions are necessary. Classically constant charge (CC) and constant potential (CP) approximations are used. However, in real systems the charge regulation takes place and those approximations fail to describe the situation at relatively short separation distances. 47 Behrens and Borkovec 45 developed a simple approach, where the charge regulation is taken into account. They summarized the charge regulation of an interface by a single charge regulation parameter p: p=lim D→∞ p(D)=C D C D +C I (15) where D is the separation distance between surfaces. Hence in the case of asymmetric system the charge regulation of each surface should be accounted by corresponding charge regulation parameter. Numerically, the condition of constant potential corresponds 2. Theory / Status of the field 18 to p = 0, and that of constant charge corresponds to p = 1. The applicability of this approach to describe experimental data for colloidal systems has been demonstrated experimentally. 48 Figure 6b shows experimental data with fits according to full Poisson-Boltzmann equation including constant charge (CC), constant potential (CP), and constant regulation approximation (CR). As can be seen all three approximations describe the interaction profile well at large separation distances. Nevertheless, at close separation the CR approximation provides the most adequate description. In a symmetrical system there are only two fit parameters to be determined, namely the diffuse layer potential ψ D and the charge regulation parameter p. 44 The colloidal probes with known parameters can be further used for analytical purposes. After the “calibration” by determining the necessary parameters ( ψ D and p) of a colloidal probe in a symmetrical system, i.e. with sphere-sphere geometry one can determine those parameters for an unknown surface by fitting the corresponding interaction profiles determined in asymmetrical system. While fitting the latter the parameters for the colloidal probe are fixed. Then the fitted value of diffuse layer potential can be converted to apparent diffuse layer charge density, i.e. oHP charge density, using equation (12). If the sample surface has acquired the surface charge due to specific ion adsorption the latter equation is also called Grahame equation, because Grahame introduced for the first time the notion of specific and non-specific ion adsorption at an interface. 49 Hence an acquisition of force profiles on a sample locally allows resolving laterally the surface diffuse layer charge density. This is a noticeable advantage of colloidal probe technique in respect to electrokinetic and electrochemical methods. As follows from eq. (8) the Debye length decreases with increasing ionic strength of solution. As shown in Figure 7 the interaction profile between poly(ethylene imine) layers changes with changing ionic strength of the surrounding electrolyte solution. This alteration of the slope in semi-logarithmic representation corresponds to the change in charge screening effect characterized by Debye length. All profiles are repulsive because the surfaces are identical and hence have likewise charged electrostatic diffuse double layers. Besides Debye length, also the effective diffuse layer potential decreases. The full Poisson-Boltzmann equation fits to the data presented in Figure 7 shows that upon 2. Theory / Status of the field 19 increase of ionic strength from 10 -4 M to 10 -1 M the diffuse layer potential reduces from 58.2 mM to 7.4 mV. However, the charge regulation parameter demonstrates very weak dependence on the ionic strength if any. Figure 7: Force-distance profiles obtained upon approach between poly(ethylene imine) (PEI) layers in aqueous solutions at pH 4 and different ionic strengths. Profiles are fitted according to the full Poisson-Boltzmann equation including constant charge (CC, dotted curve top), constant potential (CP, dotted curve bottom), and constant regulation (CR, solid line) approximations. The molecular mass of the PEI is ca. 2kDa. The fitted diffuse layer potential ψ d and regulation parameter p for a single surface in the symmetrical system are as follows: (0.1mM) ψ d = 58.2 mV and p = 0.69; (1mM) ψ d = 31.6 mV and p = 0.69; (10mM) ψ d = 15.2 mV and p = 0.76; (100mM) ψ d = 7.4 mV and p = 0.78 (reproduced from Pericet-Camara et al. 50 ). The diffuse layer properties of surfaces depend not only on the presence of ionizable groups, but also on ion-adsorption in electrolyte solution. This effect can take place on inorganic interfaces, such as clays, as well as on organic interfaces of different surface energy. It is especially pronounced for hydrophobic surfaces in aqueous solutions, where the charging mechanism remains under discussion. 51-54 This question has been addressed by various experimental techniques. On the one hand, at the presence of a positively charged acidic interface indicated some results of molecular dynamic simulations and vibrational sum-frequency spectroscopy. 51,55,56 On the other hand, the existence of basic interface is supported by electrokinetic 57-62 and other spectroscopic experimental data 54,63 and is supported by some theoretical studies 64,65 . To provide further background for precise theoretical modeling alternative experimental approaches are in demand. 2. Theory / Status of the field 20 2.6. Adhesion and short-range forces In addition to long-range forces the forces acting only during contact of two bodies are extensively studied (cf. Figure 8a). 66,67 The latter play essential role in composite manufacturing process, paper making, colloidal transport in soils, friction between solids. In solution a number of forces typically ascribed to short-range forces. 68,69 To this category may belong various combinations of solvation 70 and structural 71 forces as well as forces due to chemical bonds 72 to name just a few. They can be identified upon approach of two colloidal bodies shortly before their contact and by forces necessary to separate those bodies being in contact with each other. The short-range forces are acting primarily from the contact area (cf. Figure 8a). 16 Although the short-range forces make significant contribution in adhesive properties of a material, the adhesion comprises both shortand long-range forces. Thus, all forces contribute to the total adhesive force. Generally, the work of adhesion is described as the work needed for separation between two colloids from contact to infinity in a given medium. 9,66 Often for two solids it is given by the so-called pull-off force F a (cf. F a /R Figure 8b). The latter represent the maximum applied force needed for the sample-probe separation, which can be readily measured upon retraction in a direct force measurement experiment. To describe the pull-off force obtained in this way various continuum models of contact mechanics can be applied. Two the most commonly used are the JKR model (Johnson-Kendal-Roberts) and DMT model (Derjaguin-Müller-Toporov). 9 Their limitation is based on the assumption that elastic deformation of bodies in contact agrees with the prediction of Hertz model. 16 Furthermore, both models presume the scalability of the force with the effective radius R eff of interaction. 38 However, they consider different ranges of interaction forces to bring dominant contribution. In the JKR model it is assumed that only the short-range interactions acting in the contact area contribute to the pull-off force (Figure 8a): F a = − (3/ 2) ⋅ π R eff W a , (16) where W a is the work of adhesion per unit contact area. For the DMT model the shortrange forces are neglected, and only the long-range forces outside the contact area are assumed to contribute to the total interaction force (Figure 8a). In this case the pull-off force is given by Fa = − 2 ⋅ π Reff Wa . (17) 2. Theory / Status of the field 21 Figure 8: Adhesion force measurements with colloid probe force microscopy. (a) A scheme showing the adhesion contributions between colloidal probe and a crystalline self-assembled monolayer in liquid. (b) Example of force-distance profiles between glass surface and glass 40 mkm-sphere in electrolyte solution at pH 2, I=0.2M. Profiles obtained upon approach and upon retraction are presented. Black arrows indicate the direction of the cantilever movement during the instability jumps (adapted from Adler et al. 73 ). In order to decide, which model to apply for a particular system Daniel Maugis 67 has developed a simple indicator by means of the following dimensionless parameter λ : λ =64 3 π D 0 W a 2 R eff 4 π E tot 2 3 . (18) where E tot is the reduced elastic modulus of the system, D 0 is the equilibrium separation of the surfaces in contact, and W a is the adhesion energy predicted by the Young-Dupré theory as W a = γ sample/H 2 O + γ probe/H 2 O − γ sample/probe . (19) where γ i/j are the interfacial energies of a probe and a sample in aqueous solutions. While at λ → 0 the DMT model describes the observed forces more appropriately, at λ → ∞ ( λ > 10) the JKR would provide a better description. That means in the case of small R eff and stiff samples, where the deformation of a sample can be neglected, the DMT model can be applied. For large R eff and soft samples the JKR model is commonly used. Thus, most of the surfaces represent some intermediate situations between two limiting models and the more general Maugis theory has to be applied. By colloidal probe technique the pull-off forces can be measured together with the interaction force profile upon approach. 7,72 For example, Giesbers et al. 32 studied interactions between acidand base-functionalized surfaces on the system of two self- 2. Theory / Status of the field 22 assembled monolayers terminated in amino –NH 2 and carboxy –COOH groups. They found a strong correlation between the long-range forces and the ionization state of groups on interacting surfaces, which can be explained on the basis of DLVO theory. However, also the adhesion between –NH 2 and –COOH terminated surfaces appeared to be pH-dependent. Giesbers et al. attributed this adhesive behavior to the presence of both acid-base interactions and hydrogen bonds. The adhesion due to the former was comparable to work of adhesion calculated from DMT model. 32 Nalaskowski et al. 74 measured the interaction between polyethylene spheres and silicon wafers in electrolyte solutions. They found that the pull-off force increases either upon thermal treatment of the silicon wafer or upon silanization with a hydrophobic self-assembled monolayer of the wafer. By employing the Lifshitz/van der Waals-Lewis acid/base interaction theory a semi-quantitative explanation of the observed forces was given. 74 Those examples show that though the interpretation of pull-off forces can be successfully achieved, the separation of force contributions in adhesion remains obscure. In addition to the pull-off force the maximum sample-probe force F i determined from approach profile can reveal valuable information about different contributions in the total interaction (cf. F i /R Figure 8b). Namely, the adhesive force contribution due to diffuse layer overlap, van der Waals and hydrophobic forces can be evaluated. However, wellknown “jump-in” effect does blur such effort as shown by the black arrow on the approach curve in Figure 8b. 75 To overcome this limit a sufficiently stiff cantilever can be used for measurements, though some sensitivity is lost. 9 Alternatively, the active “feed-back” system 76 or dynamic force spectroscopy 77 has to be used. The latter two approaches appear to be most used to measure forces without instabilities. 9 To separate different force contributions to the pull-off force, various approaches have been reported. For instance, by the variation of solution composition one can separate forces with electrostatic origin. 78 The influence of the substrate’s surface chemistry can be addressed by the functionalization of surface with thin layers terminating in different groups. 79 Alternatively, the external forces may be employed to compensate for some intrinsic forces, like van der Waals or electrostatic diffuse layer forces. 8,80,81 As would be expected the superposition of those principles could further clarify the ascribing of various contributions to the pull-off force in a particular system. 48 The surface roughness makes a quantification of the adhesive properties more difficult. There are many experimental and theoretical studies where the reduction of adhesion due to roughness effect has been reported. 71,82-85 Depending on the actual system 2. Theory / Status of the field 23 the typical reduction of experimentally determined work of adhesion reaches 20 – 200 times. In order to correct for the influence of roughness a number of models have been proposed. 75,82,86,87 Approaches developed by Rumpf 86 , Rabinovich 82,87 and Cooper 75 are often used for this purpose. A recent review on the various models provides a comprehensive overview. 88 Thus, the reduction of forces acting in contact region due to surface roughness could be accounted for. However, if repulsive long-range forces dominate the sample-probe interaction, then no pull-off force or attractive force can be detected. 29 Further it should be noted, that the surface roughness influences also long-range forces. For example Valtiner et al. 89 found that the strength of the electrostatic diffuse layer decreases if the roughness of substrate is increased. In their work the interaction between golden electrodes of different roughness and atomically smooth mica covered with self-assembled monolayer was studied. It was shown that the apparent diffuse layer potential decreases drastically with the increase of rms roughness from 3 to 12Å, though further increase until 17Å produced little effect. 89 2.7. Adhesive properties of organic surfaces by AFM & Electrochemistry Tuning the adhesive properties of organic interfaces is of high importance for a number of industrial and medical applications such as water purification or in artificial hearts, which pump the blood. 47,90 Therefore studying fundamental questions of their adhesive behavior promise to increase the effectiveness of purification processes and better predict biocompatibility of synthetic organic materials. As discussed earlier the defined variation of different force contributions would provide a possibility to better understand the adhesive properties of an interface and to tune the adhesion accordingly. In particular, one system that enables flexible change of shortand long-range interaction forces is an electrode covered with organic layer (cf. Figure 9a), which is impermeable for ions and solvent molecules. A suitable layer for this purpose is self-assembled monolayer (SAM) made by employing thiol chemistry on noble metal electrode. 91-96 Changing the groups terminating an SAM may additionally alter the interfacial properties of the organic surface designed in this manner. Indeed, as Vezenov et al. 97 have shown the adhesion over self-assembled monolayers depends on the nature of terminating group. Interaction forces over a potentiostatically-controlled electrode have been studied previously either for bare gold electrodes or other electrode materials. 80,98,99 Studies with AFM on modified electrodes have been reported more rarely. 48,89,100,101 In attempt to 2. Theory / Status of the field 30 (f) the same electrode in 3.0 mM Fe 3+ (reproduced from Ref. 132 ). Self-assembled films of thiols on gold reveal highly blocking electrochemical properties after sufficient deposition time as demonstrated by cyclic voltammetry. By appropriate technique, such as template stripping methods, metal electrodes with low surface roughness can be obtained. 133 Alternative techniques include polishing and a self-cleavage. Moreover, highly homogeneous coverage of the noble metal surface can be achieved by means of self-assembled monolayers. 131 Electrodes modified in this manner have the desired functional groups and retain the roughness of the substrate, though some defects may be present. 134,135 Due to mentioned advantages SAMs are well-suited model systems for studying of various organic interfaces. 95,102 To study surface forces and phenomena at organic interfaces SAM-covered electrodes represent a well-defined surfaces. Because the functional groups terminating the SAM can alter the interfacial properties, the system appears also to be very versatile. The approach to study such model systems with direct force measurement promises to extend the knowledge about adhesive behavior and specific ion adsorption at the interface. Due to tunability of diffuse layer properties by external potential the double layer forces may vanish or compensate other forces. Thus, different force contributions in adhesion may be studied separately. 2.9. Mechanical properties of ultrathin organic films The increasing trend toward miniaturization of electro-mechanical devices leads to a decrease of functional coating thickness down to a few nanometers. 136,137 However, the simple assumption that mechanical properties remain similar to the bulk material often fails. 90 The reliability of properties such as elasticity can be crucial in applications ranging from corrosion protection to substrate-dependent adhesion of biological cultures. Therefore, the reliable estimation of ultrathin film properties is a prerequisite for designing advanced nanomaterials. AFM-based setups for measuring mechanical properties by microand nanoindentation emerged soon after the invention of AFM in 1980s, though only in the end of 1990s they became popular in both biology and material science. 9,138 Up to now most of the experiments are performed with standard AFM tips. Nevertheless, those tips have poorly defined geometry and hence the resulting data are prone to high inaccuracies. 138 To improve the accuracy of the data one solution is to employ professional nanoindentation kits, which can be combined with existing AFMs. Another 2. Theory / Status of the field 31 solution is to use colloidal probes with micrometer-sized particles of known shape. In this case the range of materials, which can be indented, is determined by the stiffness of the cantilever. 9 Due to defined geometry and adjustable stiffness of the indenter the colloidal probe technique can be considered accurate and it is in particular very versatile for nanoindentation of soft films. 23,139 Figure 13: Elasticity measurements with colloidal probe force microscopy. (a) An exemplary force versus indentation curve with theoretical fit by modified Hertz model (reproduced from Richert et al. 140 ). (b) Dependence Young’s modulus on relative humidity of Starch films with different types of plasticizers (T = 298K). The mass ratios of Starch : Glycerol : Acid are 1 : 0.25 : 0.15. The lines represent eye-guide only (adapted from Jimenez et al. 141 ). The data acquisition for indentation experiment conducted with AFM is practically identical to direct force measurements. However, the force-displacement data are converted to loading force versus indentation depth. Various systems have been studied by this technique, though the most common are biological samples and polymeric coatings. 66 For example, Hassan et al. 24 studied semi-quantitatively the elasticity of living cells and obtained images of mechanical response with a good contrast for many species. As demonstrated by Lisunova et al. 142 the Young’s modulus of polyelectrolyte multilayers varies from dozens-hundreds of kPa in the swollen state up to hundreds of MPa in dried one; while for hydrogels used as scaffolds for cell culturing the Young’s modulus is typically within 1-100 kPa range. 27,66,143 In Figure 13a an example forcedistance curve of a multilayer indented by a spherical probe is presented. 140 After taking into account bending of the cantilever, the resulting curve may be fitted by different models that assume linear or nonlinear elastic behavior of the sample. 2. Theory / Status of the field 32 To analyze indentation data obtained by the colloidal probe technique for synthetic quasi-homogeneous materials the Hertz model, based on the assumption of linear elasticity, is widely used. 138 However, this model has been criticized for its inaccuracy due to some limitations. 144 Nevertheless, Lin et al. 145 have recently shown, that this model can be used for description of the measured force versus indentation if two conditions are matched: low indentation strain (<20%) and indentation depth sufficiently higher than the surface roughness. In this case the indentation force profiles can be fitted by Hertz model with a small mean square error. Moreover, it was shown that models assuming non-linear elasticity (e.g. Mooney-Rivlin, Ogden, and Fung) provide similar description to constrained Hertz model if the number of fittable variables remains equal. 145 The nanoindentation experiments are commonly performed under ambient conditions. While some polymers can adsorb moisture from the air, the condensed water may act as a plasticizer in a polymer film. Thus, the relative humidity may determine the apparent stiffness of multilayer film. As demonstrated by Jiménez et al. 141 the relative humidity has larger effect on the stiffness of the Starch-Glycerol film, than addition of plasticizers (cf. Figure 13b) Control of moisture-related change of mechanical characteristics promises also to improve the durability of packaging materials and stability of microelectromechanical devices. 146 Nevertheless, until now few studies have addressed the mechanical response of ultrathin film systems under controlled environmental conditions. 1. Binnig, G., Rohrer, H., Gerber, C. & Weibel, E. Surface Studies by Scanning Tunneling Microscopy. Phys. Rev. Lett. 49, 57-61 (1982). 2. Binnig, G., Quate, C. F. & Gerber, C. Atomic Force Microscope. Phys. Rev. Lett. 56, 930-933 (1986). 3. Israelachvili, J. N. & Adams, G. E. 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Rabinovich, Y. I., Adler, J. J., Ata, A., Singh, R. K. & Moudgil, B. M. Adhesion between Nanoscale Rough Surfaces. J. Colloid Interface Sci. 232, 17-24 (2000). 2. Theory / Status of the field 39 83. Beach, E. R., Tormoen, G. W., Drelich, J. & Han, R. Pull-off Force Measurements between Rough Surfaces by Atomic Force Microscopy. J. Colloid Interface Sci. 247, 84-99 (2002). 84. Götzinger, M. & Peukert, W. Particle Adhesion Force Distributions on Rough Surfaces. Langmuir 20, 5298-5303 (2004). 85. Tormoen, G. W., Drelich, J. & Beach, E. R. Analysis of Atomic Force Microscope Pull-off Forces for Gold Surfaces Portraying Nanoscale Roughness and Specific Chemical Functionality. Journal of Adhesion Science and Technology 18, 1-17 (2004). 86. Rumpf, H. & Bull, F. A. Particle technology (Chapman & Hall, London, 1990). 87. Rabinovich, Y. I., Adler, J. J., Ata, A., Singh, R. K. & Moudgil, B. M. Adhesion between Nanoscale Rough Surfaces. J. Colloid Interface Sci. 232, 10-16 (2000). 88. Prokopovich, P. & Starov, V. Adhesion Models: From Single to Multiple Asperity Contacts. Adv. Colloid Interface Sci. 168, 210-222 (2011). 89. Valtiner, M., Kristiansen, K., Greene, G. W. & Israelachvili, J. N. Effect of Surface Roughness and Electrostatic Surface Potentials on Forces Between Dissimilar Surfaces in Aqueous Solution. Advanced Materials 23, 2294-2299 (2011). 90. Stuart, M. A. C. et al. Emerging Applications of Stimuli-Responsive Polymer Materials. Nature materials 9, 101-113 (2010). 91. Yamamoto, Y. Self-Assembled Layers of Alkanethiols on Copper for Protection Against Corrosion. J. Electrochem. Soc. 140, 436 (1993). 92. Gooding, J. J., Mearns, F., Yang, W. & Liu, J. Self-Assembled Monolayers into the 21st Century: Recent Advances and Applications. Electroanalysis 15, 8196 (2003). 93. Folkers, J. P., Laibinis, P. E. & Whitesides, G. M. Self-Assembled Monolayers of Alkanethiols on Gold: Comparisons of Monolayers Containing Mixtures of Shortand Long-Chain Constituents with Methyland Hydroxymethyl Terminal Groups. Langmuir 8, 1330-1341 (1992). 94. Love, J. C., Estroff, L. A., Kriebel, J. K., Nuzzo, R. G. & Whitesides, G. M. SelfAssembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chemical Reviews 105, 1103-1169 (2005). 95. Porter, M. D., Bright, T. B., Allara, D. L. & Chidsey, C. E. D. Spontaneously Organized Molecular Assemblies. 4. Structural Characterization of n-Alkyl Thiol Monolayers on Gold by Optical Ellipsometry, Infrared Spectroscopy, and Electrochemistry. J. Am. Chem. Soc. 109, 3559-3568 (1987). 3. Overview of the Thesis 46 Figure 14: (a) SEM image of a typical colloidal probe obtained by sintering procedure. (b) Lateral force versus displacement graph captured by a cantilever with attached “unbreakable” colloidal particle during the movement against periodical structure. The peaks indicate on twisting of the cantilever, while the probe sustains the pressure against hard wall. For such a probe particle-cantilever connection was reinforces by the “neck” from nanoparticles (cf. insert) the thermal renewal of sintered probe surface chemistry can be considered as another advantage of the method. Since in aqueous media the hydration of silicon oxide with silicic acid formation may occur, the possibility to reset the properties would be valuable. Indeed, at high temperature the dehydration occurs. 1 3.2. Adhesion control at organic interfaces by electrochemistry (chapter 5) Controlling adhesive properties by external stimuli such as potentials applied to an electrode is important for many applications like production of MEMS. As a model system electrodes covered with various non-ionizable SAMs were used here. Usage of stiff cantilevers with enhanced sensitivity suitable for aqueous solutions was essential to probe the full interaction range. Without loosing sensitivity it enabled to obtain interaction profiles without instabilities in the contact region. That in turn allowed 3. Overview of the Thesis 47 accurate detection of forces acting directly before the contact between the probe and a sample. Non-ionizable SAMs, which were used in this work, differed only by the terminating groups, though their potentiostatically-mediated adhesive behavior appeared to be very different. We found that the adhesion towards silica colloidal probe for the hydrophilic OH-terminated SAM emerges at potentials above the potential of zero charge (pzc). Hence adhesive forces emerged only when the electrode became oppositely charged to the negatively charged probe. By contrast, for the hydrophobic CH 3 -terminated SAM, a non-zero adhesion is present even at potentials below and above the pzc. Furthermore, flexible compensation of forces given by diffuse layer overlap enabled estimation of the solvent exclusion and van der Waals (vdW) forces. Long-range forces due to diffuse layer overlap dominate the adhesion of rough colloidal particles to the flat electrode. In the top part of Figure 15a the dependence of maximal attractive forces recorded upon approach (F i /R) and retraction, i.e. pull-off forces (F a /R) are presented, where the values are normalized to the effective radius of interaction R. The interaction force profiles were acquired between a silica colloidal probe and CH 3 -terminated electrode in electrolyte solutions. With increasing ionic strength the attractive forces generally decrease, while the increase in the applied potential leads to an increase of attractive forces. At low ionic strength the F i /R given by DLVO forces accounts for the large part of F a /R. Since we found very small vdW forces in the system, it is the electrostatic diffuse double layer contribution that dominates DLVO forces. Hence this contribution dominates the total adhesive forces at low ionic strength. Similar behavior has been demonstrated by hydrophilic SAM, where nonDLVO forces (e.i. solvent exclusion forces) are mediocre. 3. Overview of the Thesis 48 Figure 15: (a, top) Pull-off force F a /R and maximal attractive force recorded upon approach F i /R between CH 3 -terminated electrode and silica colloidal probe as a function of applied potential (statistics of ca. 100 force curves for every point). The data series for two ionic strengths are presented (pH 4.7). (a, bottom) The difference ∆ F/R between corresponding pull-off force and maximal attractive force recorded upon approach as a function of applied potential. Vertical line represents maximal difference observed at the potential of zero charge (pzc). (b) The top and bottom graphs are analogs to graph (a), but for a single-asperity silicon tip. In both graph statistics of ca. 100 force curves for every point is presented. The difference ∆ F/R between F a /R and F i /R determined by colloidal probe shows a slight dependence from the external potential (cf. Figure 15a, bottom). This behavior is not in line with solvent exclusion, but resembles more the electrocapillarity effect. However, that could be excluded by measurements with a single asperity on the same SAM (cf. Figure 15b, bottom). Moreover, the long-range forces have much less influence on the total adhesion force if probed by single asperity (cf. Figure 15b, top). Thus, the variation of ∆ F/R with applied potential for colloidal probe results probably from the instability of the cantilever. Besides force contributions, the role of surface roughness in the adhesion process has been quantified by solvent exclusion forces. The surface roughness leads to significant reduction of the theoretically predicted adhesion according to JKR theory. However, 3. Overview of the Thesis 49 detailed analysis of particle surface roughness using the Rabinovich model 2 provides a good quantitative description of the reduction factors. Thus, the increase in surface roughness results in a decrease of non-DLVO forces (i.e. solvent exclusion forces) and correspondingly of the total adhesive force. 3.3. Ion adsorption probed by direct force measurements (Chapter 6) Modified electrodes provide also a versatile model system to study the ion adsorption on non-ionizable organic interfaces. Here the long-range interaction forces have been determined and analyzed quantitatively by fits to the full solution of the PoissonBoltzmann equation. For SAM-modified electrodes the diffuse layer potential ψ D can be tuned by external potential ϕ as shown in Figure 16a. Again, the hydrophilic and hydrophobic SAMs have been studied. Additionally to the electronic potential the pH of the electrolyte solution has been varied. Figure 16: (a) Diffuse layer potential dependence of OH-terminated modified electrode on applied potential at various pH (I ca. 1 mM). Continuous curves represent global fit to the three-capacitor model. 3 (b) Dependence of potential of zero-charge (PZC) on pH of the solution for OHand CH 3 - terminated modified electrodes. The solid lines represent fits to the three-capacitor model. 3 In this work we attempted to describe the modified electrode system with a model that includes blocking SAM, adsorbed ions, and the diffuse double layer. The dependence of the diffuse layer potential from external potential should be altered significantly if ion adsorption takes place. In particular the potential of zero charge (pzc) changes due to the additional charges located at the interface (cf. Figure 16b). Nevertheless, the pzc remains indifferent to the concentration of the background electrolyte (KCl). 4 We have been able 3. Overview of the Thesis 50 to provide a semi-quantitative description of this dependence based on a simple threecapacitor model that takes into account the adsorption of hydroxyl and hydronium ions. 3 By fitting simultaneously the series ψ D (ϕ) for different SAMs we determined the adsorption constants for hydroxide and hydronium ions. Data for both OHand CH 3 - terminated electrodes suggests the specific adsorption of those ions, while the surface remains indifferent to the ions of background electrolyte. The adsorption constants are higher for hydrophobic CH 3 -terminated SAM, than for hydrophilic OH-terminated SAM. At the same time the adsorption of hydroxide ions is stronger than of hydronium. This novel approach, described in chapter 7, has possible implications in the development of ion-selective electrodes, “smart” coatings, and surface plasmon resonance sensors. 3.4. Mechanical properties of ultrathin films by nanoindentation (chapter 7) In chapter 7 the results on mechanical properties of ultrathin films determined by nanoindentation are reported. Within the project polyelectrolyte multilayer (PEM) films were prepared in the group of Prof. A. Fery by K. Trenkenschuh. 5 They were probed by colloidal probe technique at controlled humidity. The investigated PEM-films contained poly(allylamine hydrochloride) (PAH), poly(styrenesulfonate) (PSS), and poly(glutamic acid) (PGA). The latter tends to adsorb atmospheric water and change its conformation. As in similar systems the water in the PEM serves as plasticizer, hence increasing the plasticity and decreasing the stiffness of the multilayer film. We found that upon decreasing the relative humidity (RH) from 80% to 12,5% the Young’s modulus of the film increases up to two orders of magnitude (cf. Figure 17). Furthermore, the process appears to be reversible, albeit with some hysteresis. 3. Overview of the Thesis 51 Figure 17: Dependence of Young’s modulus of the (PAH/PGA 0.88 -PSS 0.22 ) polyelectrolyte multilayer with polyglutamic acid as a function of relative humidity. To analyze indentation data obtained by colloidal probe technique the measured force versus indentation profiles were compared to the modified Hertz model. 6 At low strain the indentation yields force profiles, which can be readily described by the model. Another prerequisite for compliance with the model is an indentation depth sufficiently larger than the surface roughness. The Young’s moduli determined by direct force measurements agree well with data obtained by the wrinkling metrology method. 7 References: 1. Nalaskowski, J., Drelich, J., Hupka, J. & Miller, J. D. Adhesion between Hydrocarbon Particles and Silica Surfaces with Different Degrees of Hydration As Determined by the AFM Colloidal Probe Technique. Langmuir 19, 5311-5317 (2003). 2. Rabinovich, Y. I., Adler, J. J., Ata, A., Singh, R. K. & Moudgil, B. M. Adhesion between Nanoscale Rough Surfaces. J. Colloid Interface Sci. 232, 17-24 (2000). 3. Duval, J., Lyklema, J., Kleijn, J. M. & van Leeuwen, H. P. Amphifunctionally Electrified Interfaces: Coupling of Electronic and Ionic Surface-Charging Processes. Langmuir 17, 7573-7581 (2001). 4. Rentsch, S. Direct Force Measurements Between Surfaces Under Potentiostatic Control. PhD thesis (University of Geneva, Geneva, 2008). 3. Overview of the Thesis 52 5. Trenkenschuh, K. Buildup and Mechanical Properties of Multicomponent Polyelectrolyte films. PhD Thesis (University of Bayreuth, Bayreuth, 2012). 6. Lin, D. C., Dimitriadis, E. K. & Horkay, F. Elasticity Models for the Spherical Indentation of Gels and Soft Biological Tissues. Mater. Res. Soc. Symp. Proc. 1060, 1060-LL05-07 (2008). 7. Stafford, C. M. et al. A Buckling-Based Metrology for Measuring the Elastic Moduli of Polymeric Thin Films. Nature materials 3, 545-550 (2004). 3.5. Individual Contributions to Joint Publications In this part of the overview the individual contributions of the authors to each manuscript are specified. Chapter 4 This chapter is published in Review of Scientific Instruments (2012, 83, 116103) under the title: “Mechanically and Chemically Stable Colloidal Probes from Silica Particles for Atomic Force Microscopy” by Volodymyr Kuznetsov and Georg Papastavrou I performed the experiments as well as the data analysis and wrote the manuscript. Georg Papastavrou supervised the project and participated in the writing of the manuscript. Chapter 5 This chapter is published in Langmuir (2012, 28, 48, 16567-79) under the title: “Adhesion of Colloidal Particles on Modified Electrodes” by Volodymyr Kuznetsov and Georg Papastavrou I performed the experiments as well as the data analysis and wrote parts of the manuscript. Georg Papastavrou supervised the project and wrote the final version of the manuscript. Chapter 6 This chapter is intended for submission to Journal of Physical Chemistry C under the tentative title: “Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control” 3. Overview of the Thesis 53 by Volodymyr Kuznetsov and Georg Papastavrou I performed the experiments as well as the data analysis and wrote the manuscript. Georg Papastavrou supervised the project, was involved in the scientific discussion and corrected the manuscript. Chapter 7 This chapter is published in Macromolecules (2011, 44, 8954–61) under the title: “Tuning of the Elastic Modulus of Polyelectrolyte Multilayer Films built up from Polyanions Mixture” by Katja Trenkenschuh, Johann Erath, Volodymyr Kuznetsov, Julia Gensel, Fouzia Boulmedais, Peter Schaaf, Georg Papastavrou, and Anreas Fery Katja Trenkenschuh prepared samples with polyelectrolyte multilayers, characterized them by ellipsometry and wrinkling metrology method, did data analysis and wrote parts of the paper. Johann Erath did nanoindentation experiments under ambient conditions, wrote a part of the manuscript, and was involved in the scientific discussion. My contributions were the preparation of colloidal probes for all nanoindentation experiments, performed nanoindentation experiment at controlled humidity, wrote a part of the manuscript, and was involved in the scientific discussion. Julia Gensel conducted initial experiments with ellipsometry and wrinkling metrology method, and was involved in the scientific discussion. Fouzia Boulmedais performed Fourier transformed infrared spectroscopy in the attenuated total reflection mode, wrote a part of the manuscript, and was involved in the scientific discussion. Peter Schaaf was involved in the scientific discussion concerning the results by infrared spectroscopy. Georg Papastavrou was involved in the scientific discussion concerning the nanoindentation measurements and corrected the manuscript. Anreas Fery supervised the project, was involved in the scientific discussion and corrected the manuscript. 4. Mechanically and chemically stable colloidal probes from silica particles for atomic force microscopy 55 4. Mechanically and chemically stable colloidal probes from silica particles for atomic force microscopy Volodymyr Kuznetsov and Georg Papastavrou* Department of Physical Chemistry II, University of Bayreuth, Universitätstraße 30, 95440 Bayreuth, Germany *E-mail corresponding author: [email protected] Published in Review of Scientific Instruments 2012, 83, 116103 4. Mechanically and chemically stable colloidal probes from silica particles for atomic force microscopy 62 Since the silica colloidal probes are obtained by sintering processes, they are completely inorganic. Therefore they can be used in or cleaned by various aqueous solutions or organic solvents. Additionally they can be thermally treated at temperatures of above 800° C. Such a heat-treatment for silica particles is advantageous not only in terms of the removal of organic contaminants but to provide a well-defined surface chemistry 22 . The here-presented method of sintering with small Ludox-particle (i.e. procedure C) allows thus the preparation of highly stable colloidal probes with the wellknown surface chemistry of colloidal silica. The introduction of a ‘fixation neck’ is not limited to silica particles and can be applied as well for organic or inorganic materials with lower melting points. This work has been supported by the German Research Foundation (SFB 840). The authors thank C. Kunert for the SEM images. References: 1 W. Ducker, T.J. Senden, and R.M. Pashley, Nature 353, 239 (1991). 2 H. Butt, Biophys J 60, 1438 (1991). 3 H. Butt, B. Cappella, and M. Kappl, Surf Sci Rep 59, 1 (2005). 4 T. Muster, G. Toikka, R. Hayes, C. Prestidge, and J. Ralston, Colloid Surface A 106, 203 (1996). 5 M. Kappl and H. Butt, Part Part Syst Char 19, 129 (2002). 6 R. Cain, N. Page, and S. BIGGS, Phys Rev E 62, 8369 (2000). 7 A. Fery, F. Dubreuil, and H. Mohwald, New J Phys 6, 18 (2004). 8 K. Trenkenschuh, J. Erath, V. Kuznetsov, J. Gensel, F. Boulmedais, P. Schaaf, G. Papastavrou, and A. Fery, Macromolecules 44, 8954 (2011). 9 Y. Gan, Rev Sci Instrum 78, 081101 (2007). 10 E. Bonaccurso, Investigation of Electrokinetic Forces on Single Particles, PhD thesis, Universität-Gesamthochschule Siegen, 2001. 11 O. Vinogradova, G. Yakubov, and H. Butt, J Chem Phys 114, 8124 (2001). 12 J. Ally, E. Vittorias, A. Amirfazli, M. Kappl, E. Bonaccurso, C.E. McNamee, and H.-J. Butt, Langmuir 26, 11797 (2010). 13 E. Bonaccurso, M. Kappl, and H. Butt, Phys Rev Lett 88, 076103 (2002). 14 M. Indrieri, A. Podesta, G. Bongiorno, D. Marchesi, and P. Milani, Rev Sci Instrum 82, (2011). 4. Mechanically and chemically stable colloidal probes from silica particles for atomic force microscopy 63 15 I. Popa, G. Gillies, G. Papastavrou, and M. Borkovec, J Phys Chem B 113, 8458 (2009). 16 S.-J.L. Kang, Sintering (Butterworth-Heinemann, 2005). 17 P.M. Ajayan and S. Iijima, J Am Ceram Soc 75, 999 (1992). 18 S. Rentsch, R. Pericet-Camara, G. Papastavrou, and M. Borkovec, Phys Chem Chem Phys 8, 2531 (2006). 19 cf. eq. (10) in M. Munz, J Phys D Appl Phys 43, (2010). 20 J.L. Hutter and J. Bechhoefer, Rev Sci Instrum 64, 1868 (1993). 21 R. Cannara, M. Eglin, and R. Carpick, Rev Sci Instrum 77, (2006). 22 M. Kobayashi, M. Skarba, P. Galletto, D. Cakara, and M. Borkovec, J Colloid Interf Sci 292, 139 (2005). 5. Adhesion of Colloidal Particles on Modified Electrodes 65 5. Adhesion of Colloidal Particles on Modified Electrodes Volodymyr Kuznetsov and Georg Papastavrou* Department of Physical Chemistry II, University of Bayreuth, Universitätstraße 30, 95440 Bayreuth, Germany *E-mail corresponding author: [email protected] Published in Langmuir 2012, 28(48), 16567-79 5. Adhesion of Colloidal Particles on Modified Electrodes 66 Abstract The adhesion between colloidal silica particles and modified electrodes has been studied by direct force measurements with the colloidal probe technique based on the atomic force microscope (AFM). The combination of potentiostatic control of gold electrodes and their chemical surface modification by self-assembled monolayers (SAMs) allows for the decoupling of forces due to the electrical double layers and chemical functionality of the solid/liquid interface. Adhesion on such electrodes can be tuned over a large range in dependence of the externally applied potential and the aqueous solution’s ionic strength. By utilizing cantilevers with a high force constant, it is possible to separate the various contributions to the adhesion in an unambiguous manner. These contributions comprise diffuse layer overlap, van-der Waals forces, solvent exclusion, and electrocapillarity. A quantitative description of the observed adhesion forces is obtained by taking into account the surface roughness of the silica particle. The main component in the adhesion, which is tuned by the external potential, originates from the overlap of the electrical double layers. By contrast, effects due to electrocapillarity are only of minor importance. Based on our quantitative analysis a new approach is proposed that allows tuning the adhesion force as a function of the externally applied potential. We expect this approach to have important applications for the design of microelectromechanical systems (MEMS), the development of electrochemical sensors as well as for microand nanomanipulation. Introduction Adhesion between surfaces has a strong influence on various processes such as colloidal transport in soils, wafer cleaning, colloidal aggregation, or friction between solids. 1-3 Adhesion is mediated by surface forces and represents a ubiquitous phenomenon in the colloidal domain. Typically, long-ranged interaction forces determine whether a colloidal particle can approach another surface closely enough to reach contact. 1 After this point, additional short-ranged interaction mechanisms have a strong influence on the sticking probability of a colloid or determine whether a given shear force is sufficient to remove an adhering colloidal particle. In order to tune the adhesion for industrial and technical applications, various approaches have been proposed. Most of these are based on permanently altering the surface properties, for example by polyelectrolyte coatings or monolayers of silanes or thiols. 4,5 5. Adhesion of Colloidal Particles on Modified Electrodes 67 Interaction forces of various origins can mediate adhesion. The long-ranged force contributions are summarized in the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO) and result from the interaction of the diffuse layers as well as the van-der-Waals (vdW) forces. 2 Short ranged forces are only acting in the contact area. Such forces result from solvent exclusion as well as the formation of chemical bonds and have been studied by chemical force microscopy. 6-11 In the case of polymeric interfaces, additional force contributions can arise due to steric or bridging forces. 12 With increasing miniaturization of mechanical devices down to the microand nanometer scale, the control of adhesive properties has become increasingly important for their performance and fabrication. 13 In particular, the possibility to control directly and instantaneously the adhesion would be important for many applications. Suitable external stimuli to trigger changes in adhesion can be for example electrical potential, illumination by light, or variation of the temperature. The latter two approaches have been studied extensively in recent years and typical examples are thin organic films based on PNIPAM or azo-benzene, respectively. 14,15 However, an electrode connected to a potentiostat provides for many applications a more direct and versatile approach to control surface properties. Additionally, it allows easily for computer control. Electrosorption has been studied for a long time in the field of electrochemistry. 16 By contrast, the concept of applying electric potentials to control adhesion processes on the nanoand micrometer level in the field of colloid science has been exploited only recently. 17 The development of direct force measurement techniques, in particular the atomic force microscope (AFM) or the surface force apparatus (SFA), allowed in recent years to study these interaction forces for various systems in detail. While the SFA allows determining the interaction forces for a defined contact area, the atomic force microscope offers force resolution below 100 pN and thus can detect single bond ruptures as well as stretching and detachment of single polymer chains. By modifying the surface chemistry of the AFM tip, one can probe the interaction forces between well-defined functional groups. 11 This approach is commonly referred to as chemical force microscopy. 6-11 The development of the colloidal probe technique for the AFM allowed for a combination of well-defined interaction geometries with high force sensitivity. 18,19 The colloidal probe approach has been especially useful in adhesion studies due to its great versatility in choosing the probe surface. 20 5. Adhesion of Colloidal Particles on Modified Electrodes 68 As the SFA and the AFM can be interfaced directly with an electrochemical setup, the corresponding adhesion processes can be studied in detail by direct force measurements. 21 Initial studies on bare noble metal electrodes show that the adhesion can be indeed tuned by the external potential. 22-26 Similar behavior has been observed for semi-conductors or organic interfaces. 27 However, a quantitative interpretation of the results and identification of the dominating interaction mechanism is still far from complete. Recent studies with the electrochemical SFA indicated the importance of electrocapillarity for the adhesion. 28 Furthermore, the effect of surface roughness does not only influence the interaction forces upon approach but also the adhesion. 29 In this study we provide a new approach to study the adhesion mechanisms on electrode surfaces by determining the adhesion behavior not only as a function of the external potential but also the surface chemistry of the electrode. The latter is achieved by means of self-assembled monolayers on the electrode. The interaction force profiles are measured with the colloidal probe method and are evaluated quantitatively in order to separate the long-ranged force contributions to the adhesion from short ranged contributions arising only due to the contact of the surfaces. Adhesion to electrodes has been studied previously by AFM and SFA, in particular how besides the external potential other parameters such as roughness and surface chemistry influence the adhesion behavior. 17,23,24,24-32 However, by introducing a surface modification of the electrode with different ω-functionalized thiols it is possible to decouple chemical contributions from electrostatic ones. The latter can be varied instantaneously by the external potential applied to the electrode, while the former is given by the terminating functional groups of the SAM. Thus, in difference to the aforementioned studies the interdependence of these different contributions to the adhesion of colloidal objects can be determined in detail. Experimental Methods Materials. 16-mercaptohexadecanol-1 (99%, Frontier Scientific) and 1hexadecanthiol (99%, Sigma Aldrich) were used for the electrode modification as received. Ethanolic solutions were prepared from ethanol of analytical grade. All aqueous solutions were prepared from deionized water of Milli-Q grade with a resistivity larger than 18MΩ. After regulating the pH of solution to pH 4.7 by addition of a traceable volume of 1M HCl (Sigma Aldrich), the total ionic strength has been adjusted to the 5. Adhesion of Colloidal Particles on Modified Electrodes 69 nominal values of 0.12 mM, 0.34 mM, 0.56 mM or 5 mM by addition of 1M KCl (Sigma Aldrich). Preparation of flat gold electrodes. Smooth gold substrates were prepared by a modified ‘template stripping’ method. 33 We used as substrates p-doped (<20 Ohm/cm) silicon (100) wafers (CrysTec, Berlin, Germany), which have been cleaned beforehand by a modified RCA-procedure. 34 On these cleaned Si-wafers a layer of 60 nm Au (99.99% purity) has been deposited by thermal evaporation. Directly after evaporation RCAcleaned glass slides of 11x11 mm are glued to the gold layer by means of a chemically resistant adhesive (EPO-TEK 377, Epoxy Technology Inc.), which was thermally cured for one hour at 150 °C. For the preparation of the electrodes, these glass/glue/gold sandwiched slides were mechanically separated from the wafer and immediately rinsed copiously with ethanol and then transferred to the thiol solution. Surface modification by thiols was performed in 1mM ethanolic solution of 16-mercaptohexadecan-1-ol or 1-hexadecanthiol, respectively, for at least 12 hours. The electrodes with OH-terminated SAM were rinsed with copious amount of ethanol and then with water. The electrodes with CH 3 -terminated SAM were rinsed with copious amount of ethanol and then sonicated twice in fresh ethanol in an ultrasonic bath. The electrodes were then immediately mounted in the electrochemical cell, covered with degassed electrolyte solution and AFM fluid cell has been closed. Preparation of colloidal probes and AFM-cantilever for force measurements. Tipless AFM cantilevers (NSC12, Mikromasch, Lithuania) were cleaned consecutively in a series of solvents (ethanol, aceton, chloroform, aceton, and ethanol) and were then treated in O 2 - plasma (Flecto10, Plasma Technology GmbH, Germany) at 100 W and 0.4 mbar for 90 sec. After this cleaning procedure they were coated by thermal evaporation (mini-coater, tectra, Germany) with a reflection layer consisting of 3 nm Cr (SigmaAldrich) as adhesion promoter and 60 nm Au. In order to avoid thermal drifts the tipless cantilevers have been coated from both sides. For the preparation of colloidal probes single silica particles (Bangs Laboratories, IN) with an approximate diameter of 6.8μm were attached to these cantilevers by means of a micromanipulator and UV-curable glue (Optical adhesive 63, Norland Products). We used cantilevers with nominal force constants in the range of 0.2 N/m and of 5.6 N/m. The spring constants of these cantilevers have been determined before the attachment of the colloidal particles by the 5. Adhesion of Colloidal Particles on Modified Electrodes 70 thermal noise method. 35 Directly before the force measurements the colloidal probes and normal AFM-cantilevers were cleaned by plasma treatment. AFM imaging. The surface topography of the modified and unmodified electrodes as well as for heat-treated silica particles was determined by tapping mode AFM in air (Dimension 3100 equipped with a NanoScope V controller, Bruker). These measurements were performed with cantilevers (OMCL-AC160TS, Olympus) preselected for a tip radius below 10 nm. This selection has been performed by imaging a Nioprobe standard (Aurora Nanodevices, BC, Canada). The topography of colloidal silica particles was imaged with a scan size of 1x1 μm around the apex of the particle. The surface roughness has been determined after a 2nd-order plane fit of the height image with the software package belonging to the AFM (Bruker, Research NanoScope 7.30). Figure 1: Schematic representation of the experimental setup to determine the adhesion between a colloidal silica particle attached to the end of an AFM-cantilever and a gold electrode modified by a self-assembled monolayer (SAM) terminating in non-ionizable functional groups. The electrode is connected to a potentiostat in a 3-electrode electrochemical cell with working (WE), counter (CE), and reference (RE) electrodes. Combined setup for electrochemistry and AFM. In order to measure interaction forces under potentiostatic control we constructed a custom-made electrochemical cell, which could be adapted as semi-closed fluid cell to a MFP-3D (Asylum Research, Santa Barbara, CA). 36,37 The potentiostat for the 3-electrode electrochemical cell is custom-built and is based on a design from the group of H. Siegenthaler (University of Berne, Switzerland). Comparable potentiostats have been used in potentiostatically controlled direct force measurements and electrochemical scanning tunneling microscopy. 36,38 The working electrode is the thiol-modified gold electrode and the counter-electrode is a 100 mm gold wire with a diameter of 0.25 mm (Alfar-Aesar). As reference electrode we used 5. Adhesion of Colloidal Particles on Modified Electrodes 71 an Ag/AgCl-wire, which has been placed in a circular manner around the working electrode. Dissolution of AgCl has been neglected due to the small dissociation constant (2×10 -6 M) in respect to the lowest concentration of Cl - (1.2×10 -4 M). This pseudoreference electrode has been calibrated against a commercial Ag/AgCl-reference electrode (Metrohm, Switzerland) in the same solutions as used for the direct force measurements. In order to allow comparison of our results with previous studies, we converted the potentials to the ones versus a standard calomel electrode (SCE). Immediately before the force measurements, the fluid cell was extensively rinsed with Milli-Q water. The gold counter-electrode was carefully annealed in a butane gas flame. All aqueous solutions for the electrochemical measurements have been degassed by means of nitrogen and consecutively with an HPLC-degassing unit directly before the measurements (FLOM Gastorr BG12). The performance of the thiol-coated electrodes with the electrochemical AFM-cell and the potentiostat has been verified in an independent set of experiments by acquiring cyclic voltammograms in an aqueous solution of 10mM [Fe(CN) 6 ] 4and 250mM KCl at pH 6. In cyclic voltammograms the current is acquired as a function of the externally applied potential. In the case of hexacyanoferrat the position and area of the known oxidation and reduction peaks allow confirming that parameters like electrode surface and reference electrode potential are within the specifications. Cyclovoltammograms have been also been performed for the pure electrolyte solutions used for the measurements. These voltammograms are important to determine the potential window where the thiol layer remains stable on the electrode. The formation of defects at low or high potentials is accompanied by an increase of the current. Before each set of force measurements the presence and quality of the thiol layer on the modified gold electrodes has been controlled by cyclic voltammetry. However, for these controls the same electrolyte solutions as for the acquisition of force profiles has been used and no [Fe(CN) 6 ] 4was present. During the force measurement the current in the electrochemical cell has been monitored to verify that no thiol desorption is taking place. The potential range that can be applied without occurrence of thiol desorption has been determined independent on the direct force measurements. Direct force measurements. The interaction forces were measured with an AFM equipped with a closed loop control for all three axes (MFP-3D, Asylum Research, CA). For each applied potential, a series of at least 100 approach and retraction cycles with a 5. Adhesion of Colloidal Particles on Modified Electrodes 78 The approach part of the force profiles is dominated at large separation distances by the force resulting from the overlap of the diffuse layers formed at the interfaces of the silica particle and the electrode, respectively. With increasing ionic strength this interaction is more short-ranged due to the reduced extension of the diffuse layers. The force profiles upon approach (i.e. open symbols in Figure 3) have been fitted to the full solutions of the Poisson-Boltzmann equation including charge regulation by means of the constant regulation approximation (full lines in Figure 3). 39 A quantitative evaluation of the diffuse layer properties as a function of the externally applied potential will be given in the following paragraph. For attractive interactions one can determine a force directly before the probe makes contact with the electrode (cf. Figure 3a). In the subsequent retraction of the colloidal probe, the silica particle is separated from the electrode surface and its adhesion to the electrode can be determined. Of special interest is in this respect the pull-off force necessary to separate the silica particle from the electrode. This force is indicated by in Figure 3. For the negative potential the total interaction force upon separation is repulsive (i.e. positive sign) and thus the particle would not remain attached to the electrode without externally exerted force. Depending on the applied potential, the adhesion forces can be tuned from repulsive to attractive. However, one finds qualitative differences between the hydrophilic OH-terminated SAM and the hydrophobic CH 3 -terminated SAM. For the former the force profiles upon approach and retraction coincide practically for the whole interaction range including the contact at approach and separation. By contrast, for the CH 3 -terminated SAM, the force necessary to separate the colloidal probe after contact is larger than the force upon approach just before contact with the surface. This difference is attributed to the hydrophobicity of the surface and the resulting work of adhesion due to solvent exclusion in the contact area. 7-11 In order to quantify the different force contributions, we evaluate first the long-ranged DLVO forces and compare them afterwards with the pull-off forces. Diffuse layer properties of the electrode. Figure 4 summarizes the diffuse layer potentials obtained from fits to the full solutions of the Poisson-Boltzmann equation including charge regulation as shown in Figure 3a) and b). In order to obtain the diffuse layer potential from these fits, we had to determine beforehand the diffuse layer potential and constant regulation parameter for the colloidal silica particles in a sphere/sphere geometry. 41 These values for the silica colloidal probes are compiled in the supplemental F i / R F a / R ∆ F / R 5. Adhesion of Colloidal Particles on Modified Electrodes 79 information and are in good accordance with previously published results for the diffuse layer potential of silica surfaces. 27,44,45 Each data point in Figure 4 results from a set of at least three independent measurements with different colloidal probes and electrodes. Around 100 single force profiles were acquired for each external potential and were then averaged. The diffuse layer potentials were determined from the averaged force profiles by fits to the full Poisson-Boltzmann equation. The fits have been performed by means of the so-called constant regulation approximation, which has been described in detail elsewhere. 39 Shortly, in this approximation the charge regulation between two interacting surfaces is summarized by two parameters: their diffuse layer potentials at infinite separation and a regulation parameter p for each surface. The latter summarizes the complex surface chemistry (i.e. number of functional groups and their pK´s, Stern layer capacitance) by just one parameter. Typically, the interaction forces taking into account charge regulation are falling in between the classical boundary conditions of constant charge and constant potential. By measuring the interaction forces between two silica particles in the sphere/sphere geometry these values have been obtained first in a set of completely independent measurements for the silica colloidal probe. These data are compiled in the supporting information. Recently, we demonstrated that charge regulation should be taken into account to describe the interaction force profiles upon approach for small separation distances, even for SAM-modified electrodes under potentiostatic control. 36 However, at larger separation distances the values obtained for the diffuse layer potentials do not depend critically on the regulation parameter. The first value obtained from the fits to the PB-equation is the Debye-length , which is given by (1) where εε 0 is the total permittivity of water, kT is the thermal energy at room temperature, N A is Avogadro’s number, e is the elementary charge and I is the ionic strength. The values obtained from the fits to the experimental data agree within less than 10% to the theoretical Debye-length calculated from the solution’s ionic strength as stated during preparation. The fits have been restricted to separation distances larger than the solutions’ theoretical Debye-length in order to exclude effects due to surface roughness, charge regulation, and electroviscous effects. 29,46 The dependence of the diffuse layer potential ψ D κ −1 5. Adhesion of Colloidal Particles on Modified Electrodes 80 on the externally applied potential φ shows that for a given ionic strength the diffuse layer potential of the modified electrodes varies monotonically with the externally applied potential. An increase of the ionic strength leads to a reduction of the diffuse layer potentials as can be seen most clearly for the highest ionic strength of 5mM. However, the potential of zero charge (pzc), i.e. the external potential where diffuse layer of an isolated electrode would vanish, does not depend on the ionic strength and concentration of KCl. The vertical dashed lines in Figure 4 indicate the pzc of (vs. SCE) and (vs. SCE) as determined for the CH 3 -terminated electrode and the OH-terminated electrode, respectively. The general dependence of the diffuse layer potential from the externally applied potential φ can be described in terms of a simple model of two layers, attributing a capacitance to the SAM and the diffuse layer, respectively. 36 However, in order to describe the dependence at higher external potentials accurately, a more elaborate model has to be considered. 37,47 Additionally, the adsorption of hydroxyland hydronium ions to the SAMs has to be included. 48-50 Taking into account these contributions provides an explanation of the position and shift between the pzc’s for the two different SAMs and leads to a significantly better model in describing the dependence of the diffuse layer potential on the externally applied potential. 36,37 However, such an elaborate model is outside the scope of this study and and pzc are sufficient for a quantitative analysis of the adhesion behavior. ψ D ψ D φ pzc = − 225 ± 15mV φ pzc = − 86 ± 24mV ψ D ∆ F / R 5. Adhesion of Colloidal Particles on Modified Electrodes 81 Figure 4: Diffuse layer potential versus externally applied potential for (a) CH 3 - and (b) OHterminated SAMs on gold electrodes. For each electrode the diffuse layer potentials obtained for different ionic strengths are shown. The dashed vertical lines indicate the position of the pzc. Distribution of pull-off forces. Figure 5 a) shows a series of force versus distance profiles for the retraction part of the force profile when the colloidal probe is moved away from a CH 3 -terminated electrode (I=0.34 mM). The retraction force profiles shown in Figure5a) are averaged from approximately 100 single retraction profiles in an analogous procedure as used for the force profiles of the approach part (cf. Figure 3). Again, the 5. Adhesion of Colloidal Particles on Modified Electrodes 82 interaction forces have been normalized to the effective radius R. Since a cantilever with a high spring constant has been used, the force profiles during retraction resemble closely the ones of the approach. In consequence, we observe the same dependence of the interaction forces from the externally applied potential as for the force profiles upon approach: Starting from a completely repulsive interaction at very negative potentials the overall interaction is tuned increasingly attractive by higher applied potentials. In the following analysis of the adhesion we concentrate on the pull-off force , which corresponds to the critical force necessary to separate the colloidal probe from the electrode surface. This force equals to the local minima for the retraction force profiles at about zero separation in Figure 5a and has been indicated as well in the force profiles of Figure 3. In contrast to the interaction forces that are detected several nanometers away from the electrode surface, one observes a larger scattering for the pull-off forces. This effect is well known from chemical force microscopy and can be attributed to a slight variations in contact due to surface roughness. 11 Instead, the absence of comparable scattering for the longer-ranged forces results from the larger surface area over which the interaction is averaged and thus small variations in the surface topography are smoothed out. Figure 5b) summarizes the resulting distribution of the adhesion forces as determined from the single retraction profiles for different potentials. For each potential the pull-off forces scatter around a well-defined mean value. The solid lines indicate fits to the corresponding normal distribution for each potential φ . As expected, the pull-off forces increase monotonically with the external potential. For potentials φ < 200mV the pull-off forces are positive, albeit a local minimum might be present at zero separation (cf. Figure 2a). Nevertheless, despite the local minimum of the interaction forces the sum of all interacting forces is still directed away from the surface and the particle would not adhere in a stable manner to the surface. In consequence an adhesion force of zero is attributed for the corresponding potentials (i.e. −297 mV and −397 mV). The variation of the mean pull-off forces with the externally applied potential φ is summarized in the inset for the data of Figure 5b). The error bars correspond to the standard deviations of the normal distribution. F a / R F a / R F a / R 5. Adhesion of Colloidal Particles on Modified Electrodes 83 Figure 5: (a) Averaged interaction force profiles upon retraction for an electrode modified by a CH 3 - terminated SAM for different applied potentials (I=0.34 mM). The minima in these profiles correspond to the pull-off forces, which is the force necessary to separate the colloidal particle from the electrode surface. (b) Distributions of the pull-off forces for a series of about 100 forces curves for each potential. The solid lines indicate fits to a normal distribution. (c) Compilation of the average pull-off forces at different potentials and different ionic strengths for a CH 3 -terminated electrode compiled from three independent data sets in terms of electrode and colloidal probe. (d) Analogous compilation for an OH-terminated electrode. Adhesion versus externally applied potential. In Figure 5c)-d) we compiled the dependence of the pull-off forces on the applied potential φ for at least three data sets (different combinations of colloidal probes and electrodes) and various ionic F a / R 5. Adhesion of Colloidal Particles on Modified Electrodes 84 strengths. Figure 5c) shows the data for the electrodes modified by CH 3 -terminated SAMs and Figure 5d) for OH-terminated SAMs, respectively. For both SAMs, one observes a monotonic increase of the pull-off forces and thus of the adhesion with increasing external potential after the pzc. By contrast, increase of the ionic strength by addition of the background electrolyte leads to a reduction of the pull-off forces. At high ionic strengths of about 5 mM (data not shown) the adhesion shows only a small dependence on external potential that is in agreement with diffuse layer potential shown in Figure 4. Thus, we can state that the forces due to diffuse layer overlap represent the most significant contribution to the adhesion. The diffuse layer potential depends critically on ionic strength and varies otherwise strongly with the external potential φ (cf. Figure 4) but to a smaller extent with increasing ionic strength. For both SAMs we find a clear transition from non-adhesive to adhesive behavior with increasing potential. The potentials for the transitions are independent of ionic strength. For the OH-terminated SAM, the change from adhesive to non-adhesive behavior occurs at approximately 100 mV vs. SCE (cf. Figure 5d) and thus coincides approximately with the pzc ( ) indicated by the vertical dashed line in Figure 5d). As for all potentials smaller than the pzc the overall interaction becomes repulsive (cf. Figure 4b), the transition from adhesive to non-adhesive interaction is expected to occur around the pzc. By contrast, for the CH 3 -terminated SAM one has to apply much more negative potentials (i.e. ) than the pzc ( ) in order to switch from adhesive to non-adhesive behavior. Solvent exclusion in the contact area leads to additional adhesion for the hydrophobic (i.e. CH 3 -terminated) SAM as reported previously in studies by chemical force microscopy. 6-11 Thus, one finds that much more negative potentials than the pzc on the hydrophobic SAM have to be applied in order to reach the region where no overall adhesion is taking place, as the repulsive diffuse layer forces have to compensate for the additional adhesive component by solvent exclusion. Influence of DLVO-forces on adhesion. As illustrated in Figure 3a), two types of contributions to the adhesion forces can be distinguished. Firstly, long-ranged interactions that are accounted for by DLVO-theory, namely vdW-forces and diffuse layer overlap. Secondly, short-ranged contributions, which are only present in the contact area, namely solvent exclusion and electrocapillarity. The long-ranged interaction forces are present during approach as well as during retraction of the colloidal probe. By contrast, ψ D φ pzc ≈ − 86mV φ < − 300mV φ pzc ≈ − 225mV 5. Adhesion of Colloidal Particles on Modified Electrodes 85 contributions of forces related to the contact area can be neglected as soon as the contact between the two solid surfaces is broken. Such a partition of force contributions has been proposed previously and can be approximated by simple models assuming that the interactions inside and outside of the contact area can be separately taken into account. 24,51 Here, we pursued a more direct approach, which does not require a theoretical model taking into account in a separate manner shortas well as long-ranged forces. By comparing the forces directly before contact (i.e. ) and at pull-off (i.e. ), one can separate contributions to the pull-off force originating short-ranged interactions from ones due to long-ranged forces (cf. Figure 3a) without any underlying assumptions. However, a prerequisite for this method is that the force profiles have been acquired with a cantilever of sufficiently high spring constant (cf. Figure 2) In Figure 6 we compiled a number of exemplary data sets of the interaction forces directly before contact during the approach and the corresponding pull-off forces during the retraction for various ionic strengths and both SAMs (cf. top graphs). The bottom graphs show the resulting difference . For the hydrophobic CH 3 -terminated electrode, varies between 0.2 to 0.6 mN/m with applied potential . By contrast, for the hydrophilic OH-terminated electrode one finds significantly smaller values with < 0.06 mN/m. F i / R F a / R F i / R F a / R ∆ F / R = F a / R − F i / R ∆ F( φ ) / R φ ∆ F/R φ ( ) 5. Adhesion of Colloidal Particles on Modified Electrodes 86 Figure 6: (a) Interaction forces before contact ( , filled symbols) and corresponding pull-off forces ( , open symbols) as a function of the applied potential for a CH 3 -terminated electrode. The bottom graph shows . The dashed line corresponds to at pzc for I = 5 mM. (b) Analogous data set for an OH-terminated electrode. Work of adhesion at potential of zero charge. The influence of solvent exclusion can be best studied at the pzc. At this potential the electrode’s diffuse layer practically vanishes and interaction forces due to diffuse layer overlap are minimal. However, due to F i / R F a / R ∆ F / R = F a / R − F i / R ∆ F / R 5. Adhesion of Colloidal Particles on Modified Electrodes 87 charge regulation between the surfaces a small interaction force induced by the diffuse layer of the silica particle is present. Force profiles at external potentials near the pzc show that the remaining interaction forces upon approach are extremely small (cf. force profiles for in Figure 3a and 3b). Therefore, additional contributions to the pull-off force, such as solvent exclusion, can be best quantified at the pzc. The work of adhesion due to solvent exclusion is given by: 2 (2) where , , and are the interfacial energies of the two solid/liquid and the solid/solid interface, respectively. The interfacial energies and of the two SAMs have been reported previously and corresponding values are compiled in Table 1. The here-measured static contact angles of and on the electrodes are in agreement with reported values. 4 The interfacial energy for the silica surface has been reported to be in the range of 1-15 mN/m (cf. Table 1). 54-56 It has been approximated here by as for both hydrophilic surfaces (i.e. OH-terminated SAM and silica) a closely bound layer of interfacial water is supposed to exist, which is controlling the interfacial properties as determined by adhesion studies. 10,56 This interfacial water layer is also responsible for the reduced adhesion between silica surfaces 55,56 or OHterminated SAMs 10 as determined by the colloidal probe techniques or chemical force microscopy, respectively. The observation that adhesion forces between OH-terminated SAMs in nonpolar solvents are much larger than in water further supports the assumption of a closely bound interfacial water layer. 7,10 Table 1: Compilation of interfacial energies (mN/m) (mN/m) (mN/m) (mN/m) (mN/m) γ in eq. (2) 46.0 a 1.6 a 1.6 b 19.6 c 1.6 b γ reported 44-55 d 2.0 e , 7.5 f , 1-15 g (20-35) h - a taken from ref. 10 , b approximated by , c calculated from in ref. 10 φ 2 W solv = γ SAM /H 2 O + γ SiO x /H 2 0 − γ SAM /SiO x γ −CH 3 /H 2 O γ −OH /H 2 O θ −CH 3 /H 2 O = 108 ° θ −OH /H 2 O = 21 ° γ SiOx/H 2 O γ SiO x /H 2 O ≈ 1.6mN/m γ −CH 3 /H 2 O γ −OH /H 2 O γ SiO x /H 2 O γ SiO x /−CH 3 γ SiO x /−OH γ −OH /H 2 O W −OH/H2O/−CH3 5. Adhesion of Colloidal Particles on Modified Electrodes 94 Their tip has dimensions comparable to the ultrastructure on the silicon particle (cf. Figure 7). Figure 8 shows the for such a nm-sized tip (cf. inset in Figure 8) as a function of the applied potential on a hydrophobic CH 3 -terminated electrode. In order to normalize the interaction forces for sphere/plane geometry, the effective tip radius R is determined indirectly by the forces and at pzc and I=5mM. The interfacial energies used in the calculation are the same as given in Table 1. This approach is commonly followed in chemical force microscopy and equation (3) gives R. The resulting tip radius of is in good agreement with scanning electron microscopy (SEM) images obtained after the force measurements (cf. inset in Figure 8), which show that the tip diameter is smaller than 10 nm. Figure 8: In the top graph: Interaction forces as measured by a nanometer-sized AFM-tip (i.e. single asperity). Compilation of interaction forces before contact ( , filled symbols) and pull-off forces ( , open symbols) as a function of the applied potential for different ionic strengths. In the bottom graph: Compilation of as function of the applied potential. The inset shows a SEM-image of the tip with which these measurements have been performed. The general dependence of the pull-off forces is analog to the one found for the colloidal probes (cf. Figure 6). However, with the single asperity a smaller scattering of the pull-off forces is observed, as roughness does lead to variation of the contact area. The lower graph shows the corresponding difference for each series. Within the accuracy of our measurements, no variation of is observed with and the values F a / R F a = 0 . 4 2 n N ∆ F = 0.32nN R tip ≈ 2.5nm F i / R F a / R ∆ F / R = F a / R − F i / R ∆ F / R ∆ F / R φ 5. Adhesion of Colloidal Particles on Modified Electrodes 95 scatter around . Thus, any variation of and with must be small and does not contribute significantly to the observed variation of the pull-forces with the external potential. Conclusions In this study we identified the main parameters that allow tuning the adhesion of colloidal particles on modified electrodes by an external potential. It is primarily the interplay between surface roughness and interfacial energy that establishes the extent to which the adhesion can be altered by the external potential. Especially, if one of the surfaces is highly hydrophobic, solvent exclusion contributes strongly and results in an adhesion ‘offset’. This offset has to be compensated by repulsive diffuse layer forces to switch from adhesive to a non-adhesive behavior. On the other hand, a high surface roughness reduces the ‘true’ contact area with the particle and thus diminishes the influence of surface hydrophobicity. This effect has been demonstrated by comparing the adhesion between a sharp AFM-tip and colloidal probes of comparable surface chemistry. Electrocapillarity effects do not play a major role in the adhesion on SAM-modified electrodes. It is primarily the long-ranged force due to the overlap of the electrical double layers, which provides the control of the adhesion as a function of the external potential. The charge density of the colloidal particles is in this respect an important parameter as it generates one of the two diffuse layers involved in the interaction. The presence of additional intrinsic charges from the SAM or by specifically adsorbed ions influences the diffuse layer of the electrode and additionally shifts the position of the potential of zero charge (pzc). Thus, both parameters directly influence the slope of the pull-off force in respect to the externally applied potential (cf. Figure 5). An additional parameter, albeit of less importance, is the thickness of the modification layer (i.e. SAM). The thicker this layer is, the larger is the potential drop in respect to the external potential and thus the smaller is the variation of the electrode’s diffuse layer. By tuning the composition of the SAM, e.g. by mixed SAMs, it is possible to adapt the electrode’s adhesion behavior for a given batch of colloidal particles in a rational manner to prevised applications, not also in terms of the adhesive strength but also terms of the window of electrochemical potentials. Therefore, our results should be not only of importance for electrochemical sensors but also for the design of microelectromechanical systems (MEMS) such as grippers. 66 Another feasible application ∆F/R φ pzc ( ) γ − CH 3 /H 2 O γ − CH 3 /SiO x φ 5. Adhesion of Colloidal Particles on Modified Electrodes 96 would be a new approach for the nanoand micromanipulation by AFM that would be based on switching the adhesion behavior rather than applying shear forces. Acknowledgments We thank Samuel Rentsch for helpful discussion and the conduction of the initial experiments making this study possible. The authors thank M. Borkovec (University of Geneva) and H. Siegenthaler (University of Berne) for very valuable discussions. We thank C. Kunert (University of Bayreuth) for making the SEM-images. This research has been supported by the Swiss National Science Foundation and the German Research Council (SFB 840). 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J Adhes Sci Technol 2004, 18, 1–17. (61) Ederth, T. Langmuir 2001, 17, 3329–3340. (62) Senden, T.; Drummond, C. Colloid Surface A 1995, 94, 29–51. (63) Mugele, F.; Baret, J. J Phys-Condens Mat 2005, 17, R705–R774. (64) Ekeroth, J.; Konradsson, P.; Bjorefors, F.; Lundstrom, I.; Liedberg, B. Anal Chem 2002, 74, 1979–1985. (65) Antelmi, D.; Connor, J.; Horn, R. J Phys Chem B 2004, 108, 1030–1037. (66) Dejeu, J.; Bechelany, M.; Rougeot, P.; Philippe, L.; Gauthier, M. ACS nano 2011, 5, 4648–4657. 5. Adhesion of Colloidal Particles on Modified Electrodes 100 Supporting information Interaction forces between silica particles in the sphere/sphere geometry. In order to determine unambiguously the diffuse layer potential of the modified electrodes, we determined first the diffuse layer properties of the silica colloidal probe. These measurements have been performed in a completely symmetric system, which consists of two silica spheres. Thus, these measurements have been performed in the sphere/sphere geometry as presented in a previous publication. 1 Figure S1: Exemplary force versus distance profiles upon approach between a silica colloidal probe and a colloidal silica particle immobilized on a glass slide. The force profile has been averaged from about 50 single force profiles. The solid line indicates the fit to the full Poisson-Boltzmann equation including the charge regulation. By contrast, the dashed lines represent the solution to classical boundary conditions of constant charge (top) and constant potential (bottom), respectively. Figure S1 shows an exemplary interaction force profile for two silica particle at pH 4.7 and a total ionic strength of I=0.34 mM. Before acquiring the force profiles the colloidal particles are aligned axially by optical microscopy and so-called force volume plots, where the lateral position of the colloidal probed is varied on a lattice. Both colloidal particles are prepared in an identical manner. Thus, a comparable surface chemistry of both particles is ensured. Fitting Poisson-Boltzmann equation with constant regulation approximation. The force average force profiles acquired in the sphere/sphere geometry have been fit to the full solutions of the Poisson-Boltzmann equation. Here, we included besides the classical boundary conditions of constant charge and constant potential also the so-called constant 5. Adhesion of Colloidal Particles on Modified Electrodes 101 regulation approximation. 2 This approximation summarizes the charge regulation of the interacting surface by the diffuse layer potentials of the isolated surfaces ψ D and a regulation parameter p, which is defined as: p=C d C d +C i (S-1) where C i is the inner layer capacitance and C d is the diffuse layer capacitance given by C d = εε 0 κ cosh(e ψ D / 2k B T) where ε ε 0 is the total permittivity of the water, k B T the thermal energy and e the elementary charge. The inverse Debye-length κ is given by equation (1). The regulation parameter takes typically values between 0 and 1. The former value corresponds to the boundary condition of constant potential (p=0) and the latter to the one of constant charge (p=1). The values for the regulation parameter obtained from the fits scatter substantially. However, here we limited the fit interval to separations larger than one Debye-length κ −1 . Therefore, the regulation parameter has no substantial effect on the diffuse layer potentials obtained from the fits to the constant-regulation approximation. We find that the diffuse layer potentials are in general agreement with the inverse Grahame-equation: ψ D =2k B T easinh e σ 2k B εε 0 κ      (S-2) where σ is the diffuse layer charge density of the isolate silica particles. The values for ψ D and p resulting from the fits are compiled in Table S1. The obtained diffuse layer potentials are in good general agreement with values reported in other studies. In order to determine the diffuse layer potentials of the electrodes at different external potentials, we used the values compiled in Table S1 as fixed parameters for the silica particles and fitted the diffuse layer potential ψ D (and the regulation parameter) of the electrode at a given ionic strength. Further details concerning this approach, in particular in respect to the regulation parameter for an electrode under potentiostatic control, can be found elsewhere. 3,4 5. Adhesion of Colloidal Particles on Modified Electrodes 102 Table S1: Compilation of diffuse layer potential and regulation parameters for silica colloidal probes at different ionic strengths Ionic strength 0.12 (mM) 0.34 (mM) 0.56 (mM) 5.0 (mM) ψ D (mV) -58.0 -57.3 -53.0 -37.6 p 0.80 0.70 0.60 0.10 References: (1) Rentsch, S.; Pericet-Camara, R.; Papastavrou, G.; Borkovec, M. Phys Chem Chem Phys 2006, 8, 2531–2538. (2) Pericet-Camara, R.; Papastavrou, G.; Behrens, S.; Borkovec, M. J Phys Chem B 2004, 108, 19467–19475. (3) Rentsch, S.; Siegenthaler, H.; Papastavrou, G. Langmuir 2007, 23, 9083–9091. (4) Rentsch, S. Direct Force Measurements Between Surfaces Under Potentiostatic Control, Ph.D. thesis, University of Geneva, Faculty of Sciences, 2008. 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 103 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control Volodymyr Kuznetsov and Georg Papastavrou* Department of Physical Chemistry II, University of Bayreuth, Universitätstraße 30, 95440 Bayreuth, Germany *E-mail corresponding author: [email protected] Intended for submission in Journal of Physical Chemistry C in 2013 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 110 Figure 2: Interaction forces between silica particles in the sphere geometry for different pH-values but constant ionic strength (I=1.2 mM). The fits to the PB-equation with different boundary conditions are represented by the dashed (CC constant charge, CP constant potential) and full (CR charge regualtion approximation) lines. The force profiles for the interaction between two silica particles show for all pHvalues repulsive forces over the full separation distance and can be described by the overlap of the two diffuse layers originating from the particles. At separations larger than about 10-15 nm, the interaction force decays exponentially as indicated by the straight lines in the semi-logarithmic representation of Figure 2. The decay constant is given by the by the inverse of the Debye-length κ -1 with (1) where εε 0 is the total permittivity of water, kT is the thermal energy at absolute temperature, N A is Avogadro’s number, e is the elementary charge and I is the ionic strength. The interaction force in Figure 2 have been normalized to the effective radius R eff (2a) with the radii R CP and R S , of the colloidal probe and the immobilized particle, respectively. In the case of sphere-plane geometry, as for the electrodes, equation (2) reduces to R eff =R CP , where R CP is the radius of the colloidal probe. In this manner the κ −1 = εε 0 kT 2N A e 2 I 1 R eff =1 R CP +1 R S 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 111 experimental force profiles F(D)/R eff can be evaluated quantitatively by the Derjaguin equation with the free interaction energy W int (D) of two infinite plates at separation D by (2b) where W int (D) for the silica surface results from overlap of their diffuse layers and is thus given by the solutions of the Poisson-Boltzmann (PB-) equation. The dashed lines in Figure 2 represent fits to the full solutions of the PB-equation with the classical boundary conditions of constant charge (CC) and constant potential (CP). The solid lines represent fits to the constant regulation approximation (CR) that takes into account the charge regulation between the two surfaces. 39 In this approximation the influence of surface chemistry of the surfaces is summarized by the diffuse layer potential ψ D at infinite separation and a regulation parameter p that is defined by 39 (3) Here, C I is the inner layer capacitance and C D is the diffuse layer capacitance. The latter is given by (4) where ψ D is the diffuse layer potential at infinite separation of the surfaces. 39 Commonly, the regulation parameter ranges from 0 to 1, where the p=0 corresponds to CP and p=1 to CC. In order to describe the interaction profiles over the full range of separations, one has to take into account charge regulation. Charge regulation is also important to describe the interaction force between a silica colloidal probe and an electrode. 28 However, the diffuse layer potentials obtained from the fits are not significantly influenced from p as long as the interaction force profiles are evaluated at separation distances larger than about κ -1 . The absence of attractive force at small separations due to van-der-Waals forces results from the surface roughness of colloidal silica particles and has been observed previously. 40,41 Variation of colloidal probe properties with pH. By fitting interaction force profiles like the ones shown in Figure 2, the diffuse layer potentials ψ D and the regulation parameters p can be determined for the different pH-values. Hover, the sign of the diffuse layer potentials cannot be inferred from the interaction forces in a symmetric system but F D ( ) = 2 π R eff W int (D) p=C D C I +C D C D = εε 0 κ cosh e ψ D 2kT      6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 112 it is known to be negative. 43,44 In Table 1 the results from at least 5 pairs of particles and two different colloidal probes are summarized. Table 1: Diffuse layer potentials and regulation parameters for the silica colloidal probes at different pH.values and I = 1.2 mM pH 3.5 4.7 5.5 8.0 9.5 (mV) -30.2 ± 2.3 -44.2 ± 4.2 -44.9 ± 6.0 -45.7 ± 3.8 -47.4 ± 3.1 p 0.60 ± 0.10 0.50 ± 0.05 0.50 ± 0.20 0.60 ± 0.25 0.60 ± 0.20 The diffuse layer potentials in Table 1 are in good general agreement with the values reported by other groups. 45,46 However, the silica surface chemistry is highly susceptible to preparation history and conditions. Thus, diffuse layer potentials reported in the literature vary significantly. The increase and successive leveling-off of ψ D for high pHvalues follows the trend expected from surface chemistry of silica according to 1-pK or 2pK dissociation models for the ionization behavior. 47 In the pH-range studied here the regulation parameter changes only slightly and the observed variations are within the accuracy of the method. The values for the regulation parameter of p = 0.3-0.8 are in general agreement with calculations based on a 1-pK model. 39 The average values summarized in Table 1 for ψ D and p are used in the following for the quantitative analysis the interaction force profiles on the SAM-modified electrodes. Interaction profiles over SAM-modified electrodes. Figure 3 shows a selection of exemplary force profiles on SAM-modified electrodes acquired with colloidal probes whose diffuse layer properties have been determined previously. The measurements compiled in Figure 3 have performed on two different electrodes that were either modified by OH-terminated SAMs (cf. Figure 3a,b) or a CH 3 -terminated SAMs (cf. Figure c,d). The measurements have been performed in solutions of different pH and additionally the potential applied to the electrodes has been varied over a wide range. From the full range of pH-values ranging from pH 3.5 to pH 9.5 examples slightly acidic (i.e. pH 5.5, cf. Figure a,c) and slightly basic (i.e. pH 8.0, cf. Figure b,d) pH-regime are shown.. For each pH-value a series of different potentials ϕ i has been applied to the electrode, ranging from approximately ϕ 1 =-225mV (vs. SCE) to ϕ 8 =+475mV (vs. SCE). Each force profile shown for a given potential ϕ i is obtained by averaging about 100 ψ D 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 113 single force curves acquired at this protential. These potentials may vary slightly (<5%) for the reference electrode used in the study (Ag/AgCl-wire). Its potential against commercial Ag/AgCl-electrode was in-situ determined in the AFM cell before and after the experiments under used solution conditions. Figure 3: Exemplary force versus distance profiles upon approach between a silica colloidal probe and an electrode with (a, b) OHand (c, d) CH3-terminated SAMs at different applied potentials. They were acquired in acidic (pH 5.5: a, c) and basic (pH 8.0: b, d) solutions. The force profiles have been averaged from about 100 single force-distance curves. The applied potentials ϕ are given versus a saturated calomel electrode (SCE). 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 114 Let us first consider the electrode modified by the OH-terminated SAM. At pH 5.5 the force profiles show qualitatively a similar behavior as reported previously for pH 4.7. 28,29 At very negative potentials ϕ the interaction is repulsive (cf. ϕ 1 - ϕ 3 in Figure 3a). Due to the negative charge of the colloidal probe such repulsive interaction is expected. With increasing ϕ the interaction becomes always less repulsive until finally a transition from repulsive to attractive behavior (cf. ϕ 4 - ϕ 5 Figure 3a) takes place. In this potential interval one find also the potential of zero charge ϕ pzc , which corresponds to the potential that leads to a vanishing diffuse layer. Upon further increase of ϕ the interaction becomes progressively attractive (i.e. ϕ 6 - ϕ 8 Figure 3a). The arrows in Figure 3 indicate instabilities of the cantilever where it jumps directly to the electrode´s surface,. It occurs if the gradient of acting attractive force becomes larger than the spring constant. Figure 3b) shows the interaction force profiles for the same electrode modified by an OH-terminated SAM at pH 8.0. The dependence of the profiles under these slightly basic conditions is comparable to the one under the slightly acidic conditions discussed before (i.e. pH 5.5). However, the forces profiles at comparable potentials are slightly different and seem to be shifted to more negative potentials. This is most clearly visible for ϕ 5 and ϕ 6 near to the ϕ pzc , which is shifted. In terms of diffuse layer potential this shift will be discussed later in detail. For the electrode modified with the CH 3 -terminated SAM the pH of the solution has a much stronger influence on the interaction force profiles. Under acid conditions at pH 5.5 (cf. Figure 3c) the interaction force profiles are generally attractive (i.e. ϕ 2 - ϕ 8 in Figure 3c), only for the lowest potential of ϕ 1 =-218 mV a slightly repulsive interaction between the electrode and the negatively charged silica probe can be observed. The ϕ pzc for the electrode with the CH 3 -terminated SAM is shifted to more negative potentials at pH 4.7 in respect to the OH-terminated SAM as previously reported. 28,29 Thus, the same effect can be observed also at pH 5.5 for these two types of modified electrodes. By contrast, under basic conditions the interaction profiles are completely repulsive, independently from the applied potential. Therefore, a ϕ pzc is not reached and even at the highest positive ϕ 8 =+475mV the interaction force profiles remain completely repulsive, albeit reduced in strength compared to the smaller potentials (i.e. ϕ 1 - ϕ 7 in Figure 3d). Diffuse layer potentials of the electrodes. The force profiles as a function of the applied potential and pH have been analyzed quantitatively. The procedure is analogous 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 115 to the one presented for the silica particles (cf. Figure 2) and is illustrated in an exemplary manner in Figure 4. The combination of silica colloidal probes and SAM-modified electrodes represents an asymmetric combination of surfaces. Therefore, the parameters and for the colloidal probe are fixed according to the values compiled in Table 1 and the diffuse layer potential and the regulation parameter of the electrode are determined by the fit to the full PB-equation with the charge regulation approximation. Figure 4 demonstrates the fit quality to PB-equation at the different boundary conditions of the interaction force profiles obtained at pH 8.0 for an electrode with an OH-terminated SAM. The two boundary conditions of constant charge (CC, p=1) or constant potential (CP, p=0) for the electrode’s surface show large deviations from the force profiles at small separations. As previously reported, 29,35 one has also to take charge regulation for the electrodes surface into account, despite the fact that it is connected to a potentiostat and therefore CP boundary condition would be expected (cf. solid line in Figure 4). The occurrence of charge regulation at the surface of an electrode connected to a potentiostat is compatible with the presence of a layer of adsorbed ions at its interface. However, the three boundary conditions give practically the same result at large separation (i.e. 1-2 × κ -1 ) between the probe and the electrode and thus have no large influence on the diffuse layer potentials of the electrode as obtained from the fits. From the fits to the PB-equation with the charge regulation approximation shown in Figure 4 one obtains diffuse layer potentials of ψ (ϕ 1 )= -16.8 mV, ψ(ϕ 5 )= +14.3 mV, and ψ(ϕ 8 )= +61.2 mV, respectively. ψ CP D p CP ψ D p ψ D 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 116 Figure 4: Exemplary force versus distance profiles upon approach between a silica colloidal probe and an OH-terminated electrode (I = 1.2mM, pH 8.0) with fits to the full PB equation with different boundary conditions. The solid lines represent the charge regulation approximation (CR), while the dashed lines represent constant charge (CC) and constant potential (CP) boundary conditions, respectively. Diffuse layer potentials vs. pH. Figure 5 summarizes the diffuse layer potentials obtained from the fits of the single interaction force profiles as shown in Figure 4. Figure 5a) shows the results for the electrode modified by an OH-terminated SAM and Figure 5b) for the ones for an electrode with a CH 3 -terminated SAM, respectively. Both graphs plot the diffuse layer potential as a function of the applied potential. The different pHvalues are represented by different symbols, while ionic strength is constantly I=1.2 mM. Each data point has been obtained from the diffuse layer potentials of at least three different data sets with different colloidal probes. 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 117 Figure 5: Diffuse layer potential versus externally applied potential for electrodes modified by an OH- (a) and a CH3- (b) terminated SAM, respectively. The data points correspond to the diffuse layer potentials determined at different applied potentials and pH-values. The lines represent global fits simultaneous at all pH-values according to three-capacitor model for each SAM. The resulting fit parameters are compiled in Table 2. The diffuse layer potentials on the electrode with the OH-terminated SAM increase monotonically with applied potential for all pH-values. Such monotonic behavior is expected from the force profiles in Figure 3 a),b) and the general shape of curves for vs. are similar to the ones reported previously for OH-terminated electrodes at pH 4.7 and different ionic strength. 28,29 However, the values for the potential of zero charge ϕ pzc ψ D φ 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 118 shift to higher potentials with increasing pH and thus OH - -concentration. In the case of pH 4.7 (and I=1.2 mM) ϕ pzc =230 ± 7 mV (vs. SCE) is in good agreement with the one reported previously for the same type of electrode and pH. 29 For the electrodes with a CH 3 -terminated SAM one finds instead a more pronounced variation of vs. as a function of of pH. Only for a small regime of pHvalues (3.5 < pH < 8.0) one can observe the presence of ϕ pzc in the potential regime accessible in the experiments as otherwise desorption of the SAM is occuring. In this regime ϕ pzc is shifting with pH to higher values but for more acidic (i.e. < pH 4.7) or basic conditions (i.e. > pH 5.5) no reversal of the sign of can be obtained for the CH 3 - terminated SAM. For the acidic regime the interaction is in these cases either completely attractive and for the basic regime completely repulsive. Under the most basic conditions at pH 9.5 the diffuse layer potential remains practically constant and independent from the applied potential . The main trends of vs. in Figure 5 are in good agreement with preferential ion adsorption of hydroniumor hydroxide-ions on SAM-modified electrodes 4,21,48 : At low pH, i.e. high concentration of OH 3+ and low concentration of OH - , a net positive charge is resulting on the interface due to adsorption of OH 3+ . By contrast, at high pH, i.e. high OH - - and low OH 3+ - concentration, one observes a negatively charged ion layer at the interface. The adsorption of hydroniumas well as hydroxyl-ions is clearly pronounced for the hydrophobic interface, which is in agreement with theoretical studies. 21 Influence of the background electrolyte. At this point it is important to verify that the background electrolyte does not adsorb preferentially to the SAM-modified electrodes. Figure 6 summarizes the dependence of the diffuse layer potential on the applied potential for various total ionic strengths at pH 4.7 for the CH 3 -terminated electrode. becomes weaker with increasing ionic strength and thus KClconcentration. This behavior is expected on basis of the Gouy-Chapman-Stern theory 49 and has been observed for the same SAM-modified electrodes previously. 28,29 It is evident that the potential of zero charge remains constant at 224 ± 15 mV, independently from the total ionic strength. Thus, we can conclude that adsorption of potassium (K + ) or chloride (Cl - ) ions to the SAM can be neglected in comparison to hydroniumor hydroxyl-ions. ψ D φ ψ D ψ D φ ψ D φ ψ D φ ψ D φ ( ) 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 119 Figure 6: Diffuse layer potential versus externally applied potential for CH3-terminated electrode at pH 4.7 and different ionic strengths. The full symbols are determined at I=1.2 mM, while the open symbols at different ionic strengths have been reported elsewhere.29 The lines indicate calculations based on the three-capacitor model with the parameters compiled in Table 2. Model for diffuse layer properties of SAM-modified electrodes. In the following we are presenting a simple model for the electrode/SAM/solution interface in order to describe preferential ion adsorption to SAM-modified electrodes under potentiostatic control. Despite its simplicity this model allows to capture the essential features outlined in the previous paragraphs. This model has been presented in similar form first by Duval et al. in order to describe ion adsorption at oxide layers of metal electrodes. 42 It has been then adapted to SAM-modified electrodes. 35 Figure 7 outlines in a schematic manner the composition of the different layers at the interface: (i) the gold electrode connected to the potentiostat with an electronic charge at its interface, (ii) the self assembled monolayer (SAM) with thickness dielectric constant , (iii) the layer of preferentially adsorbed ions with a charge density at the interface between SAM and electrolyte solution, and (iv) the diffuse layer with the diffuse layer charge density . With these layers different potentials are associated, where ϕ corresponds to the externally applied potential, ψ a the potential at the interface of the SAM and ψ D the diffuse layer potential. σ e d SA M ε SA M σ i on σ D 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 126 and the values from Table 2.. The horizontal dashed lines indicate the potential range in which the SAMs are stable. 37 In order to compare our results with the comprehensive studies by streaming potential of Werner and coworkers 4,17,31 , we determined additionally the diffuse layer potentials at open circuit conditions. These measurements were performed again by direct force measurements, however, the potentiostat has been disconnected from the electrochemical cell (data not shown). The resulting diffuse layer potentials as determined from the fits to the PB-equation with CR-boundary conditions are compiled in Table 3. Table 3: Diffuse layer potentials determined by direct force measurements under open circuit potential conditions at I = 1.2 mM pH 3.5 4.7 5.5 8.0 9.5 (mV), SAM-CH 3 30.3 ± 11.0 28.4 ± 4.3 20.9 ± 11.2 -34.2 ± 4.6 -48.6 ± 5.1 (mV), SAM-OH 22.7 ± 5.8 19.0 ± 5.3 -1.1 ± 4.7 -18.9 ± 2.9 The open circuit potentials as determined by direct force measurements confirm the trend found for both electrodes. Under acidic conditions positive charge is accumulated by hydronium adsorption to the SAMs, while under basic conditions a negatively charge ion layer results from the adsorption of hydroxyl-ions. The latter effect is more pronounced, in particular for the hydrophobic CH 3 -terminated SAM. From Table 3 the isoelectric point (iep) can be estimated, which corresponds to the pH where the diffuse layer charge vanishes. We find, that the pH iep lies between 5.5 and 8.0 for hydrophobic as well as for hydrophilic interfaces. This corresponds to the findings of Dicke et al. of pH iep close to 6.0-7.0 by AFM for an hydrophobic SAM, which is consistent with our data. 30 The potentials in Table 3 are also in line with streaming potential measurements, which show the same but a somewhat lower isoelectric point at pH iep 3.5-4.0, which might be attributed to the position of shear plane. 17 Conclusions By employing colloidal probe force microscopy in combination with electrochemical setup the diffuse layer properties of modified electrodes were determined at different applied potentials and solution composition. As a result the variations of diffuse layer ψ OC D ψ OC D 6. Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control 127 potential on hydrophobic and hydrophilic interfaces with electric potential and pH were deduced. Because the background electrolyte does not contribute to the charging of the surface, the found pHand potential-controlled changes in the diffuse layer properties may be attributed to the specific interfacial adsorption of hydronium and hydroxide ions. The adsorption is more pronounced on hydrophobic interface than on hydrophilic. The simple three-capacitor model provides semi-quantitative description of the data. Therefore we used a novel approach to quantify the ion adsorption on non-conductive organic surfaces, that has possible implications in the development of selective electrodes, “smart” coatings and membranes with ion selective properties. Acknowledgments: The authors thank Michal Borkovec, Hans Siegenthaler and Samuel Rentsch for the fruitful discussions and in particular Samuel Rentsch for the initial experiments leading to this study. This research has been supported by the Swiss National Science Foundation (SNSF) and the German Research Foundation (DFG SFB 840). 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Interfacial Water on Hydrophobic Surfaces Recognized by Ions and Molecules. Physical Chemistry Chemical Physics 13, 15925 (2011) 7. Tuning of the elastic modulus of polyelectrolyte multilayer films built up from polyanions mixture 133 7. Tuning of the elastic modulus of polyelectrolyte multilayer films built up from polyanions mixture Katja Trenkenschuh a , Johann Erath a , Volodymyr Kuznetsov a , Julia Gensel a , Fouzia Boulmedais b , Peter Schaaf b , Georg Papastavrou a , and Anreas Fery a, * a Department of Physical Chemistry II, University of Bayreuth, Universitätstraße 30, 95440 Bayreuth, Germany b Institut Charles Sadron, Université de Strasbourg, Centre National de la Recherche Scientifique, UPR 22, rue du Loess 23, 67034 Strasbourg Cedex, France. *E-mail corresponding author: Andreas.Fer[email protected] Published in Macromolecules 2011, 44(22), 89548961 7. Tuning of the elastic modulus of polyelectrolyte multilayer films built up from polyanions mixture 134 Abstract In this paper we report on the mechanical characterization of polyelectrolytes multilayer (PEM) films prepared from poly(glutamic acid)-poly(styrene sulfonate) (PGAPSS) blends, deposited in alternated spray deposition with poly(allylamine hydrochloride) (PAH). The polyanion composition of the blended film was first investigated using Fourier transformed infrared spectroscopy in the attenuated total reflection mode. The monomer molar fraction of PGA in the film increases almost linearly as a function of x, i.e. the monomer molar fraction of PGA in the sprayed polyanion solution. The mechanical properties of the blended (PAH/PGA x -PSS 1-x ) n film were measured using two methods: wrinkling metrology method and the colloidal probe atomic force microscopy technique. We demonstrate that the Young´s modulus of the PAH/PGA x -PSS 1-x multilayer films can be systematically controlled by the chemical composition of these films, depending on x. Measurements indicate that increasing the monomer molar fraction of PGA in the blended film results in a decrease in film modulus up to three orders of magnitude as compared to the PAH/PSS system. At a monomer molar fraction of PGA in the film around 0.7 (corresponding to x = 0.7), this system shows such a transition . We also show that for a given x the elastic properties of these films can be significantly affected by the humidity conditions. For (PAH/PGA 0.88 -PSS 0.22 ) film, the Young’s modulus of the film varies from several hundred of MPa to some kPa only by altering the relative humidity from 12.5% to 80%. Introduction Polyelectrolyte multilayer (PEM) films prepared by the layer-by-layer (LbL) technique became very popular since the concept was introduced by Decher et al. in the early 1990s. 1 The method relies on the sequential adsorption of oppositely charged polyelectrolytes to construct thin multilayered films. The adsorption results in charge overcompensation after each polyelectrolyte deposition. This allows the alternated assembly of oppositely charged polyelectrolytes. Numerous polymeric materials with different functional groups are available for the multilayer construction which resulted in numerous applications such as controlling wetting properties or interactions with biological systems, 2 anticorrosion coatings, 3-5 free-standing membranes, 6-10 osmotic pressure sensors, 11 and to build up microand nanocapsules. 12, 13 Adjusting mechanical properties of PEMs is desirable for most applications mentioned above. This can be 7. Tuning of the elastic modulus of polyelectrolyte multilayer films built up from polyanions mixture 135 achieved within a limited range by variation of solution conditions during adsorption (pH, 14, 15 ionic strength 16 ), by changing the molecular weight of used polyelectrolytes, 17 by cross-linking of the film 18, 19 or by adding a linear growing capping multilayer films on an exponential growing one 20 . In order to tune mechanical properties over many orders of magnitude using the same chemical building blocks, synthetic approaches relying on the use of random copolymers of controlled ratio of charged and uncharged monomers have been used. 21-23 It has been also shown that the elastic modulus of PEM films can be significantly affected by changing the ambient relative humidity (RH). 24 Using blends of either polyanions 25-31 or polycations 32, 33 provide a potentially interesting alternative to tune several properties of PEMs without synthesis of novel molecular compounds. To control the mechanical property of PEMs, our approach relies on using blends of components, which are known to result in very different mechanical properties where pure components are used. We study LbL films prepared with weakly charged poly(allylamine hydrochloride) (PAH) and two polyanions, poly(styrenesulfonate, sodium salt) (PSS) and a polypeptide poly(L-glutamic acid, sodium salt) (PGA). Our motivation is to investigate the effect of polyanion mixing ratio and the humidity on the mechanical properties since in the literature a huge difference in elastic modulus of (PAH/PSS) n and (PAH/PGA) n films is reported. In previous work, the Young’s modulus of PAH/PSS capsules was estimated to be between 1.3 and 1.9 GPa. 34 Nolte et al. reported for the PAH/PSS multilayer system a modulus of 2.7 ± 0.3 GPa 24 and Gao et al., who measured hollow polyelectrolyte capsules, found that the Young’s modulus for the PAH/PSS system ranges between 500 and 700 MPa. 35 Boudou et al. reported for the PAH/PGA system the elastic constant value of 118 ± 34 kPa as measured by AFM nanoindentation in liquid. 19 In this study, we show that the different chemical nature of these two polyanions, PGA and PSS, strongly affects the elasticity of the blended PAH/PGA x -PSS 1-x PEM films, with x representing the molar fraction of the monomer repeat unit of PGA in the polyanion solution. The polyanion composition of the blended film was first investigated using Fourier transformed infrared spectroscopy in the attenuated total reflection mode. The monomer molar fraction of PGA in the film increases almost linearly as a function of x. The film modulus decreases up to three orders of magnitude by transition from the PSS behaviour to the PGA behaviour. Interestingly this transition occurs around x = 0.7. This behavior is remarkable, because for many systems preferential adsorption of one