Can Polyoxometales Be Considered as Superchaotropic Ions?

HAL Id: hal-02384922 https://hal.science/hal-02384922 Submitted on 28 Nov 2019 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Can Polyoxometalates be considered as super-chaotropic ions? Carlos Drummond, Leonor Pérez-Fuentes, Delfi Bastos-González To cite this version: Carlos Drummond, Leonor Pérez-Fuentes, Delfi Bastos-González. Can Polyoxometalates be considered as super-chaotropic ions?. Journal of Physical Chemistry C, In press, �10.1021/acs.jpcc.9b08324�. �hal02384922� Can Polyoxometalates be considered as super-chaotropic ions? Carlos Drummonda,b, Leonor Pérez-Fuentesc, Delfi Bastos-González*c a CNRS, Centre de Recherche Paul Pascal (CRPP), UPR 8641, F3300, Pessac, France b Université Bordeaux, CRPP, F-33600 Pessac, France c Biocolloid and Fluid Physics Group, Department of Applied Physics University of Granada, Av. Fuentenueva S/N, 18071 Granada (Spain) e-mail: [email protected] Abstract In this work we have studied the influence of different anionic species, including typical Hofmeister ions (citrate3and SCN-), hydrophobic ions (tetraphenyl borate, Ph4B-) and polyoxometallic clusters (Keggin-type POM, PW3-), on the electrophoretic mobility of colloidal particles of different nature, charge and wettability. We focused the attention on the adsorption properties of PW3in comparison with Ph4Bbecause both species have previously termed as super-chaotropes. However, their different structural properties make it difficult to reconcile the same classification for both anions. We found that the influence of Ph4Bis alike and surpasses the characteristic effect of chaotropic anions (SCN-), in particular interacting more strongly with hydrophobic interfaces. On the contrary, we observed clear POM adsorption on the different particles investigated, regardless of their charge or wettability. These observations are complemented by direct adsorption measurement combining Quartz Crystal Microbalance and Atomic Force Microscopy, confirming the strong and irreversible self-assembly of POM on different substrates. Our results indicates that polyoxometallic anionic clusters should better be described as small colloids instead of as large anions. The exceptional adsorption properties of POMs clusters on any kind of interface suggest that their behavior goes beyond the classical Hofmeister effects and, therefore, these species should not be included in the Hofmeister series as super-chaotropic anions. Introduction Most of the relevant interactions of macromolecules or colloids in water occur in presence of ions. The type and amount of ions in solution can modulate the macromolecular interaction, which can even be changed from attractive to repulsive, modifying their conformation, stability and activity.1,2 The importance of these ion-specific interactions, whose origin is far more complex than the pure ionic valence, have attracted the attention of many groups. The first researcher that studied in a systematic way the influence of different ions in the interaction of proteins was Franz Hofmeister. For this reason, the Specific Ion Effects (SIE) are customarily known as Hofmeister Effects (HE) and the classification of the ions in systematic sequences refers as Hofmeister series (HS) or lyotropic series. After these initial studies, researchers have found a plethora of interacting systems and properties where SIE or HE manifest.3 In addition, a number of ions have been added and classified into the HS during the last years. Figure 1 shows an up-to-date representative direct HS for anions; as SIE are typically more marked for anions than for cations, the specific effect of negatively charged ions has been investigated to a larger extent. Nowadays, it is widely accepted that ionic hydration is an important parameter determining ionic specificity. 1,4,5 In accordance, the classification of the ions in the HS of Figure 1 is related with their degree of hydration. Na+ and Clare usually considered as indifferent ions in the HE. Anions on the right of Clare less hydrated than Cland are known as chaotropes and anions on the left of Clare more hydrated than Cland are known as kosmotropes. Originally, the terms kosmotrope and chaotrope were related with the capability of the ions to modify the structural arrangement of water, but these ideas are still controversial (see the analysis of E. Leontidis5 about these terms). However, kosmotrope and chaotrope terms are widely used in the studies of HE; we apply them exclusively in relation with the ionic hydration degree. In this sense, kosmotropicity is associated with a highly hydrated ion while chaotropicity is related with a poorly hydrated one. It has also been shown that the ionic character is strongly associated with the Bcoefficient of the linear term in the Jones-Dole viscosity equation; larger positive B values are typically observed for more kosmotropic ions, while negative B values point to chaotropicity. 6,7 On the contrary, attempts to classify the kosmotropic or chaotropic character of the ions only considering their size and/or polarizability has been unsuccessful. For example, the halide sequence F-, Cl-, Brand I- (from more kosmotropic to more chaotropic) can be suitable explained considering both size and polarizabilities parameters. However, if the corresponding halate ions (ClO3-, BrO3and IO3-) are considered, ionic size and polarizability cannot explain accurately their position in the HS; BrO3and IO3have clearly shown a kosmotropic character.1,8,9 It has been increasingly acknowledged that ions are not the only players in SIE. Ionspecific effects deal with macromolecules, substrates or interfaces which exert some influence on the surrounding water. Thus, the effects of hydration or wettability of the interface and their interplay with the ionic environment should be considered in a comprehensive description of HE. In this sense, SIE have been qualitatively explained taking into account that chaotropic ions tend to accumulate on poorly hydrated (hydrophobic) surfaces while they are excluded from the highly hydrated (hydrophilic) ones. Conversely, kosmotropic ions tend to accumulate on hydrophilic surfaces while they are excluded from the hydrophobic ones.1,2,5,10,11,12 Figure 2 shows a representation of this accumulation/exclusion mechanism, which reinforces the importance of the water structure around both, ions and surfaces, to explain HE. However, the difficulty of properly describe and introduce the terms of hydration into a theoretical model (in particular for the interfaces) has hindered the emergence of a general theory able to explain HE. Recently, other type of ions, called super-chaotropes, has been added to the HS. These ions have similar effects than regular chaotropic ions, but to a much greater extent; for this reason, they have been located in extreme positions of the HS (see Figure 1). Examples of these ions are the tetraphenylborate (Ph4B-) (see Figure 3a) anion or the tetraphenylarsonium (Ph4As+) cation. Different studies have demonstrated how these ions strongly accumulate on hydrophobic surfaces while they are excluded from the hydrophilic ones, and how this accumulation augments as the hydrophobic character of the surface increases.2,8,13,14 This behavior, which parallels and goes beyond what is regularly observed for inorganic ions in the original HS (e.g. SCNor ClO4-), has motivated labeling the tetraphenyl ions as super-chaotropic. These ions has also been called “soft matter disruptors”, due to their manifest ability to disrupt the structure and phase transitions of many soft matter systems.15 The inclusion of big hydrophobic monovalent charged ions in the HS reinforces the idea of the fundamental role that the solvent ion-interface interaction plays in the origin of the IES, and their connection with the hydrophobic interaction.2 For monovalent ions, ion hydration and the hydrophobic/hydrophilic character of the surfaces are essential to define the accumulation or exclusion of the ions from the interfaces and, hence, to determine their position in the HS. Other type of anionic species called polyoxometalates, POMs, have been recently classified as super-chaotropes and introduced in the HS.16 These charged metal-oxide clusters are characterized by high valences (three and higher) and very well defined molecular structures (see Figure 3b). POMs have found numerous applications in different fields as catalysis, lithography, medicine, etc. where their interaction with the interfaces plays a fundamental role.17 Naskar et al. were the firsts to classify these species as super-chaotropes by studying the interaction of several POMs with nonionic surfactants.16 They concluded that POMs have a tendency to adsorb on hydrophilic surfaces. More recently, part of these authors extend the study to estimate the superchaotropic character of different type and charged POMs anions according to their affinity for polar surfaces,18 although no adsorption in water-alkane interfaces was observed. A somehow different conclusion was reached by Kobayashi et al.,19 who studied the interaction of POMs with different charges (3-, 4and 6-) and several lipid monolayers to analyze the interplay between the electrostatics and hydrophobic interactions. They found that when working with the compressed lipid monolayers (hydrophobic environment) the chaotropic nature of the POMs was manifested. Chaumont et al. carried out molecular dynamic simulations of POMs with chloroform, ionic liquids and graphite and they observed that these anions tend to accumulate to some extent at the interface of the different liquids and surfaces.20 These studies clearly show that POMs have interfacial activity, and seem to adsorb or accumulate on both hydrophobic and hydrophilic surfaces. However, these results related to the interaction of POMs with surfaces would be in contradiction with the definition of super-chaotrope ion proposed above for the tetraphenyl ions. This is, super-chaotrope ions should strongly accumulate on hydrophobic surfaces but they should be excluded from the hydrophilic ones, a tendency that is also observed for the monovalent chaotropic inorganic ions that belongs to HS (e.g. SCNor ClO4-). In a different perspective, K. Assaf and W. Nau support in a recent paper that the chaotropic effect is a generic driving force for supramolecular assembly, “orthogonal” to the hydrophobic effect.21 In their description, the chaotropic effect would be the force responsible for the strong interaction that very large anionic groups like dodecaborate, B12X122and B12X11Y2- (X= H, Cl, Br, I and Y= SH), or POMs show with hydrophobic interfaces. In accordance, these groups should be classified as super-chaotropic ions. For these authors, the classification of ions as kosmotropic, chaotropic or super-chaotropic depends on hydration parameters previously proposed by Marcus. 6,22 In this sense, superchaotropic ions are characterized by enthalpically driven processes with a negative entropic contribution. These super-chaotropic anions would be introduced in the extended HS between chaotropic and hydrophobic ions (e.g. Ph4B-), which should not be considered as super-chaotropic. As previously commented, the classification of the inorganic ions belonging to Hofmeister series are related to their hydration degree. Considering hydration parameters, Ph4Bis a hydrophobic anion, poorly hydrated, and its behavior matches the tendency observed for chaotropic ions. On the contrary, PW3is considered a hydrophilic ion,5 highly hydrated and, hence, its behavior should be akin to that showed for kosmotropic ions. These different views indicates that there is not a clear and unified concept about the meaning of super-chaotropic ion, or even of the Hofmeister series. This lack of consensus generates more confusion when it comes to understanding specific ion effects, which are already extremely complex, obstructing the development of a long-awaited general theory capable of explaining HE. It seems reasonable to explore the rationality of expanding the Hofmeister series to any type of ionic species; are all ionic effects Hofmeister effects? This study aims to contribute to the clarification of these concepts and to deepen in the origin of such phenomena. The goal of this work is to study the electrokinetic behavior of several interfaces with different hydrophobic/hydrophilic character in presence of a typical Kegging POM, PW12O403- (PW3-). PW3contains a central phosphorus atom bonded to four oxygen atoms in tetrahedral configuration. These oxygen atoms are shared with 4 W3O13 groups (Figure 3b) arranged around the P-centred tetrahedron. Thus, the cluster has a quasi-spherical structure of ca. 0.8 nm.23,24 Similarly to other heteropolyacids, 12-tungstophosphoric acid (H3PW12O40) is an extremely strong acid; thus, complete deprotonation of the anionic cluster will occur, even at very acidic conditions.25 The results obtained with PW3are compared with those obtained in presence of the tetraphenyl anion, Ph4B-, a large hydrophobic anion (radii ca. 0.5 nm) with four phenyl rings bonded to a central boron atom (fig. 3a),13 and with archetypal chaotropic (SCN-) and kosmotropic (citrate3-) anions. In addition, Quartz Cristal Microbalance and Atomic Force Microscopy were used to reinforce the conclusions. All these results allow us to explore the role of the hydrophobicity of an interface in the ion-surface interaction and to revisit the concept of super-chaotropic anions. Methods Reagents and proteins All the salts in this study were of analytical grade and used as received. Sodium phosphotungstate hydrate, Na3PW12O40, (purity ≥ 99.9%) and sodium tetraphenylborate, NaThB4, (purity ≥99.5%) were purchased from Sigma-Aldrich; sodium citrate, Na3C6H5O7, was obtained from Scharlau. Proteins from Sigma Aldrich: Bovine Serum Albumin (BSA) (purity ≥98%), β-lactoglobulin (purity ≥90%, mixture of A and B genetic variants) and β-casein (purity ≥98%). Olive oil was obtained from Sigma Aldrich. Epikuron 145 V, was kindly provided by CargilIbérica SL. Protasan® Cl 113, and medium-molecular-weight chitosan chloride salt with a deacetylation degree of 85%, was supplied from FMC Biopolymer Novamatrix (Norway). Several buffers of ionic strength lower than 2 mM were used for particle synthesis: the solution with pH 4 was buffered with acetic acid. In each case, the pH was adjusted by adding NaOH. In addition, we used a buffered solution at pH 7 with bis-Tris in order to dissolve the β-casein protein. The pH of this solution was adjusted by adding HCl; the ionic strength of the solution was 2.4 mM. Non-buffered solution at pH 3 was prepared by adding dilute HCl to deionized water. Deionized Milli-Q water was used throughout. Nanoparticles Eight different types of colloidal surfaces were used in this work. Two polystyrene latexes prepared by the emulsion polymerization method in absence of surfactants were used as hydrophobic particles. One of them carried a negative charge on the surface due to the presence of sulfonate groups (d=138±7 nm),26 and the other one had positive charge coming from amine groups (d=475±4 nm).27 The electrophoretic mobility of the positively charged latex was larger than 3.10-8 m2V-1s-1 for pH values between 3 and 10. Both latexes were supplied by IKERLAT polymers. Negatively charged monodisperse silica hydrophilic particles (d=150±9 nm), were prepared by a seeded-growth protocol.28 Positively charged chitosan nanocapsules (d=281±6 nm), with isoelectric point ca. pH 7.5, were synthesized as described before.29 As intermediate positive hydrophobic system, modified silica particles were obtained by silanization of silica particles by using 3-aminopropyltriethoxysilane (APTES), which provides amine groups to the surface. All the details concerning particle synthesis and characterization can be found in the corresponding references. The other three surfaces were obtained by adsorbing BSA, βlactoglobulin and β-casein onto the negative latex. Briefly, each protein was added to an aqueous solution at the desired pH containing latex particles with a total polystyrene area of 0.3 m2. The protein concentration used was high enough to guarantee a maximum coating. Incubation was carried out at 25ºC for 21 h. Then, samples were centrifuged at 25.000  g for 30 min, and the pellets were re-dispersed and stored at the desired pH. More details can be found elsewhere.14,30 Electrophoretic mobility The measurements of electrophoretic mobility were carried out using a Zetasizer Nano Z device (Malvern Instruments). The particles were diluted to a concentration of around 1010 particles per cm3. The reported data were the average of three measurements; the standard deviation was always lower than 5%. Mobility-based isotherms were measured for the different particle-ion combinations; the low-salt limit equals the electrophoretic mobility of bare particles at pH 3. All the mobility measurements were performed at pH 3 (adjusted by addition of HCl in absence of buffer) except for the case of sodium citrate; in this case the pH was a function of salt concentration. Quartz Crystal Microbalance with dissipation monitoring, QCM-D The adsorption of POM on silica was measured in a commercial quartz crystal microbalance (QCM-D E1, Q-Sense). The principles of the technique have been extensively described in the literature.31 Briefly, the resonance frequency f of a quartz resonator is measured; a change in the effective mass of the resonator due to material adsorption translates into a variation in the resonance frequency, Δf. In addition, the damping of the oscillation of the crystal was measured, and used to calculate the “dissipation factor”, D (inverse of the quality factor of the resonance peak).32 The measured Δf and ΔD can be related to the thickness and viscoelastic properties of the material adsorbed on the quartz crystal by using adequate models.31 We used 5.0 MHz quartz resonators with gold electrodes, which were coated with silica thin layers. At the Acknowledgments The authors thank the financial support granted by the project FIS201680087-C2-1-P from the Spanish Ministerio de Economía y Competitividad, Plan Nacional de Investigación, Desarrollo e Innovación Tecnológica (I + D + i).The authors are indebted to Dr. S. Ravaine and Dr. P.-E. Rouet for providing the calibrated silica nanoparticles References (1) Bastos-González, D.; Pérez-Fuentes, L.; Drummond, C.; Faraudo, J. Ions at Interfaces: The Central Role of Hydration and Hydrophobicity. Curr. Opin. Colloid Interface Sci. 2016, 23, 19–28. (2) Calero, C.; Faraudo, J.; Bastos-González, D. Interaction of Monovalent Ions with Hydrophobic and Hydrophilic Colloids: Charge Inversion and Ionic Specificity. J. Am. Chem. Soc. 2011, 133 (38), 15025–15035. (3) Zhang, Y.; Cremer, P. S. Interactions between Macromolecules and Ions: The Hofmeister Series. Curr. Opin. Chem. Biol. 2006, 10 (6), 658–663. (4) Lo Nostro, P.; Ninham, B. W. Editorial: Electrolytes and Specific Ion Effects. 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Crystallization of Keggin Heteropolyanions via a Two-Step Process in Aqueous Solutions. J. Am. Chem. Soc. 2016, 138 (23), 7282–7288. Figure 1. Extended Hofmeister series for anions Figure 2. Schematic of the accumulation/exclusion mechanism of ions on intefaces. As an example we have considered positive surfaces, so that anions act as counter-ions. For simplicity, co-ions are not included. a) Accumulation of chaotropic (poorly hydrated) ions on hydrophobic interfaces. b) Exclusion of komostropic (highly hydrated) ions on hydrophobic interfaces. c) Exclusion of chaotropic ions on hydrophilic interfaces. d) Accumulation of kosmotropic ions on hydrophilic interfaces. Figure 3. Chemical structure of a) Tetraphenyl borate (Ph4B-) and b) Kegging POM (PW12O403-) anions used in this work. Figure 4. Electrophoretic mobility (µe) as a function of Ionic strength in presence of POM. (a) POM anions acting as counterions. ■ cationic latex, ▲ silica-NH2 particles, ● chitosan particles. (b) POM acting as co-ion. ▼ silica particles, ◊ anionic latex. Figure 5. Electrophoretic mobility (µe) as a function of ionic strength in presence of Ph4B-. ■ cationic latex, ▲silica-NH2 particles, ●chitosan particles. ▼silica particles, ◊ anionic latex. Figure 6. Electrophoretic mobility (µe) as a function of ionic strength a) POM and b) Ph4Banions (data taken from reference 11). ■ bare latex, ◊ latex with β-casein, ● latex with BSA, ▲ latex with β-lactoglobulin, Figure 7. Electrophoretic mobility (µe) as a function of ionic strength for (a) hydrophobic latex and (b) hydrophilic chitosan particles. ● SCN-, ▲ PW3- , ■ Ph4B-, ▼citrate3Figure 8. Mean thickness of the adsorbed PW3layer on silica as a function of ion concentration in solution, measured by QCMD. The arrow indicates the final thickness of irreversibly adsorbed ions measured after rinsing with pH 3 water. Figure 9. 5x5 µm2 tapping mode AFM micrographs of freshly cleaved mica surfaces exposed to PW3solutions of concentrations a) 1 mM, b) 10 mM and c) 100 mM for 12 hours, after rinsing with water at pH 3 and drying with nitrogen gas.