Influence of the AOT Counterion Chemical Structure on the Generation of Organized Systems

1 Influence of the AOT counterion chemical structure in the generation of organized systems Cristian M. O. Lépori*[a], N. Mariano Correa[b,c], Juana J. Silber[b,c], R. Darío Falcone[b,c], Manuel López-López[d] and M. Luisa Moyá[e] [a] Instituto de Física Enrique Gaviola (IFEG), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) – Universidad Nacional de Córdoba (UNC), Medina Allende s/n, Ciudad Universitaria, X5016LAE Córdoba, Argentina. [b] Departamento de Química, Universidad Nacional de Río Cuarto (UNRC), Agencia Postal # 3, X5804BYA Río Cuarto, Argentina. [c] Instituto para el Desarrollo Agroindustrial y de la Salud (IDAS), CONICET – UNRC. Agencia Postal # 3, X5804BYA Río Cuarto, Argentina. [d] Departamento de Ingeniería Química, Química Física y Ciencias de Materiales, Centro de Ciencia y Tecnología, Universidad de Huelva, Campus ‘El Carmen’, Facultad de Ciencias Experimentales, E-21071 Huelva, Spain. [e] Departamento de Química Física, Universidad de Sevilla, c/ Prof. García González 1, 41012 Sevilla, Spain. * Corresponding-Author: Dr. Cristian M. O. Lépori. E-mail: [email protected] Page 1 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 ABSTRACT The impact of the imidazolium counterions structure on the organized systems formed by the surfactant 1,4-bis-2-ethylhexylsulfosuccinate, AOT, both in aqueous solutions and in non-polar solvents is investigated. With this in mind, we investigated if the ionic liquid-like (IL-like) surfactant 1-ethyl-3-methylimidazolium 1,4-bis-2- ethylhexylsulfosuccinate, emim-AOT, forms direct micelles or vesicles in water. Dynamic Light Scattering (DLS), zeta potential, conductivity, fluorescence and UV- Visible spectroscopy measurements were performed to characterize the organized systems in aqueous solution. We also studied the self-aggregation of emim-AOT, 1- butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate, bmim-AOT, and of 1- hexyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate, hmim-AOT, in nonpolar solvents. The results obtained showed that the IL-like surfactant emim-AOT forms direct micelles in water, as Na-AOT does. However, emim-AOT aggregates are larger, have a lower surface charge, are more stable, and have a more polar and less fluid micellar interface than Na-AOT micelles. It was also observed that emim-AOT and hmim-AOT form reverse micelles in non-polar solvents. The size of the imidazolium cations dramatically influences on the size of the reverse micelles as well as on their ability to solubilize water. KEYWORDS: ionic liquid-like surfactant, organized systems, Na-AOT, emim-AOT. Page 2 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 INTRODUCTION Organized systems are important and promising in the progress of various branches of modern science and technology.1,2 For example, they are used as detergent, in the industries of dyes, cosmetic and pharmaceutical.3 The existence of different microenvironments in these systems is one of the reasons for their applications in different areas, such as chemical catalysis,4–8 enzymatic hydrolysis,9–12 and nanoparticle synthesis,13–16 among others.17 In this context, nanomedicine has become in recent years one of the most promising fields concerning the diverse applications of organized systems. For example, they are frequently used as drug delivery systems.7,18–21 Also, the biomimetic characteristics presented by these systems can be highlighted, since they can be considered "simplified models" that simulate the behavior of highly complex structures, such as biological membranes.22,23 The surfactant sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT, Scheme 1) can form direct micelles in aqueous solution,24–26 with a critical micellar concentration value ~ 10-3 M.27–30 These aggregates have been used as a dispersing medium for hydrophobic dyes in an aqueous medium,31 for the synthesis of nanoparticles,16,32 among others.33 Furthermore, reverse micelles formulated in non-polar organic solvents can be created using also Na-AOT as surfactant. Depending mostly on the non-polar organic medium and temperature, these systems have the ability to disperse large amounts of water. For example, it is possible to achieve a W0max = 12 using benzene,34 while in n-heptane is possible to reach a value close to W0max = 6035,36 (W0 = [Water]/[Surfactant]). These properties are responsible for their numerous applications, as was mentioned above.4–6,9,11,13–15,17 Ionic Liquids (ILs) are synthetic compounds with interesting properties such as low toxicity, friendly to the environment and very versatile.37–39 Chemically, the ILs are Page 3 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 salts with a low melting point (< 100 ºC), composed only of ions. They also present very low vapor pressure, excellent thermal and chemical stability, easy recycling, and wide possibility to vary their properties such as polarity and hydrophobicity, through suitable cation and anion modification.40 In this sense, the possibility to create ILs with amphiphilic properties have been increased in the last years.41–43 These novels compounds, named IL-like surfactants, were able to formulate direct micelles, reverse micelles and vesicles as examples of organized systems.39,41–47 Furthermore, the IL-like surfactants derived from the imidazolium group are very interesting due to the unique properties of such cation: i) delocalization of the positive charge on the heterocyclic ring, which produce great stability. ii) The possibility of substituting the hydrogen attached to the carbon atom located between the two nitrogen atoms, for a substituent that allows the rest of the imidazolium ring to act as a ligand catalyst for metals. iii) Change the lipophilic character of the cation by substitution with alkyl groups at the ring nitrogen’s. Besides, thinking in future applications such as the design of catalysts, the flat geometry of the imidazolium ring can also be a significant aspect to consider.48– 50 Recently46,51 we reported the synthesis of two IL-like surfactants 1-butyl-3- methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate and 1-hexyl-3- methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT and hmim-AOT respectively, Scheme 1). In comparison with Na-AOT, these IL-like surfactants showed very different properties.46,51 Particularly, we studied the physicochemical properties of the aggregates formed by bmim-AOT and hmim-AOT in different solvents.46,47 Taking into account, the effect of the AOT counterion structure on the selfassembly process is investigated. With this goal, the IL-like surfactant 1-ethyl-3- methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (emim-AOT, Scheme 1) was Page 4 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 synthesized. Later, the emim-AOT aqueous solutions were investigated performing several experiments in order to acquire information about size, surface charge, degree of dissociation, critical micellar concentration (cmc), aggregation number (Nagg), micropolarity and microviscosity. Thus, emim-AOT aggregates were investigated using Dynamic Light Scattering (DLS), zeta potential and conductivity techniques. Additionally, by using three molecular probes such as pyrene (Scheme S1, Supplementary Material), 2,2'-[[4-[(4-nitrophenyl)azo]phenyl]imino]bisethanol52–54 (DR9, Scheme S1) and 1,3-dipyrenylpropane55,56 (P3P, Scheme S1), valuable information about the cmc, Nagg,57,58 polarity and viscosity of their microenvironment can be obtained.58,59 The self-association of emim-AOT and hmim-AOT in the nonpolar solvents n-heptane and benzene was examined and the results compared with those already reported on reverse micelles of bmim-AOT and Na-AOT.46,47,60 Page 5 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 6 Scheme 1. Molecular structures of the surfactants Na-AOT, emim-AOT, bmim-AOT and hmim-AOT. EXPERIMENTAL SECTION Materials 1-Ethyl-3-methylimidazolium chloride (emim-Cl), bmim-Cl, hmim-Cl, Na- AOT, dichloromethane (DCM), benzene, n-heptane, N-hexadecylpyridinium bromide (CPyBr) and DR19 were from Sigma-Aldrich (> 99% purity) and kept under vacuum prior to use. P3P and pyrene were purchased from Molecular Probes, Inc. and used as received. The IL-like surfactants used, bmim-AOT and hmim-AOT, were obtained following the experimental procedures previously described.46,51 Emim-AOT was prepared following the same methodology (fully described in Supplementary Material section),46,51 using as reactants emim-Cl and Na-AOT. Methods Detailed experimental procedure for all the samples preparations; DLS, zeta potential, conductivity and emission experiments are shown in the Supplementary Material Page 6 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 section. RESULTS AND DISCUSSION As a first experiment, the solubility of emim-AOT in water was investigated. This IL-like surfactant was soluble in water until 2 x 10-2 M, forming transparent solutions which remain stable over time. Similar behavior was reported by the Na-AOT surfactant and can suggests the formation of small size aggregates with low capacity to scatter the light.61 We previously observed by DLS that the surfactants bmim-AOT and hmim- AOT form vesicles in water.51 In this work aqueous solutions of emim-AOT in water were prepared, in a concentration range from 1 x 10-4 M to 2 x 10-2 M, and the samples were analyzed by DLS. Table 1 shows the apparent hydrodynamic diameter, dapp, values obtained for emim-AOT in water at 298 K. In Figure S3 (Supplementary Material), the DLS intensity plot for emim-AOT in water at [surfactant] = 1 x 10-2 M is depicted. The DLS data show the formation of aggregates with diameters ranging from 1.2 - 1.4 nm, with low polydispersity indexes (within the range 0.1 - 0.2). Moreover, within the range of 6 x 10-3 M – 2 x 10-2 M the sizes were independent of the surfactant concentration. The sizes found for the emim-AOT aggregates and the detection of these aggregates from [emim-AOT] = 6 x 10-3 M, are good evidences that the surfactant self-aggregates forming direct micelles,23 as makes the surfactant Na-AOT.25,26,28 The DLS results seem to indicate that the critical micellar concentration is close to 6 x 10-3 M.23 Page 7 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 Table 1. dapp and polydispersity indexes (PDI) values of emim-AOT micelles in water at different surfactant concentrations. T = 298 K. [Surfactant] (10-2 M) dapp (nm) PDI 0.6 1.4 ± 0.1 0.2 0.8 1.3 ± 0.1 0.2 1.0 1.3 ± 0.1 0.2 1.2 1.2 ± 0.1 0.1 1.6 1.3 ± 0.1 0.1 2.0 1.3 ± 0.1 0.1 Zeta potential measurements were carried out to determine the surface charge of these aggregates and the corresponding values are listed in Table 2. Table 2. Zeta potential values (mV) of Na-AOT and emim-AOT micelles in water at different surfactant concentrations. T = 298 K. [Surfactant] (10-2 M) Na-AOT emim-AOT 0.6 -51 ± 2 -37 ± 2 0.8 -52 ± 2 -36 ± 2 1.0 -49 ± 2 -38 ± 2 1.2 -51 ± 2 -37 ± 2 1.6 -52 ± 2 -40 ± 2 2.0 -50 ± 2 -39 ± 2 Data in Table 2 show zeta potential values of -51 mV and -38 mV for micelles of Na-AOT and emim-AOT, respectively. Thus, the anionic part of the polar head of Page 8 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 9 both surfactants appears to be exposed to the outer region of the micelle in both cases. However, the larger zeta potential value found for Na-AOT micelles compared to emim-AOT micelles could indicate the presence of a smaller concentration of positive counterions in the micellar interface of the water/Na-AOT systems than in the emim- AOT aggregates. This may due to the emim+ cation which is more hydrophobic than the Na+ cation, and therefore it prefers to be part of the micellar aggregate than to be solvated in the aqueous phase. To check this effect, the degree of counterion dissociation of micelles, ,62 of emim-AOT and Na-AOT was determined by measuring the conductivity of the medium at different concentrations of surfactant, as shown in Figure 1 and Figure S3 (Supplementary Material), respectively. Figure 1. Variation of the conductivity with [surfactant] in the micellar system water/emim-AOT. T = 298 K. The  values were determined using the method described by Carpena et al.62 Thus, we found for emim-AOT direct micelles a value of  = 0.73, while for the Na- AOT micellar system is  = 0.92. These results show that the emim+ counterion, in the 0.000 0.002 0.004 0.006 0.008 0.010 0 100 200 300 400  (S cm-1) [emim-AOT] (M) Page 9 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 16 Scheme 2. Schematic representation of the direct micelles of Na-AOT surfactant (A) and IL-like surfactant emim-AOT (B). A A B B Page 16 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 17 Considering that Na-AOT and emim-AOT form direct micelles, while bmim- AOT and hmim-AOT form vesicles in water, the counterion nature has a clear influence on the type of aggregates formed in aqueous solution. With this in mind, information about the effect of the counterion on the self-aggregation of the IL-surfactants in nonpolar solvents was also investigated. Na-AOT and bmim-AOT form reverse micelles in these media and present several differences.46,47 Therefore, our interest was focused in the study of the self-aggregation of emim-AOT and hmim-AOT in different non-polar solvents. Firstly, non-polar solvents such as benzene and n-heptane were selected to evaluate the solubility of emim-AOT and hmim-AOT. Emim-AOT was soluble at least up to 0.1 M in benzene, but not in n-heptane, whereas hmim-AOT was soluble in both solvents. Next, water aliquots were added to the surfactants/non-polar solutions in order to investigate the maximal amount of polar solvent (W0max) able to be dissolved. In Table 4 the results obtained in all the systems evaluated that provide clear and stable ternary mixtures at [surfactant] = 0.1 M is summarized. Additionally, the results found in Na-AOT and bmim-AOT micellar systems are also included. Table 4. Values of W0max obtained in different systems at [surfactant] = 0.1 M. T = 298 K. Non-polar solvent W0max Na-AOTa emim-AOT bmim-AOTb hmim-AOT n-Heptane 60 - - 14 Benzene 12 11 5 2 a Value obtained from ref. 60. b Value obtained from ref. 46. Page 17 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 18 As can be seen in Table 4, the values of W0max depends on the surfactant used. This result indicates that the counterion nature considerably influences the amount of solubilized water. For benzene/surfactant systems it can be seen that W0max decreases with increasing the hydrocarbon tail length of the imidazolium counterion, while in the n-heptane/surfactant micellar systems at a [surfactant] = 0.1 M, only the IL-like surfactant hmim-AOT can dissolve water. The hmim+ counterion appears to act as a kind of co-surfactant. Therefore, the impact of the different counterions (imidazolium vs Na+) on properties such as the size or the interactions of water molecules at the interface is interesting to explore. The solutions consisting of benzene/emim-AOT/water, benzene/hmim- AOT/water and n-heptane/hmim-AOT/water were studied by the DLS technique in order to investigate the formation of reverse micelles. Table 5 shows the dapp and PDI values of the DLS experiments that were performed at a surfactant concentration of 0.1 M for the emim-AOT and hmim-AOT solutions in non-polar solvents at W0 = 5. These working conditions were chosen to compare the data obtained in this work with those corresponding to the micellar systems formed by Na-AOT and bmim-AOT in the same solvents.46,47 It should be noted that the dapp value of the benzene/hmim-AOT/water system was performed at W0 = 2, since this micellar system does not allow to prepare homogenous solutions at W0 larger than this value (Table 4). Page 18 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 19 Table 5. Apparent hydrodynamic diameters (dapp) and polydispersity indexes (PDI) for the benzene/surfactant/water and n-heptane/surfactant/water reverse micelles at W0 = 5. T = 298 K. Surfactant benzene n-heptane dapp (nm) PDI dapp (nm) PDI Na-AOT 2.9 ± 0.3a 0.05 5.1 ± 0.1a 0.05 emim-AOT 3.5 ± 0.2 0.04 - - bmim-AOT 3.9 ± 0.2b 0.03 - - hmim-AOT 5.3 ± 0.1c 0.02 14.1 ± 0.1 0.01 a Value obtained from ref. 60. b Value obtained from ref. 46. c Value at W0 = 2. The apparent hydrodynamic diameter values listed in Table 5 make evident the existence of reverse micelles for emim-AOT and hmim-AOT surfactants in benzene.72,73 Furthermore, the low PDI values observed for the studied micellar systems suggest that the particles are essentially monodisperse.74 It is interesting to note that the dapp values of these IL-like surfactant micelles are higher than those of Na-AOT in benzene. Besides, the dapp value is higher as the larger the size of the of imidazolium cation is. In this regard, it is interesting to consider that when the effective packaging parameter of the surfactant, p, diminishes, the diameter of reverse micelles increases.71 The values of v and lc are similar for both the IL-like surfactants and Na-AOT.46 The substitution of Na+ by the imidazolium cations can produce an increase in the a value because the alkyl tails of the counterions would be close to the chains of AOT favoring the hydrophobic interactions and producing an increase in dapp. Data in Table 5 show that the droplet sizes of the hmim-AOT reverse micelles Page 19 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 20 are larger in n-heptane than in benzene. This result could be explained by considering the different penetration of the two non-polar solvents at the interface. It is well known that in Na-AOT reverse micelles, the penetration of the aromatic solvents into the interface is larger than aliphatic solvents.13,60 This phenomena can alter the p value and, consequently, the sizes of reverse micelles. Thus, benzene penetrates deeper into the interfacial zone in Na-AOT reverse micelles than those of n-heptane. Consequently, at similar W0 value the Na-AOT/benzene reverse micelles are smaller than Na-AOT/nheptane reverse micelles.60 Furthermore, the micelle-micelle interactions and the water structure inside reverse micelles can be altered by the type or composition of the nonpolar phase.13,60 Thus, it was found that when aromatic solvents are used, inter-micellar interactions are weaker than in aliphatic solvents. As consequence of this fact, a notable impact on the practical applications of reverse micelles as nanoreactors can be produced.13,60 For example, in the synthesis of nanoparticles one of the key steps that controls the synthetic pathway is the coalescence process. In this sense, the inter-micelle interactions can be controlled by the nature of the non-polar solvent.75 CONCLUSIONS Our investigations demonstrate that the substitution of Na+ cation by alkylimidazolium cations on the AOT surfactant alter significantly the type and characteristics of the organized systems that they form, both in water and in non-polar solvents. The IL-like surfactant emim-AOT forms direct micelles, while bmim-AOT and hmim-AOT can spontaneously form unilamellar vesicles in water, as previously shown. The emim-AOT micelles are larger, with less negative surface charge, a lower degree of ionization, and lower critical micellar concentration than those of Na-AOT. As a Page 20 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 21 consequence, they have a higher Gibbs energy of micellization. Emim-AOT direct micelles also present a lower fluidity of the micellar interface and a greater capacity to solubilize low polarity molecules than Na-AOT ones. In non-polar solvents emim-AOT forms reverse micelles in benzene, while hmim-AOT does in both benzene and n-heptane. It was observed that, depending on the imidazolium cation nature, the micellar systems have a different capacity to solubilize water in their interior and that the aggregates have different sizes. These results show the importance of the surfactant counterion nature in the behavior of surfactants, converting them into ILs and determining the aggregates formed in the self-association process. Indeed, it is a very important factor to consider when designing new surfactants for different applications in which aggregates play a key role, such as drug nanocarriers, nanoreactors in the synthesis of nanoparticles or in different chemical reactions. ASSOCIATED CONTENT Supplementary Material. Full experimental methodology, molecular structures of the probes, NMR characterization of emim-AOT, DLS data of aqueous solution of emim- AOT, conductivity measurements and determination of cmc with pyrene of Na-AOT aqueous solution, are available free of charge. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes Page 21 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 22 The authors declare no competing financial interests. ACKNOWLEDGEMENTS We gratefully acknowledge the financial support for this work by the Consejería de Conocimiento, Investigación y Universidad de la Junta de Andalucía (P12-FQM- 1105), FQM-274, the Consejo Nacional de Investigaciones Científicas y Técnicas (PIP CONICET 112-2015-0100283), Universidad Nacional de Río Cuarto (PPI-UNRC 2016- 2018, 2020-2024), Agencia Nacional de Promoción Científica y Técnica (PICT 2018- 0508), and Ministerio de Ciencia y Tecnología, Gobierno de la Provincia de Córdoba (PID 2018). N.M.C., J.J.S. and R.D.F. hold a research position at CONICET. C.M.O.L. thanks CONICET for a research fellowship and Asociación Universitaria Iberoamericana de Postgrado (AUIP) and Consejería de Economía y Conocimiento de la Junta de Andalucía for a fellowship, with which Dr. Lépori made the research stay at the University of Seville (Spain). REFERENCES (1) Mclntire, G. L.; Dorsey, J. G. 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Langmuir 2010, 26 (12), 15802–15806. (26) Sheu, E. Y.; Chen, S.; John S. Huang. Structure and Growth of Bis(2-Ethylhexyl) Sulfosuccinate Micelles in Aqueous Solutions. J. Phys. Chem. 1987, 91 (12), 3306–3310. (27) Chatterjee, A.; Moulik, S. P.; Sanyal, S. K.; Mishra, B. K.; Puri, P. M. Thermodynamics of Micelle Formation of Ionic Surfactants: A Critical Assessment for Sodium Dodecyl Sulfate, Cetyl Pyridinium Chloride and Dioctyl Page 25 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 1 GRAPHICAL ABSTRACT The IL-like surfactant emimAOT forms direct micelles in water, while bmimAOT and hmimAOT form vesicles in water. The three IL-like surfactants form reverse micelles in non-polar organic solvents. Page 32 of 32 ACS Paragon Plus Environment Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60