Effect of cationic structure of surface active ionic liquids on their micellization: A thermodynamic study

Abstract

In this work the influence of cationic structure of surface active ionic liquids (SAILs) on their micellization process in water is investigated by isothermal titration calorimetry (ITC) at temperatures between 278.15 and 318.15 K. First, 1-dodecylimidazolium bromide ([C12im]Br) was studied, and then the length of alkyl chain on the N3 position is varied from methyl- (1-dodecyl-3-methylimidazolium bromide ([C12mim]Br)), ethyl- (1-dodecyl-3-ethylimidazolium bromide, [C12eim]Br), to butyl- (1-dodecyl-3-butylimidazolium bromide, [C12bim]Br). In next step, the N3 was replaced by S atom in 3-dodecylthiazolium bromide ([C12thiaz]Br) as an analogue of [C12im]Br. Due to problems at synthesis and low solubility of other thiazolium analogues no further experiments on these systems were possible. ITC experimental data were analyzed by the help of an improved model, yielding simultaneously the values of critical micelle concentration, cmc and enthalpy, ∆MHθ by the fitting procedure together with the degree of counter ion binding, β, aggregation number, n, standard heat capacity, ∆Mcpθ and Gibbs free energy, ∆MGθ of micellization. It was found that the investigated systems behave mainly like common ionic surfactants and already investigated SAILs: the micellization process of investigated systems was detected as endothermic at low temperatures and exothermic at high temperatures; the values of cmc are decreasing with the increasing length of alkyl chain on N3 position. Between [C12thiaz]+ and Br− weaker interactions were found in comparison to those between [C12im]+ and Br− what can be ascribed to more expressed aromaticity of thiazolium ring. ∆Mcpθ values were further discussed, regarding the removal of water molecules from contact with nonpolar surface area upon micelle formation. It can be concluded, that the ethyl- and butyl- chain on N3 position are folded during the micellization process and thus they are (at least) partially removed from contact with water.