Modulation of cationic surfactant micellization by imidazolium-based ionic liquids: A thermodynamic study

Abstract

Understanding the modulation of ionic surfactant properties in the presence of ionic liquids (ILs) is fundamental for expanding the application of that class of compounds. Although the effect of ILs on the micellization process of anionic surfactant has gained broad attention, its effect on cationic surfactants aggregation processes has been underexplored. This work investigated the effect of 1-alkyl-3-methylimidazolium chlorides (C_nmimCl, with n = 2 or 4) and NaCl on the micellization of alkylpyridinium chlorides (C_nPyCl, with n = 12 or 16) cationic surfactants using Isothermal Titration Calorimetry (ITC) and electrical conductivity. The reduction in the critical micellar concentration (CMC) in the presence of the electrolytes depends similarly on the stoichiometry between the surfactant and the electrolyte in the solution before micellization, independent of the electrolyte nature. However, the more hydrophobic the ionic liquid and the surfactant, the more intense the effects of electrolyte concentration on the micellar dissociation degree (α) and the micellization parameters (${∆G}_{\mathrm{mic}}^o,{∆H}_{\mathrm{mic}}^o$, and ${T∆S}_{\mathrm{mic}}^o$). The presence of C₂mimCl or C₄mimCl made the micellization process of both surfactants more exothermic, associated mainly with the maximization of non-dispersive interactions among monomers within the micelles and reduction of repulsive electrostatic interactions among surfactant head groups. Additionally, kosmotropic effects and preferential solvation of the surfactant, more intense for C₄mimCl, actuate at high concentrations of this ionic liquid, intensifying the decrease of ${∆H}_{\mathrm{mic}}^o$. This underscores that, despite both the imidazolium cation and the surfactant being positively charged, dispersive interactions between them play an important role in efficiently modulating the aggregation process.