Variation of micellization, thermodynamic, and surface properties of sodium dodecyl sulfate in aqueous media using 1,3-disulfo-2-alkyl imidazolium chloride ionic liquids

Three direct –NSO₃H functionalized Brønsted acidic ionic liquids of 2-alkyl-1,3-disulfo imidazolium chloride with varied sizes of alkyl substituents (Me-, Et-, nBu-) were utilized to explore the effects of ionic liquids on aggregation behavior of anionic surfactant sodium dodecyl sulfate in aqueous media at various concentrations of ionic liquids and temperatures (288.15 K, 293.15 K, 298.15 K, 303.15 K, and 308.15 K). Critical micelle concentrations (CMCs) of the IL-surfactant systems obtained from conductivity measurement were found to be in good agreement with the CMC values of surface tension and UV–visible spectroscopy techniques. These CMCs values were used to calculate the thermodynamic parameters of IL-surfactant solutions such as standard free energy of micellization (ΔG°_m), standard enthalpy of micellization (ΔH°_m), and standard entropy of micellization (ΔS°_m). Continuous decrease of the CMC values was observed with increasing concentrations of the ILs as well as temperatures. Packing parameters calculated from the surface tension measurement displayed small spherical shape for all the mixed micellar systems. Structural changes of the IL-surfactant solutions were also observed using FT-IR spectroscopic method. Increased positive inductive effect (+ I) of 2-alkyl substituent of the imidazolium cation of ionic liquid showed stabilizing effect on the micelle formation by lowering of more negative zeta potential values of the IL-surfactant systems.

Graphical Abstract