Properties of aqueous micellar solutions in the presence of ionic liquid

Abstract

The physicochemical properties of aqueous surfactant solutions above their critical micelle concentrations (cmc) are governed by the micellar assemblies present within the solution. Due to their distinct structural features and properties, ionic liquids (ILs) can effectively and favorably modify the properties of aqueous micellar solutions. Effect of addition of a prototypical ‘hydrophobic’ IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), which exhibits only ∼2wt% miscibility in water at ambient conditions, on the properties of aqueous micellar solutions of four different surfactants, namely, nonionic triton X-100 (TX-100), anionic sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS), and zwitterionic dodecylsulfobetaine (SB-12), is assessed. Surface tension, density, refractive index, and dynamic viscosity along with fluorescence probe-reported microfluidity of the four aqueous micellar solutions are investigated in the absence and presence of up to 5wt% [bmim][PF6]. Density and refractive index of aqueous micellar solutions increase linearly with wt% [bmim][PF6]. Increasing [bmim][PF6] results in decrease in surface tension of anionic/zwitterionic micellar solutions; on the contrary, the surface tension of nonionic TX-100 micellar solution increases. This is due to the negative adsorption exhibited by [bmim][PF6] within aqueous micellar TX-100 solution. There is an initial increase, in general, in the dynamic viscosity of the four micellar solutions as [bmim][PF6] is added; however, this increase is way more dramatic for SDBS. This is attributed to a drastic increase in the size of the micelles as a result of the presence of both [bmim][PF6]-SDBS electrostatic as well as cation-π interactions. Response from the microfluidity probe does not necessarily scale with the dynamic viscosity of the micellar solutions as [bmim][PF6] is added. The water penetration into the palisade layer of the [bmim][PF6]-added anionic micelles, where the microfluidity probe solubilizes, is the reason for this outcome. The aqueous micellar media is perturbed significantly in the presence of IL and the extent of the changes in the properties is contingent on the identity of the surfactant.