Modulating properties of aqueous sodium dodecyl sulfate by adding hydrophobic ionic liquid

Altering and modifying important physicochemical properties of aqueous surfactant solutions is highly desirable as far as potential applications of such systems are concerned. Changes in the properties of aqueous solutions of a common anionic surfactant sodium dodecyl sulfate (SDS) are assessed in the presence of a common and popular ‘hydrophobic’ ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6). Upon addition of up to ∼0.10 wt%∼0.10 wt% bmimPF6, a dramatic decrease in critical micelle concentration (cmc) is accompanied by an increase in the degree of counterion dissociation (α  ) and micellar aggregation number (NaggNagg) indicating micellar growth. However, in the range 0.10 wt% ⩽ bmimPF6 ⩽ 2.00 wt%, relatively gradual decrease in α   and NaggNagg is observed along with no change in cmc. Significantly decreased microfluidity of the aqueous SDS solutions on addition of bmimPF6 is indicated by a fluorescence microviscosity probe 1,3-bis-(1-pyrenyl)propane which suggests partitioning of bmimPF6 into the SDS micellar phase. Behavior of solvatochromic fluorescence probes, pyrene, pyrene-1-carboxaldehyde, and 2-(p-toluidino)naphthalene-6-sulfonate, confirms interaction, and possible complexation, between IL bmimPF6 and anionic micellar surface. Increased solubility of bmimPF6 with increasing SDS concentration further confirms SDS-bmimPF6 interactions. Presence of strong electrostatic attraction between bmim+ and anionic micellar surface is proposed to be the most dominant reason for these observations. All-in-all, unique role of a hydrophobic ionic liquid bmimPF6 in modifying the properties of aqueous anionic sodium dodecyl sulfate is demonstrated.

Unique role of a hydrophobic ionic liquid toward modifying physicochemical properties of aqueous anionic sodium dodecyl sulfate is demonstrated.Figure optionsDownload as PowerPoint slide