Effect of ionic liquid on the micellization behavior of bile salts in aqueous medium

Interfacial and aggregation behavior of sodium cholate (NaC) and sodium deoxycholate (NaDC) in aqueous ionic liquid (IL, 1-butyl-3-methylimidazolium tetrafluoroborate) medium were investigated. Studies were also carried out in the presence of NaBF₄. Both IL and NaBF₄ reduce the critical micelle concentration (CMC) of bile salts, determined by surface tension, conductance, and fluorescence spectroscopic techniques; effect of NaBF₄ is more prominent than IL. CMC of NaC was higher than NaDC because of its higher hydrophilicity. IL, in addition to its salt-like behavior, results in associative interaction with the bile salts. Surface excess (Г_max), surface pressure at the CMC (π_CMC) were evaluated from surface tension data. Г_max and π_CMC values gradually decreased with increasing the IL and NaBF₄ concentration. Negative Gibbs free energy change for micellization (ΔG ⁰_mic) and interfacial adsorption (ΔG ⁰_ads) indicate spontaneity of micellization and interfacial adsorption, however to different magnitude. Fraction of counter ion dissociation, evaluated from conductance data, systematically decreased with increasing IL and NaBF₄ concentration. Aggregation number, determined by fluorescence quenching method, increased with increasing IL and NaBF₄ concentration that are in consonance with the size of the aggregates, determined by dynamic light scattering study. IL induced the formation of larger facial aggregates by bile salts; the effect was more imperative in case of NaDC than NaC. In case of NaBF₄ the opposite trend was recorded. Such systems are considered to have potential application where the dual benefit of both a surfactant and an IL are warranted.