Self-aggregation of ionic liquids in aqueous media: A thermodynamic study

Aggregation process of the ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4], 3-methyl-1-octylimidazolium tetrafluoroborate [C8mim][BF4], 3-methyl-1-octylimidazolium chloride [C8mim][Cl], and N-butyl-3-methylpyridinium chloride [C4mpy][Cl], in the aqueous solutions has been characterized by electrical conductivity, density, and speed of sound measurements at 298.15 K. The critical aggregation concentration (CAC), the standard Gibbs energy of aggregation ΔGagg.∘, isentropic compressibility κs and changes in the isentropic compressibility upon aggregation Δκs,agg for the ionic liquids were derived from the experimental data. The dependence of the CAC, ΔGagg.∘, and Δκs,agg on the nature of the cations and anions has been examined. The aggregation number n for various ILs was determined by applying the mass action theory to the concentration dependence of electrical conductivity and compressibility. Aggregation properties showed fairly good agreement with our previously reported results of 1H NMR and fluorescence spectroscopy.