Aggregation Behavior of Pyridinium-Based Ionic Liquids in Aqueous Solution

Aggregation of the ionic liquids 1-butylpyridinium tetrafluoroborate, 1-butylpyridinium triflate, 1-butyl-2-methylpyridinium tetrafluoroborate, 1-butyl-3-methylpyridinium tetrafluoroborate, 1-butyl-4-methylpyridinium tetrafluoroborate, 1-butyl-3-methylpyridinium dicyanamide, and 1-octyl-3-methylpyridinium tetrafluoroborate in aqueous solution has been characterized at 298.15 K through density, ρ, speed of sound, u, and conductivity, σ, measurements. In addition, apparent molar volumes, Vφ, and isentropic compressibilities, κs, have been calculated from the experimental data. To characterize the formation of aggregates, the critical aggregation concentration of the ionic liquids, cac, the degree of ionization of the aggregates, β, and the standard Gibbs energy of aggregation, $\Delta G_{\mathrm{m}}^{\circ}$, have been obtained, with good agreement between results derived from the different methods. The dependence on the structural variation of these ions has been analyzed by comparing the results obtained for this series of ionic liquids.