Micellization and Thermodynamic Study of 1-Alkyl-3-methylimidazolium Tetrafluoroborate Ionic Liquids in Aqueous Solution

Three surfactant-like ionic liquids, 1-dodecyl-3-methylimidazolium tetrafluoroborate (C12mimBF4), 1-tetradecyl-3-methylimidazolium tetrafluoroborate (C14mimBF4), and 1-hexadecyl-3-methylimidazolium tetrafluoroborate (C16mimBF4) had been systematically studied by conductivity measurement, surface tension, steady-state fluorescence measurement and ultraviolet (UV) absorption spectra at 298.15 K. Micellization of the three ILs were certified by the above methods and aggregation number of micelles (Nagg) were determined by pyrene fluorescence quenching method. A comparison of CnmimBF4 with different alkyl chain lengths shows that that the critical micelle concentration (cmc) decreased remarkably with the increase of alkyl chain length, but the surface tensions at cmc (γcmc) were approximately the same, except for that of C16mimBF4. The effects of temperature, inorganic salt, and organic alcohols on the cmc of CnmimBF4 (n = 12, 14, 16) aqueous solution were also investigated. The results showed that the cmc values assumed a trend of increase along with the increase of temperature, and decreased remarkably with the presence of inorganic salt. For alcohol–water systems at lower temperature, the cmc values increased with the presence of short chain alcohols (ethanol, 1-propanol), but decreased or were invariable with long chain alcohols (1-butanol, 1-pentanol). Finally, the micellization thermodynamic parameters (ΔGm°, ΔHm°, ΔSm°) of CnmimBF4 (n = 12, 14, 16) showed that the micelle formation process of C12mimBF4 was entropy-driven, and the micellization behavior of C14mimBF4 and C16mimBF4 was found to be an enthalpy-driven process in the investigated temperature range.