Micellization, surface activities and thermodynamics study of dialkylpyridinium [C₁₆pymC_n][Br] (n = 1–4) in aqueous solutions

The pyridinium-base ionic liquids surfactants: N-hexadecyl-3-alkylpyridinium bromide [C₁₆pymC_n][Br] (n: the carbon numbers of short alkyl side chain, n = 1, 2, 3, 4) were synthesized and characterized by NMR(¹H, ¹³C) and FTIR. The thermal properties of the [C₁₆pymC_n][Br] showed a same transition from a crystalline phase to a thermotropic liquid-crystalline phase and then to an isotropic liquid phase, and were thermally stable in the desired temperature range. The critical micelle concentration (CMC) values and surface tension of [C₁₆pymC_n][Br] gradually decreases with the increasing length of the side chain from 1 to 4. The temperature dependence of CMC shows typical U-shaped. The surface chemical parameters of all the synthesized surfactants showed moderate surface activity. The calculated thermodynamic parameters showed that the micellization processes of [C₁₆pymC_n][Br] are spontaneous, endothermic at low temperatures and exothermic at high temperatures. For the micellization process of [C₁₆pymC_n][Br] (n = 1–3) were entropy-driven in the investigated temperature range. The micellization process of [C₁₆pymC₄][Br] changes from entropy-driven to enthalpy-driven at high temperature. The standard heat capacity were further discussed. It can be concluded that propyl- and butyl-chain of meta-position can be folded and thus at least partially removed from contact with water.