Micellization, surface activities and thermodynamics study of pyridinium-based ionic liquid surfactants in aqueous solution

Abstract

The micellization and surface activity properties of long-chain pyridinium ionic liquids n-alkyl-3-methylpyridinium bromide ([C_nmpy][Br], n: the carbon numbers of hydrophobic tails, n = 12, 14, 16) in aqueous solution were systematically investigated through electronic conductivity measurement, surface tension, and ultraviolet-absorption spectra. The surface chemical parameters and thermodynamics parameters were obtained. The [C_nmpy][Br] ionic liquids exhibit higher surface activities than conventional surfactants with corresponding alkyl chain lengths. The effects of inorganic salts (LiBr, NaBr, MgBr₂), organic alcohols (C₂H₅OH, C₃H₇OH, C₄H₉OH, C₅H₁₁OH) and temperature on the critical micelle concentration (CMC) values of [C_nmpy][Br] aqueous solutions were also investigated. The CMC values remarkably decreased with the addition of inorganic salts. The CMC values increased slightly in the presence of ethanol, but decreased gradually as the chain length of the alcohol increased. The CMC values assumed a trend of decreasing and then increasing with the increase of temperature. The calculation results of thermodynamic parameters show that both adsorption and micellization processes of [C_nmpy][Br] are spontaneous; the enthalpy of [C₁₂mpy][Br] is negative at 293.15 K and becomes negative with temperature increasing. For [C₁₄mpy][Br] and [C₁₆mpy][Br] this transition occurs at 288.15 K and the micellization process is entropy-driven in the investigated temperature range.