Phase diagrams of alkyltrimethylammonium surfactants in some polar solvents

Abstract

Phase diagrams of alkyltrimethylammonium surfactants and different polar solvents were determined or collected from the literature. Hexadecyltrimethylammonium bromide forms with water a solution phase L, a hexagonal liquid crystalline phase E, a cubic phase I, and a lamellar phase D, with increasing surfactant concentration. In glycerol, the same aggregation sequence was found. With ethylene glycol, the existence regions of the liquid crystalline phases were found to be considerably smaller. The alkyl chain length of the surfactant also influences the extensions of the liquid crystalline phases. With a shorter alkyl chain the size of the existence regions was found to decrease, in particular with ethylene glycol as solvent. The size of the existence regions for the liquid crystalline phases with hexadecyltrimethylammonium bromide and water, glycerol, formamide, ethylene glycol, and N-methylformamide, respectively, was correlated to the solvent/hydrocarbon interfacial tension. A high interfacial tension suggests a considerable solvophobic interaction which favors extensive surfactant aggregation. The interfacial tension decreases in the order water > formamide ≈ glycerol > ethylene glycol > N-methylformamide, the same sequence as for the size of the existence region of the hexagonal phase E.