Thermal study on the interaction of crystalline surfactant with water: Octadecyltrimethylammonium chloride-water system

Abstract

Thermoanalytical studies on the binary system octadecyltrimethylammonium chloride (OTAC)-water were carded out; the system was found to exhibit transition curves with a stepwise decreasing T c , particularly in the low-water-content region. The gel phase, which appears when the sample is cooled to a temperature below the T c transition, was allowed to exist in a metastable supercooled state down to −100°C. The gel phase was converted to the stable coagel phase by a suitable annealing treatment accompanying the two-step process at different temperatures. The coagel phase thus obtained was transformed into the gel phase at the elevated transition temperature ( T gel), indicating that the gel phase exists in the stable state in a specified temperature region between the T gel and T c transitions. Furthermore, thermal analysis of the aggregation state of the water molecules indicated that the gel phase interposed the intermediate water phase, corresponding to the so-called “loosely bound water,” between the bilayers of the OTAC molecules. This water was released outside the bilayers by the annealing treatment, existing as bulk free water and coexisting with the resulting new coagel phase. Conversely, at the elevated T gel transition, this water was again intercalated between the bilayers, existing as interlamellar water. Thus, the T gel transition was shown to accompany changes in the aggregation state of the water as well as in that of the OTAC molecules.