CALORIMETRIC INVESTIGATIONS OF THE STABILITY OF OCTADECYLTRIMETHYLAMMONIUM HALIDE WATER-SYSTEMS

The thermodynamic stability of the gel phase of the binary systems of water and homologous series of octadecyltrimethylammonium halides of different counterions, i.e., chloride (OTAC), bromide (OTAB), and iodide (OTAI), was systematically investigated. The gel phases in the present three systems were revealed to exist in the thermodynamic stable or metastable states; the stability of the gel phases relative to the coagel phase increased in the order OTAI < OTAB < OTAC, that is, in the order of an increase in the hydration interaction of these counterions. In accordance with this difference the mode of phase transition also differed among the three systems. On the assumption that the OTAI- and OTAB-water systems exhibit a provisional phase transition of the coagel to gel, the present result was discussed from the enthalpy viewpoint on the basis of pseudolattice (|ΔHl|) and hydration (|ΔHh|) enthalpies operating in the polar head groups of the surfactant molecules. The assumed enthalpy change associated with this transition was shown to increase in the order OTAC < OTAB < OTAI, indicating that the hydration enthalpy of the halide counterions is a predominant factor in determining the thermodynamic stability of the gel phases in the three systems, which is related to the mode of phase transition. © 1990 American Chemical Society.