Differential scanning calorimetry (DSC) has been used to study the phase behavior of aqueous solutions of a homologous series of cationic surfactants, tetradecyl-, hexadecyl-, and octadecyltrimethylammonium bromide. Three separate endothermic transitions were observed by the heating scans of DSC. They are in turn from the higher temperature the transitions (i) from the coagel phase (or the so-called hydrated solid) to the micellar solution; (ii) from the metastable coagel, which is one of the polymorphic forms of the hydrated solid, to the supercooled micelle; and (iii) from the gel phase (metastable) to the supercooled micelle. The metastable gel phase appears when the micellar solution is supercooled and is allowed to stand for a short time at 5°C. A sufficiently long storage time (1-6 days) at 5°C is necessary to complete the transformation to the stable coagel phase. After storage at -20°C the metastable coagel phase appears. The enthalpy and entropy changes were determined for individual transitions. The difference in the enthalpy changes between first and second transitions is attributable to the difference in the dissolution enthalpy from the stable or metastable coagel to the singly dispersed surfactant solution.
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