Thermal characterization of the mode of phase transition in the dioctadecyldimethylammonium bromide - water system in relation to the stability of its gel phase

The binary system of water and dioctadecyldimethylammonium bromide (DODAB) having a double hydrocarbon chain was found to exhibit a complex and characteristic phase transition. That is, when the sample is cooled to a temperature that is not low enough to crystallize the coexisting water, the mode of phase transition of the present system was classified into three types according to water content: at a water content below 67 wt %, only the T_c transition of coagel to liquid crystal reversibly appears on cooling and heating; at a water content of 67-93 wt %, the additional T_c* transition of gel to liquid crystal, which also reversibly appears on thermal cycling, takes place and the fraction occupied by the T_c* transition increases with increasing water content, in competition with the T_c transition; and above 93 wt % water content, only the T_c* transition reversibly appears. On the other hand, when the sample is cooled to a temperature as low as -20°C (low enough to crystallize the coexisting water), the T_c* transition is no longer realized in the heating direction and is replaced by the T_c transition. The present mode of phase transition was investigated from the thermodynamic viewpoint, and it was revealed that the stability of the metastable, supercooled liquid crystal phase relative to the coagel phase is a factor to determine the mode of the T_c. and T_c* transitions.