Reversibility of Thermal Transitions in Dilute Dioctadecyl-Dimethyl-Ammonium Bromide Vesicles

Dioctadecyl-dimethyl-ammonium bromide (DODAB) vesicles can be characterized by their differential scanning calorimetry (DSC) thermograms comprised of two endotherms at T (s) a parts per thousand 36 A degrees C and T (m) a parts per thousand 45 A degrees C in the heating, ascribed respectively to the subgel-to-gel and gel-to-liquid crystalline transitions, and two exotherms at T'(m) a parts per thousand 40 A degrees C and T'(s) a parts per thousand 16 A degrees C in the cooling, ascribed respectively to the liquid crystalline-to-gel and gel-to-subgel transitions. It has been reported but not proved that the T (m)-transitions, the T'(m)-transitions, the T (s)-transitions, and the T'(s)-transitions are reverse to each other, displaying hystheresis Delta T (m) a parts per thousand 5 A degrees C and Delta T (s) a parts per thousand 20-25 A degrees C, respectively. By investigating the effects of the initial scanning temperature (T (i)) on the transition enthalpies (Delta H (m), Delta H (s), Delta H'(m) and Delta H'(s)), we have seen that these transitions are the reverse to each other and display different kinetics.