REORGANIZATION OF ALKYL CHAINS IN VESICLES FORMED IN AQUEOUS-SOLUTIONBY DIALKYLDIMETHYLAMMONIUM BROMIDE, R(2)N(-) WHERE R=C12H25, C14H29, C16H33 OR C18H37()ME(2)BR()

Differential scanning calorimetry (DSC) scans are reported for aqueous solutions containing vesicles formed by four dialkyl-dimethylammonium bromides; R(2)N(+)Me(2)Br(-) where R=dodecyl (DDAB), tetradecyl (DTAB ), hexadecyl (DHAB) or octadecyl (DOAB). Electron micrographs of DTAB (ag; 2 x 10(-3) mol dm(-3)) confirmed that these solutions, prepared a t 50 degrees C, contain vesicles, with radii ca. 700 nm. DSC scans of these solutions, initially cooled to 5 degrees C and then scanned with increasing temperature, showed no evidence of an extremum in isobaric heat capacity as a function of temperature, associated with a chain-p acking transition in the vesicular bilayer at a characteristic tempera ture, T-m. However, clear evidence for such a transition near 29 degre es C was obtained after this solution had been held at 5 degrees C for periods up to 11 h. With increase in the time during which the DTAB s olution was held at 5 degrees C, the recorded signal associated with t he melting temperature increased in intensity. However, there was agai n no evidence for this transition if the solution was cooled to 5 degr ees C and the heat capacity dependence on temperature immediately re-s canned. The patterns are discussed in terms of a kinetic control, duri ng cooling, of the packing of dialkyl chains in the bilayers. A simila r pattern was observed for DDAB (aq) where T-m=15.8 degrees C. In the case of DHAB (aq) where T-m=28.1 degrees C, no kinetic features were a pparent using DSC to study the gel to chain-packing transition.