Reorganisation of alkyl chains in vesicles formed in aqueous solution by dialkyldimethylammonium bromide, R2N+Me2Br- where R = C12H25, C14H29, C16H33 or C18H37

Differential scanning calorimetry (DSC) scans are reported for aqueous solutions containing vesicles formed by four dialkyldimethylammonium bromides; R2N+Me2Br- where R = dodecyl (DDAB), tetradecyl (DTAB), hexadecyl (DHAB) or octadecyl (DOAB). Electron micrographs of DTAB (aq; 2x10-3 mol dm-3) confirmed that these solutions, prepared at 50°C, contain vesicles, with radii ca. 700 nm. DSC scans of these solutions, initially cooled to 5°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-packing transition in the vesicular bilayer at a characteristic temperature, Tm. However, clear evidence for such a transition near 29°C was obtained after this solution had been held at 5°C for periods up to 11 h. With increase in the time during which the DTAB solution was held at 5°C, the recorded signal associated with the melting temperature increased in intensity. However, there was again no evidence for this transition if the solution was cooled to 5°C and the heat capacity dependence on temperature immediately re-scanned. The patterns are discussed in terms of a kinetic control, during cooling, of the packing of dialkyl chains in the bilayers. A similar pattern was observed for DDAB (aq) where Tm = 15.8°C. In the case of DHAB (aq) where Tm = 28.1°C, no kinetic features were apparent using DSC to study the gel to chain-packing transition.