Influence of polar head groups on the gel to liquid-crystal transition in vesicular and lipid bilayer systems

Bilayer systems in aqueous solutions formed by the C-18-bilayer systems, dimethyldioctadecylammonium bromide (DOAB), sodium dioctadecylphosphate (DOP) and distearoyl phosphatidylcholine (PC) undergo well characterised gel to liquid-crystal transitions. Differential scanning microcalorimetric data yield melting temperatures, enthalpies of fusion and patch numbers for these systems which probe the effects of the polar head groups, since in each of these systems the chain lengths are similar. The patterns confirm unambiguously that the stability of vesicular systems reflects a balance between the stabilising effects of alkyl chain-alkyl chain cohesive interactions and the unfavourable charge-charge interactions between the polar head groups.