Density measurements through the gel and lamellar phase transitions of di-tetradecanoyl- and di-hexadecanoyl-phosphatidylcholines: Observation of slow relaxation processes and mechanisms of phase transitions

Density measurements on aqueous dispersions of C14:0/C14:0 and C16:0/C16:0 phosphatidylcholines have been carried out to give information on the mechanism of the phase transitions [main transition: lamellar phase (Lα) to gel (Pβ′); pre-transition: gel (P β′) to second gel phase (Lβ′)]. Quite remarkably, for both systems we observe that repeated temperature cycles give different measurements, with a slow relaxation to apparent equilibrium, at least for some of the phases. We also observe that there are marked reductions in lipid density just above the main transition that indicate the spontaneous formation of 'lipid patches'. The density measurements allow the calculation of the relative contributions to the phase transitions from changes in van der Waals energy (ΔUvdw) and chain configurations (ΔU Rot). Both contribute to the main transition whilst only changes in the van der Waals energy are involved in the pre-transition. © 2005 Taylor & Francis.