Specific heats of lipid dispersions in single phase regions.

Differential scanning calorimetry has been used for the first time to measure the specific heat, Cp, as a function of temperature in the single phase regions above and below the main phase transition temperature, Tm, for dispersions of saturated phosphatidylcholines and phosphatidylethanolamines. Within error limits Cp, when expressed per gram, does not vary in any systematic way with chain length or headgroup. Its temperature dependence in both single phase regions qualitatively resembles that of n-alkanes. Contributions to Cp from intrachain vibrations and interchain van der Waals' interactions have been calculated and account for nearly all the measured Cp at temperatures above Tm. However, these contributions do not yield the observed temperature dependence below Tm. It is conjectured that such a temperature dependence arises from the unhindering of chain vibrations as the lipids undergo thermal expansion, and the result of a preliminary calculation which supports this conjecture is presented.