Thermal expansion and compressibility coefficients of phospholipid vesicles. Experimental determination and theoretical modeling

The isobaric thermal expansion (α = (1/V)(∂V/∂T)P) and the isothermal compressibility (β = (-1/V)(∂V/∂P)T) coefficients of different phospholipid vesicles were investigated at various temperatures, as a function of the chemical structure of the lipid polar head groups and ionic concentration. Several head groups having different charge distribution and size were considered. Measurements were performed in both the gel (Lβ′) and liquid-crystalline (Lα) phases of the lipid matrix; the transition temperature between the two phases was revealed through the sharp discontinuity of the thermal expansion and compressibility coefficients. The different structures of the lipid-water interfaces cause large variations in the α and βvalues, the larger figures referring to the less ordered Lα phase. Moreover, in some lipid systems (e.g., phosphatidylserine) and in some mixtures of them, an anomalous temperature dependence of α and βwas detected. The addition of calcium ions to bilayers containing negatively charged lipids shows dramatic effects, shifting the Lβ′ → Lα transition temperature toward higher values, lowering the volume variation accompanying such a transition, and reducing the thermal expansion and compressibility coefficients. On the basis of a thermodynamic model of the lipid-water interface the experimental results were rationalized and some possible explanations of the anomalous temperature dependence of the expansion and compressibility coefficients were suggested. © 1990 American Chemical Society.