Compressibility of lipid mixtures studied by calorimetry and ultrasonic velocity measurements

Ultrasonic velocity and heat capacity temperature profiles of various lipid mixtures have been recorded with high accuracy. This included mixtures of phophatidylcholines with different chain length as well as phosphatidylcholine mixtures with diacyl glycerides. Following previous studies relating the heat capacity to the isothermal compressibility of lipids close to the chain melting transition, we found that the measured ultrasonic velocities are very similar to those calculated from the heat capacity. This implies that we are able to determine the compressibility changes from the excess heat capacity and the heat capacity changes from ultrasonic velocity measurements. The sound velocity and heat capacity traces are discussed with respect to the phase diagrams of the lipid mixtures.