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.

A method is described to precisely measure the sound velocity of liquids. The construction of a resonator cell for the simultaneous measurement of a sample liquid and a reference liquid is presented. The non-ideal properties of the cavity resonators are carefully considered for a superior evaluation of the resonance frequency data. Also presented is a low-priced electronic set-up, designed for the computer-controlled determination of the complex transfer functions of the cavity resonators and also for automatic temperature monitoring and control. Possible sources of errors are discussed and some representative results are presented in order to illustrate the repeatability of the method and the accuracy of the sound velocity data relative to a reference liquid.