An ultrasonic study of the thermotropic transition of dipalmitoyl phosphatidylcholine

The acoustic absorption of sonicated and unsonicated suspensions of dipalmitoyl phosphatidylcholine has been studied as a function of temperature at frequencies between 0.6 and 4.0 MHz. The absorption at a fixed frequency passes through a maximum at a temperature which is dependent on the sample preparation and history and somewhat dependent on the frequency of observation, with sonicated vesicles exhibiting a temperature of maximum absorption 2--3 degrees C below that found for unsonicated dispersions. The temperature dependence of the absorption at fixed frequency suggests a cooperative process while the frequency dependence of the absorption at fixed temperature indicates that more than a single relaxation time is required to describe the acoustic absorption.