Thermotropic Phase Transition and Ultrasonic Absorption in Lecithin Bilayer Vesicle Preparations

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Abstract
Temperature dependent ultrasonic absorption spectra (frequency range 0.3‐150 MHz) of homogeneous lecithin bilayer vesicle preparations in the range of their liquid/liquid‐crystalline phase transition provide information on equilibrium shifts for various states of the lipid system. Furthermore the frequency dependence of the absorption reveals time constants of elementary processes which cause the ultrasonic absorption. From our measurement data the following conclusions can be drawn: The thermotropic phase transition in our lipid system is not a one‐step reaction but incorporates various processes. Predominant changes of the ultrasonic absorption with temperature, correlated with cooperative processes at the lipid/water interface, occur well below the temperature of the “calcrimetric” main effect. Relaxation times of these processes are between 10 and 200 ns. The observed ultrasonic absorption appears to be larger than expected values from equilibrium (“static”) data (e. g. dilatometry and calorimetry); consequently in this case the ultrasonic absorption indicates processes with compensating effects, which can only be resolved by “dynamic” methods on the time scale. These kinetic observations are compared with results from calorimetry, density‐ and viscosity‐measurements on the same lipid preparations. A model mechanism for the thermotropic phase transition in lecithin bilayer vesicles is suggested.

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