FLUID-GEL INTERPHASE LINE TENSION AND DENSITY-FLUCTUATIONS IN DIPALMITOYLPHOSPHATIDYLCHOLINE MULTILAMELLAR VESICLES - AN ULTRASONIC STUDY

Temperature dependence of sound velocity and absorption in dilute susp ensions of dipalmitoylphosphatidylcholine multilamellar vesicles have been measured at 7.2 MHz in the temperature range 20-55-degrees-C. Rel axational characteristics of heterophase fluctuations of density in th e vicinity of the main phase transition have been determined, i.e. rel axational sound velocity, absorption, and effective relaxation time. F luid-gel interphase line tension in the fluid bilayer has been evaluat ed for the first time from the acoustic data by means of a simple mode l based on Frenkel's theory of heterophase fluctuations in the vicinit y of first-order phase transitions. The theory focuses on the formatio n of clusters of a new phase inside the parent phase. The line tension was found to be 5.6 X 10(-7) erg/cm. The rate constants of cluster cr eation and growth have also been evaluated. Ultrasonic absorption spec tra calculated by means of the model are in agreement with known wide- frequency-range acoustic spectroscopy data.