Ultrasonic measurements of two-component lipid bilayer suspensions

Two-component lipid bilayers of dipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine were studied by measuring ultrasonic velocity and absorption at 3 MHz. The phase diagram of the two-component lipid bilayers is discussed based upon the transition anomalies of the ultrasonic velocity as well as absorption, and it is suggested that this binary system has two critical points. The bulk modulus of lipid bilayers was determined from the ultrasonic velocity to be (2.2-3.0) x 10(10) dyne/cm2, whereas the bulk viscosity calculated from the absorption was 10-20 P except for the transition regions.