Recent studies of the properties of phospholipid dispersions in water indicate that a higher-order phase transition occurs that involves a spontaneous transformation from a unilamellar liquid-crystal state to a suspension of large, unilamellar vesicles when the ambient temperature increases. It has been suggested that the unilamellar vesicles that form are a critical state. The thermodynamic properties of this transformation have been inferred primarily from the properties of air-water surface films in equilibrium with the dispersed phospholipid phase. A more direct and conceptually simpler analysis of the thermodynamic properties of this higher-order transition may be obtained by measuring the temperature dependence of the heat capacity of the lipid dispersions. Since transformations of this type are believed to be intimately involved in the assembly of cell membranes, we developed an extremely sensitive differential heat-conduction calorimeter for measuring heat capacities of aqueous-membrane lipid dispersions. Using this instrument, we have measured a small heat-capacity change during the phase transition at the critical temperature. We have begun work on two other independent measurements to confirm the nature of the transition: (1) a fluorescence system that measures the abrupt increase in surface area of the lipid during the transition from multilamellar to unilamellar, and (2) X-ray diffraction measurements of the intermolecular spacing during the transition.
