Spontaneous formation of large (1 to 10 microns) unilamellar vesicles in aqueous phospholipid dispersions has been reported to occur as a critical phenomenon. Some of the salient properties of the unilamellar state are as follows: (1) For each phospholipid or mixture of lipids, the unilamellar structure forms only at a characteristic singularity in temperature, the critical temperature, T*. (2) The critical temperature is generally higher than Tm, the gel-liquid crystal transition temperature. (3) At temperatures above and below T*, multilamellar structures (liposomes) form. These properties of the critical unilamellar state were obtained primarily from studies of the equilibrium lipid films that form at the air-water surface of phospholipid dispersions. To complement these thermodynamic studies, we are using calorimetry to observe the transition from multilamellar liposomes to the critical unilamellar state around T*. An extremely sensitive heat conduction calorimeter is used to make the measurements. Preliminary heat capacity data using an aqueous dispersion of a phospholipid, DMPC, indicate that a transformation occurs between 28.8 and 29.O degrees C. This result agrees with previous data derived from the thermodynamic properties of DMPC films at the air-water interface in equilibrium with the dispersion. From the preliminary calorimetric measurements, we have determined that the energetics of forming the unilamellar state at T* from liposomes is consistent with the work for the reverse process of bilayer adhesion. Calorimetry thus can provide a direct measurement of the work of bilayer adhesion, as well as the critical temperature, T*.
