Chain-melting phase transition in dipalmitoylphosphatidylcholine foam bilayers

Microscopic horizontal foam bilayers (Newton black films) from dipalmitoylphosphatidylcholine (DPPC) have been studied by the microinterferometric method of Scheludko and Exerowa. The foam bilayers were formed from DPPC dispersions in water and in a water/ethanol (52.5:47.5, v/v) mixture in the presence of 0.15 M NaCl. The high ethanol concentration strongly facilitates their formation. The bilayer thickness and the critical bulk concentration C-c of DPPC required for the formation of a stable microscopic foam bilayer from the water/ethanol mixture were measured in the range 35-45 degrees C. Both parameters indicate cooperative changes in the state of the foam bilayers. These changes take place at the temperatures of the bulk chain-melting phase transitions, as determined by differential scanning calorimetry (DSC) for both aqueous and water/ethanol DPPC dispersions. The critical DPPC concentration C-c for the water/ethanol dispersions changes between 55 and 140 mu g ml(-1) in the range 35-45 degrees C. However, measurements by DSC show that decreasing the lipid concentration to 2.5 mu g ml(-1) in both water and water/ethanol mixtures does not affect the enthalpy, temperature and width of the bulk phase transition of DPPC. This is an indication for a mechanism of foam bilayer formation which involves adsorption of whole vesicles to the air-solution interface, followed by their subsequent spreading on the surface. A concentration-temperature phase diagram of DPPC foam bilayers that defines the regions of gaseous (ruptured), gel and liquid crystalline foam bilayers has been constructed.