An alternative view of phospholipid phase behavior at the air-water interface. Microscope and film balance studies

Pure-lipid films at the water interface have surface-pressures vs. area isotherms that are often interpreted as involving first-order phase transitions from a condensed region to a liquid-expanded region. Two phases are presumed to coexist in the intermediate part of the isotherm. We constructed a film balance that could be placed on the stage of an epifluorescence microscope. A dipalmitoyl phosphatidylcholine film containing a low concentration of a fluorescent lipid probe showed an inhomogeneous fluorescence distribution in the so-called liquid-expanded region of the isotherm. Only the intermediate and condensed regions could be prepared so as to be optically homogeneous below 25 degrees C. We investigated membrane flow and lateral lipid diffusion in the membrane on the trough. The isotherms and isochores were measured. The results require, at least, a modified description of the monolayer structure in various regions of the isotherms. The solid-condensed region corresponds to a gel phase of the lipids where there is no flow in the membrane, lateral diffusion is low, the compressibility is low, and the membrane is optically homogeneous. The "liquid-condensed/liquid-expanded" region appears to be a homogeneous membrane where lateral diffusion and membrane flow are both rapid. This is a region of high compressibility. The "liquid-expanded" region is not homogeneous as seen under the microscope, and the flow of the surface layer can be very fast.