Critical mixing in monolayer mixtures of phospholipid and cholesterol

We report the observation of two-dimensional fluid-fluid immiscibility in monolayer mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol spread at an air-water interface. Coexisting fluid domains were visualized by fluorescence microscopy in monolayers that were stained with low concentrations of a fluorescent lipid probe. Monolayers with cholesterol concentrations in the range 0-30 mol % show reversible two-phase to one-phase transformations at characteristic densities and surface pressures. For cholesterol concentrations near 30 mol %, this isothermal transition is characterized by fluctuations in the shape of the domain boundaries, indicating the presence of a critical point at this composition.