Liquid-liquid immiscibility in lipid monolayers.

Some binary lipid mixtures form coexisting liquid phases when spread at the air/water interface. This work describes the pressure-composition phase diagrams of binary mixtures of four unsaturated phosphatidylcholines with dihydrocholesterol. These four binary mixtures have critical compositions of approximately fifty mole percent, and average critical exponents of 0.25 +/- 0.07. The data can also be approximated by a regular solution thermodynamic model, yielding parameters for the non-ideality of these mixtures.