A theoretical model for lipid monolayer phase transitions.

We present a theoretical model for the liquid-expanded to liquid-condensed phase transition observed in many phospholipid monolayer films. The total two-dimensional pressure in the model is the sum of the hydrocarbon chain pressure and the surface pressure. The hydrocarbon chain pressure is calculated in an exteded version of a model published earlier. The surface pressure results from a lowering of the surface tension in the monolayer over that of pure water, thus producing a force on a Langmuir float. When these two contributions are added, pi/A isotherms are obtained which have slope discontinuities very similar to those observed experimentally. The results indicate that a successful model for lipid phase behavior must consider the interactions between head groups and water as well as cooperative hydrocarbon chain melting.