the critical temperature, and (ii) includes the effect of proteins which are adsorbed into the monolayer. Assuming that the proteins are completely incorporated into the monolayer, this simplistic model leads to an entropic force which tends to localize the protein at the boundary between LE and LC domains. Depending on the energetic preference of the protein for the LE or LC phase, the protein will be either dissolved in the LE or in the LC domain, or, if there is no pronounced preference, will be localized at the boundary.

Phase separation in amphiphilic layers is also observed for freely suspended multicomponent bilayers[@Bloom]. Here, the coexisting phases are distinguished by their compositions. The most important examples include mixtures of phospholipids with cholesterol[@Thewalt] and mixtures of different phospholipids[@Wu], and in both cases the coexisting phases are in a fluid state. These phenomena are of great biological interest since biological membranes are always multicomponent mixtures and lateral organization into domains is supposed to play an important functional role. We note that our results apply directly to these situations as well, although we will limit our terminology to the situation of coexisting dense and dilute phases for one-component systems at the air-water interface. For 