A new theory of the liquid condensed–liquid expanded phase transition in lipid monolayers

Abstract

A new model of the liquid condensed (LC) – liquid expanded (LE) phase transition in lipid monolayers is presented which accounts for the observed nonzero slopes of the pressure area isotherms across the transition. This model proposes that small cooperative units or "domains" of lipids with extended hydrocarbon chains in the LC phase gradually melt into large regions of lipids with disordered chains as the lateral pressure is decreased. Thus, the domain melting is coupled to the lipid chain order–disorder transition and hence, to the lipid chain order parameter [Formula: see text] where N signifies a domain of N molecules. In order to obtain [Formula: see text], an established Ising-like model of lipid monolayers is solved in the Bethe–Peierls approximation. By fitting two free parameters to one isotherm, we are able to calculate isotherms for other temperatures and find good agreement with experiment.