Phase transitions in phospholipid monolayers at air–water interfaces

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Abstract
In this study we present a unified theoretical description of neutral, saturated, phospholipid monolayers at air–water interfaces. This model encompasses the liquid-condensed (LC) to liquid-expanded (LE) and liquid-expanded to surface gas (SG) phase transitions observed in such systems. The model is a lattice model of lipid hydrocarbon chains which allows for the introduction of free volume. The lipid chains can be upright in a ground or excited state or collapsed relative to the substrate. Furthermore, the chains interact via short range potentials due to steric, van der Waals, and dipolar forces. We show that the LC/LE phase transition is to be understood as a chain melting transition and through the growth of lipid domains across the transition. We further show that the LE/SG transition involves the creation of large amounts of free volume into which the lipid chains can collapse.