Second-order phase changes in phospholipid monolayers at the oil/water interface and related phenomena

Abstract

Surface pressure against area isotherms for phospholipid monolayers at the oil/water interface display second-order phase transitions which resemble the ones between “condensed” and “expanded” monolayers at air/water interfaces. Behaviour at lower surface densities indicates a strong repulsive interaction between the phospholipid molecules at the oil/water interface. A statistical mechanical model is developed in which the monolayer molecules occupy sites on a two-dimensional lattice and adopt one of two possible orientational states. By appropriate choice of the interaction energies of the molecules in these states and the use of order–disorder statistics the features characteristic of a second-order phase transition are obtained for isotherms at both interfaces. From the point of view of our present theory what is termed the “expanded” state is in fact a fluid phase whose degree of order is lower than that of the “condensed” state. The choice of a large overall attractive interaction energy between nearest-neighbours yields the liquid–vapour phase transition which has been observed in experiments at the air/water interface whereas with an overall repulsive interaction the liquid–vapour transition is absent, as in the oil/water case.