AN INTERPRETATION OF THE MULTIPLE FLUID FLUID TRANSITIONS IN LIQUID SUPPORTED AMPHIPHILE MONOLAYERS

We describe an extension of the generalized regular solution model of a liquid supported monolayer of long chain amphiphile molecules in which chain-chain interactions outside the surface layer are included as well as the effects of chain flexibility in the surface and in the space above the surface layer. Self-consistent equations for the distribution of molecular configurations and the surface density and pressure are derived by using a model free energy expression. Two first order fluid-fluid phase transitions are predicted for three different models for the chain-chain interactions outside the surface. These transitions are identified as the observed gas-to-"liquid-expanded" and liquid-expanded-to-"liquid- condensed" phase transitions in a monolayer, the first to be associated with the condensation of the in-surface portions of the amphiphile chains and the second to be associated with the condensation of the out of surface portions of the amphiphile chains. This interpretation comes from the observation of the different distributions of the molecular configurations in the coexistence regions for the two transitions and is reinforced by the analysis of the sensitivity of the equation of state to the parameters used in the model. The theory leads to a simple and qualitative but accurate physical picture of the successive fluid-fluid transitions of the monolayer. © 1990 American Institute of Physics.