THE INFLUENCE OF THE STRUCTURE OF THE SURFACE OF A LIQUID ON THE PROPERTIES OF A SUPPORTED MONOLAYER

This paper describes a simple lattice model of a liquid supported monolayer of long chain amphiphile molecules. The new feature which this model possesses is incorporation of the structure of the surface of the support liquid. The liquid-monolayer interface is represented as a multilayer region, and its structure is described in terms of the associated inhomogeneous density distributions of the support liquid molecules and of the segments of the chain molecules. The properties of this diffuse surface-amphiphile monolayer model are compared with the properties of the smooth surface-amphiphile monolayer model described in our previous work [J. Chem. Phys. 92, 1427 (1990)]. The general behavior of the monolayer equation of state, including the prediction of two successive fluid-fluid phase transitions, is preserved in the present model. We find that the existence of the diffuse surface structure of the supporting liquid leads to an increase in the number of the amphiphile chain segments that lie in the surface region, particularly at low surface densities. We also find that the multilayer character of the diffuse surface region can alter the equation of state of the monolayer at high surface densities. The two layer diffuse surface-amphiphile monolayer model supports a change in the slope of the surface pressure versus area isotherm in the very high surface density region, which can be ascribed to a conformational "freezing" of the chain molecules. The theory illustrates the qualitative features of the system which define the close interplay between the structure and properties of the monolayer and the structure of the surface of the support liquid. © 1990 American Institute of Physics.