THERMODYNAMICS OF A FLUID CONFINED TO A SLIT PORE WITH STRUCTURED WALLS

In this article we extend our previous thermodynamic analysis of films confined to slit pores with smooth walls (i.e., plane-parallel solid surfaces without molecular structure) to the situation in which the wa lls themselves possess structure. Structured-wall models are frequentl y employed to interpret experiments performed with the surface forces apparatus (SFA), in which thin films (1-10 molecular diameters thick) are subjected to shear stress by moving the walls laterally over one a nother at constant temperature, chemical potential, and normal stress or load. The periodic structure of the walls is reflected in a periodi c variation of the shear stress with the lateral alignment (i.e., shea r strain) of the walls. We demonstrate by means of a solvable two-dime nsional model that the molecular length scale imposed by the structure of the walls precludes the derivation of a simple mechanical expressi on for the grand potential analogous to that which herds in the smooth -wall case. This conclusion is borne out by the results of a grand-can onical Monte Carlo simulation of the three-dimensional prototypal mode l consisting of a Lennard-Jones (12,6) fluid confined between fcc (100 ) walls. Criteria for the thermodynamic stability of thin films confin ed by structured walls are derived and applied to the SFA.