ON THE UNIQUENESS OF STRATIFICATION-INDUCED STRUCTURAL TRANSFORMATIONS IN CONFINED FILMS

This article is devoted to an investigation of structural transformati ons which occur in molecularly thin films confined between planar stru ctured and unstructured rigid solid surfaces as the number of fluid st rata parallel with the walls changes under conditions of fixed chemica l potential mu and temperature I: The transformations become manifest as periodic maxima in the transverse isothermal compressibility kappa( parallel to) computed in grand canonical ensemble Monte Carlo simulati ons via fluctuations in film density. The transformation occurs at nea rly identical characteristic separations s(z) of the surfaces regardle ss of whether these are discrete (i.e., composed of individual atoms) or smooth in transverse dimensions. The transformation is caused mainl y by confinement to spaces of molecular dimension and is unique, that is independent of derails of the film-wall interaction. Because of the inhomogeneity of a film between structured surfaces in transverse dim ensions a definition of kappa(parallel to) in terms of density fluctua tions is precluded in general. However, by considering only restricted thermodynamic transformations on a length scale defined in units of t he lattice constants characterizing the wall structure, such a definit ion is possible introducing kappa(parallel to) as a density fluctuatio n-related quantity in a coarse-grained sense. Variations of the local density of the film around the transition points suggest to perceive t he phase transition as stratification-induced, because it happens afte r the new stratum has begun to form.