STRATIFICATION-INDUCED ORDER-DISORDER PHASE-TRANSITIONS IN MOLECULARLY THIN CONFINED FILMS

By means of grand canonical ensemble Monte Carlo simulations of a mona tomic film confined between unstructured (i.e., molecularly smooth) ri gidly fixed solid surfaces (i.e., walls), we investigate the mechanism of molecular stratification, i.e., the tendency of atoms to arrange t hemselves in layers parallel with the walls. Stratification is accompa nied by a heretofore unnoticed order-disorder phase transition manifes ted as a maximum in density fluctuations at the transition point. The transition involves phases with different transverse packing character istics, although the number of layers accommodated between the walls r emains unchanged during the transition, which occurs periodically as t he film thickens. However, with increasing thickness, an increasingly smaller proportion of the film is structurally affected by the transit ion. Thus, the associated maximum in density fluctuations diminishes r apidly with film thickness.