THE IMPACT OF DISCRETE WALL STRUCTURE ON STRATIFICATION-INDUCED STRUCTURAL PHASE-TRANSITIONS IN CONFINED FILMS

Molecular mechanisms by which a fluid closely confined between plane-p arallel solid walls tends to order itself in layers parallel with the walls (i.e., stratifies) are investigated by a grand canonical ensembl e Monte Carlo method. The walls are composed of individual atoms distr ibuted across each wall according to the (100) plane of a face-centere d cubic (fee) crystal. Wall atoms are either rigidly fixed (model A) o r thermally coupled (model B) to the film, that is the walls are ''sof t'' on account of intermolecular interactions. As for a film between u nstructured (i.e., molecularly smooth) walls [Schoen et nl. J. Chem. P hys, 101, 6865 (1994)] stratification is accompanied by a subtle phase transition manifested as a maximum in density fluctuations at the tra nsition point where packing characteristics of film molecules change i n transverse directions. Thus, the transition involves phases with dif ferent degrees of transverse fee (100)-like order induced by the walls which act like templates. If the transition involves films comprising only one and two layers, the mechanism of stratification is qualitati vely similar to the one previously reported (see above): Well off the transition point the degree of fee (100)-like order is nearly identica l for one- and two-layer films and higher than at the transition point where the film is least ordered. The mechanism of stratification-indu ced phase transitions is different if it involves thicker films which tend to gain fcc (100)-like order more or less abruptly at the transit ion point. If wall atoms are not thermally coupled the film may solidi fy under geometrically favorable conditions. Solidification is not obs erved in model B under identical thermodynamic conditions. (C) 1996 Am erican Institute of Physics.