Fluids in micropores .5. Effects of thermal motion in the walls of a slit-micropore

Previous articles in this series have concerned the prototypal slit-pore with rigin walls, in which a Lennard-Jones (12,6) monatomic film is constrained between two plane-parallel walls comprising like atoms fixed in the face-centered-cubic (fcc) (100) configuration. The behavior of molecularly thin films in the rigid-wall prototype is governed by the template effect, whereby solid films can form epitaxially when the walls are properly aligned in the lateral directions. In this article the influence of thermal motion of the wall atoms on the template effect is investigated. The walls are treated as Einstein solids, the atoms moving independently in harmonic potentials centered on rigidly fixed equilibrium positions in the fcc (100) configuration. The force constant f(c) is a measure of the stiffness of the walls, the rigid-wall limit being f(c) = infinity. Formal thermodynamic and statistical mechanical analyses of the system are carried out. The results of grand canonical ensemble Monte Carlo simulations indicate that for values of f(c) characteristic of a soft (e.g., noble-gas) crystal dynamic coupling between wall and film has a substantial influence on such equilibrium properties as normal stress (load) and interfacial tensions. In general, the softer the walls (i.e., the smaller the value of f(c)), the weaker the template effect and hence the softer and more disordered the confined film. (C) 1996 American Institute of Physics.