Associating fluids with four bonding sites against solid surfaces: Monte Carlo simulations

Abstract

Abstract We present a Metropolis Monte Carlo study of an associating fluid near a solid surface. Each fluid molecule is modelled as a spherical hard core with four highly anisotropic square-well sites placed in tetrahedral symmetry on the hard core surface. The molecules are placed between two parallel walls, a large enough distance apart to enable a bulk fluid region to appear. We consider flat walls with and without association sites, and molecules with association energies of several strengths. We report the density of molecules and the fraction of unbonded molecules as a function of distance from the wall over a range of molecular densities and temperatures. The profiles at the hard wall show two competing effects. Monomers, which suffer no drop in configurational entropy as they approach the wall, are affected only by the collision forces and tend to pile up at the wall. At lower temperatures the molecular attractions cause the molecules to cluster amongst themselves rather than accumulate against the wall. As expected, the walls with active sites have greater wall densities and larger adsorbed clusters than the hard walls. Finally, we present a theory for determining the fraction of the monomers as a function of distance from a hard wall, and compare this theory with the results of the simulations.