Monte-Carlo Simulations of Adsorption Equilibria at States Near Bulk Fluid Phase Boundaries

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We discuss the use of Monte Carlo simulations to study adsorption on solid surfaces from fluids and fluid mixtures at states near to coexistence boundaries of the bulk fluid. The work is focused primarily upon the relationship between the adsorption isotherms and the nature of the contact between the coexisting fluid phases and the solid surface, and the behavior associated with the occurrence of wetting transitions in the system. Previously we have used Monte Carlo simulations in an ensemble with constant normal pressure to study prewetting transitions in adsorption from a single component undersaturated vapor in the approach to saturation. In this paper we present comparisons of our simulations results with predictions from density functional theory. We also discuss the use of simulations to determine more precisely the conditions for phase equilibrium at the prewetting transition through determination of the surface tension at the fluid-solid interface. p] We present results which extend these studies to the case of adsorption from a liquid mixture with a miscibility gap. In this work we have considered a symmetric Lennard-Jones mixture (σ11 = σ22 = σ12; ε11 = ε22 = ε,ε12 = 0.75ε). We describe Monte Carlo simulations of this mixture in contact with a wall which preferentially adsorbs one of the components. At several temperatures below the consolute temperature we studied adsorption at the solid surface for various compositions in the approach to coexistence from the phase rich in the weakly adsorbed component. Within the limited precision of the results obtained thus far, the system shows behavior suggesting complete wetting at higher temperatures and partial wetting at lower temperatures. Density functional theory predicts a first order wetting transition with a prewetting line for this system.







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