Determination of absolute adsorption for argon on flat surfaces under sub- and supercritical conditions

• A new method, based on fraction of success, is developed to determine the boundary of the adsorbed phase.
• Adsorbed phase of supercritical gas on a surface is confined to 1 or 2 layers above the surface.
• Absolute adsorbed density approaches a constant at sufficiently high pressures.
• Adsorbed phase is densified at high pressure and its thickness slightly decreases, akin to the densification of the bulk fluid.

A new method, employing computer simulation, is proposed for the determination of the location of the interface separating the adsorbed phase from the adjacent gas phase, giving a means to calculate the “absolute” amount adsorbed. The method involves monitoring the fraction of successful insertions of molecules into differential volumes in the simulation box. By applying the concept of equal areas, as implemented in the determination of the Gibbs dividing surface, for the profile of the fraction of success versus distance, we are able to determine the location of the interface bounding the adsorbed phase. This allows us to find (1) the thickness of the adsorbed phase, (2) the absolute surface density (absolute loading) and (3) the volumetric density of the adsorbed phase, as functions of pressure. Knowing the absolute surface density as a function of pressure at different temperatures, we are able to calculate the heat of adsorption as a function of loading, using the Clausius–Clapeyron equation and to show that this is consistent with the heat obtained from the fluctuation formula in grand canonical Monte Carlo simulations.

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