(238c) Adsorption on Molecularly Rough Surfaces

Surface heterogeneity affects significantly the efficiency and selectivity of porous adsorbents and catalysts. However, most of the adsorption theories and simulation techniques assume a standard thermodynamic model of the Gibbs' dividing solid-gas interface, which is molecularly smooth. This assumption gives rise to an artificial layering of the adsorbed phase near the surface that is displayed in oscillating density profiles and leads to step-wise adsorption isotherms and oscillating solvation pressures, which are not observed in experiments. I will present a novel approach to a quantitative description of the effects of surface hererogeneity and microporosity on adsorption in micro- and mesoporous materials based on the quenched solid density functional theory (QSDFT). In QSDFT, the surface roughness is quantified by the roughness parameter, which represents the thickness of the surface ?corona? ? the region of varying solid density. QSDFT quantitatively describes adsorption/desorption isotherms of simple gases and vapors on various carbons and silica materials in a wide range of relative pressures. The adsorbate density profiles predicted by QSDFT are shown to be in quantitative agreement with the density profiles revealed from the in-situ synchrotron XRD studies of Ar adsorption on MCM-41. Thus, QSDFT reconciles the results of adsorption and XRD measurements. In addition to a better understanding of the fundamentals of adsorption processes on heterogeneous surfaces, an important practical outcome of the proposed approach is envisioned in modeling of selectivity of adsorption separation processes and developing novel methods for calculating pore size distributions, meso- and micropore volumes, surface roughness, and other structural parameters of micro-mesoporous materials employed in separation technologies.