Molecular thermodynamics of physical adsorption in heterogeneous solids

This dissertation is about the molecular modeling of fluids adsorbed in disordered porous materials. The goal is to obtain the relationship between the adsorption thermodynamics and adsorbent microstructure. This was accomplished by using molecular models that exhibit realistic three dimensional descriptions of the structural and energetic heterogeneities of the adsorbent. In these models the adsorbent is treated as a matrix of particles with a predetermined spatial arrangement. An important focus of the present work is on how the adsorbent microstructure affects the isosteric heat of adsorption. In these molecular models, microstructural variations can be made in several ways. These include: (i) changing the translational order of the matrix; (ii) the matrix particle connectivity; (iii) the porosity; (iv) the surface roughness. Our studies for a model of methane in silica xerogel show that the translational order of the matrix particles has the most significant effect. Surface roughness is also important and we have investigated several ways of incorporating this into the models. The effects of adsorbate molecular shape was investigated using two molecular models of ethane: (i) a single site model; (ii) a two site model. The effect of the adsorbate molecular shape on the adsorption isotherms is more pronounced at low temperature compared to that at high temperature. The nonspherical model tends to have a higher heat of adsorption. The differences in the adsorbate microstructures are similar to those found in the bulk fluids (e.g. a \u27shoulder\u27 in the site-site distribution functions). We have investigated the adsorption of methane-ethane mixtures in silica gel. Adsorption isotherms, heats of adsorption, and selectivities from the molecular model were calculated. These were compared with the ideal adsorbed solution (IAS) theory. It was found that the IAS theory gives very good predictions from low to moderate bulk pressures. Our work includes the first theoretical calculation of the component isosteric heats of adsorption for a mixture in a heterogeneous solid