Adsorption Hysteresis in Porous Media

244 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1993.The phenomenon of adsorption hysteresis in porous media is studied from a microscopic point of view via computer simulations based on statistical mechanics principles.Grand canonical Monte Carlo (GCMC) simulations of adsorption in single open slit-like and cylindrical pores predict that hysteresis exists in single pores as well as in a porous medium. The presence of hysteresis in single open pores is due to the existence of metastable states along the adsorption process. Desorption in single open pores occurs via a receding meniscus and the pore emptying takes place at an undersaturation value virtually at which true coexistence between the gas-like and liquid-like phase occurs.Density Functional Theory calculations of adsorption in simple pore networks, i.e., a collection of interconnected pores, demonstrate pore blocking effects during desorption. For interconnected pores the appearance of the hysteresis loop depends on the geometry of the individual pores, i.e., shape and size, as well as the topology of the pore network, i.e., the connectivity.A novel simulation method is developed to study diffusion limitation associated with inhomogeneous fluid behavior. Local control of the chemical potential in a small control volume is implemented in GCMC and Molecular Dynamics (GCMD) simulations. Local GCMC and GCMD results are found to be in excellent agreement with global GCMC results and analytical solutions of several models.Local GCMD is applied to the study of adsorption in a single open slit-like pore. It indicates that diffusion is the rate determining step during an adsorption process. Diffusion into the pores is found to involve adsorption onto the outer surface as an intermediate step.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD