Unified Network Model for Adsorption–Desorption in Systems with Hysteresis

The problem of equilibrium and kinetics for adsorption] desorption of condensable ®apors in porous media is studied experimentally and theoretically. For adsorption, the ( network model for diffusion based on pore blocking theory with percolation in the ) network added by effecti®e medium approximation is further impro®ed. A new predicti®e model based on properties of the Bethe lattices is proposed to account for the existence of liquid-filled ‘‘blind’’pores that result in a decrease in the total diffusion rate. ( ) For desorption, a new ‘‘shell and core’’ or shrinking core representation of the network model is proposed. Information from adsorption] desorption equilibria is needed to compute the thickness of the shell in which desorptionre®aporation occurs for concentrations higher than the percolation threshold. These models form a unified equilibrium-kinetics theory for gas-porous solid systems that exhibit hysteresis. The models are applied to the systems silica gel-water ®apor and Vycor glass-nitrogen. Concentration-dependent Fickian diffusi®ities for these systems ha®e been measured for both adsorption and desorption branches. The adsorption model successfully predicts the experimental data with a maximum in diffusi®ity. The desorption model correctly predicts the concentration dependence of diffusi®ity with a steep minimum at the percolation threshold.