Pore size distribution of porous glasses: A test of the independent pore model

Using the Kierlik and Rosinberg fundamental measure theory, we test the density functional theory method for determination of pore size distributions from adsorption data for porous glasses. The glasses chosen for study are model glasses prepared by a quench molecular dynamics method that mimics the experimental synthesis process and are completely characterized at the molecular level. The density functional method involves two approximations: (a) the glasses can be regarded as made up of a distribution of nonconnected pores of simple geometry, which we refer to as the independent pore model, and (b) the adsorption isotherms for these nonconnected pores can be described by the density functional theory. Using simulated adsorption isotherm data for the glasses and adsorption isotherms for the pores of simple geometry calculated by the density functional theory, a regularization method is used to determine the pore size distribution from the adsorption data. These calculated pore size distributions, as well as the adsorption isotherms for the materials, are compared with the exact geometric pore size distributions for the material and with the simulated isotherms. Both slit-shaped and cylindrical pores are used in the density functional theory method. It is found that a unique geometry is not able to accurately describe the whole adsorption isotherm. The use of slit-shaped pores gives overall better results, although the low-pressure regime is more accurate when cylindrical pores are used; reasons for this are discussed. The pore size distributions from the density functional theory are in reasonable agreement with the geometrical ones, giving the same shape and mean pore width and similar porosities in the four materials. Since it is known that the density functional theory gives excellent results for the adsorption isotherms (approximation b above), this comparison tests the independent pore model directly.