Correlation functions of adsorbed fluids in porous glass: a computer simulation study

We have prepared a series of model porous glasses using a recently developed molecular dynamics procedure (GELB, L. D. and GUBBINS, K. E., 1998, Langmuir, 14, 2097) that mimics the experimental processes that produce Vycor and controlled pore glasses. Using Grand Canonical Monte Carlo simulations we have measured nitrogen adsorption isotherms in these models and determined the radial distribution functions and structure factors for the adsorbate at several different pressures. These correlation functions show long-wavelength fluctuations which at low density are controlled by the glasses surface-surface correlation functions and at high density are controlled by the glasses void-void correlation functions. The structure factor data at low density can be used to extract a characteristic pore diameter which agrees well with that obtained by a geometrical analysis of the pore structure. Significant qualitative differences in S(k) are found between 30% porous and 50% porous materials.