SORPTION IN ENERGETICALLY HETEROGENEOUS MODEL SILICA SYSTEMS

Grand canonical ensemble Monte Carlo (GCEMC) simulation results for a Lennard-Jones (12-6) vapor adsorbing in model microporous silica structures are presented and discussed. The silica media are modeled as randomly distributed interconnected nonoverlapping silica microspheres with the latter treated in two different ways: (i) as structureless particles (smeared admolecule/silica microsphere interactions) and (ii) as fully structured entities (the sorbed particles interact with the individual atoms of the SiO2 matrix). Sorption isotherms are determined for fractional monolayer coverages in the range 10(-5) < theta < 0.5 at three different temperatures and porosities and the influence of solid surface structure is investigated using these data in conjunction with the profiles for the admolecule number density as a function of energy within the micropore volume. These density profiles correspond to the kernel of the Fredholm equation for energetically heterogeneous systems and, coupled with complementary studies of the first few local virial coefficients of the pore fluid, the simulation results reported here suggest that a number of assumptions currently employed in the characterization of the energetic heterogeneity of microporous materials may require review.