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 struc tures are presented and discussed. The silica media are modeled as ran domly distributed interconnected nonoverlapping silica microspheres wi th the latter treated in two different ways: (i) as structureless part icles (smeared admolecule/silica microsphere interactions) and (ii) as fully structured entities (the sorbed particles interact with the ind ividual atoms of the SiO2 matrix). Sorption isotherms are determined f or fractional monolayer coverages in the range 10(-5) < theta < 0.5 at three different temperatures and porosities and the influence of soli d surface structure is investigated using these data in conjunction wi th the profiles for the admolecule number density as a function of ene rgy within the micropore volume. These density profiles correspond to the kernel of the Fredholm equation for energetically heterogeneous sy stems and, coupled with complementary studies of the first few local v irial coefficients of the pore fluid, the simulation results reported here suggest that a number of assumptions currently employed in the ch aracterization of the energetic heterogeneity of microporous materials may require review.