ADSORPTION OF GAS-MIXTURES IN MICROPORES - EFFECT OF DIFFERENCE IN SIZE OF ADSORBATE MOLECULES

Adsorbed solutions of molecules of unequal size (CCl2F2 and CO2 on act ivated carbon) show negative deviations from ideal mixing. The size ef fect was studied by molecular simulation of a Lennard-Jones fluid mixt ure adsorbed in zeolite 13X. The molecules forming the binary mixture are non-polar and have a 2.4 ratio of saturation capacities (the same as CC2F2 and CO2) In the molecular simulation, adsorption occurs on th e energetically homogeneous surface of a spherical shell. Binary adsor ption is centered on separate shells with the larger molecule in the i nner shell. At high coverage, the simulations yield moderate negative deviations from ideal mixing similar to experimental data. Both molecu les can enter the cavity, but adsorbed-phase nonidealities are caused by the exclusion of the larger molecule from part of the micropore spa ce accessible to the smaller molecule. The partial exclusion effect is calculated from the cavity radius and molecular diameters of the adso rbate molecules.