BINARY-MIXTURES OF SIMPLE FLUIDS IN STRUCTURED SLIT MICROPORES

The grand canonical Monte Carlo method is used to study a binary mixtu re of Lennard-Jones atoms confined to an atomically structured slit mi cropore which is in thermodynamic equilibrium with its bulk phase coun terpart. In one example, the mixture consists of atoms of two distinct sizes, but with the same minimum depth in potential energy. In anothe r example a binary mixture of different size atoms is again considered , but in the latter case the larger atom has a deeper potential energy minimum. Three mechanisms are found which influence selective adsorpt ion of a mixture species: (i) liquid-like fluid layering, (ii) inplane solid-like ordering and (iii) molecular sieving. The large atoms are completely eliminated from the pore when the wall separation is physic ally too small for the large atoms to fit, or when both species physic ally fit in the pore and the small component epitaxially aligns with t he surface or freezes. Complete elimination of the small species is no t observed. A significant excess of large atoms in the pore relative t o the composition in the bulk phase is found only when the large atoms attain transverse order within the fluid layers. The adsorption of th e large component is either enhanced or reduced depending on the relat ive magnitude of the potential energy well depth of the fluid and wall species.