Self-diffusion in fluids in microporous solids

Enskog’s kinetic theory of tracer diffusion has been generalized recently to strongly inhomogeneous fluids and is here applied to self-diffusion in fluids in slit pores, i.e., fluids confined between parallel, flat, solid walls. In pores narrower than ten molecular diameters, the diffusivity deviates significantly from its value in bulk phase at the same temperature and chemical potential. Because of the confining pore walls, the fluid tends to form layers. The pore diffusivity oscillates as a function of pore width, a local minimum occurring when the packing of a given number of fluid layers is favored and a local maximum in the region of transition between these favored packing widths. The theory predicts fluid density distributions in good agreement with computer simulations of a similar fluid. The predicted diffusivities are in good qualitative agreement with computer simulations, although quantitatively the predicted oscillations are sharper than those observed in the computer simulations. This work represents the first application of Enskog’s kinetic theory of diffusion in strongly inhomogeneous fluids.