Fluids in micropores. III. Self-diffusion in a slit-pore with rough hard walls

Self-diffusion coefficients for a model slit-pore consisting of Lennard–Jones (12,6) fluid constrained between two plane-parallel rough hard walls have been computed by standard (microcanonical) molecular dynamics. Fluid molecules collide impulsively with the walls, their speeds remaining unchanged, but the directions in which they rebound from the walls are chosen probabilistically so that the walls behave as Maxwell’s ‘‘perfectly adsorbing’’ walls. For comparison, fluid contained in the same thermodynamic state by a pore with smooth walls has also been simulated. The local density profiles, which are the same in both rough and smooth-walled pores, show that the pore fluid forms distinct layers parallel with the walls. The rate of transverse diffusion (parallel with walls) within a given fluid layer depends markedly on its distance from the walls and on the nature of the walls. Transverse diffusion within the layer in contact with the rough wall is slower than that within the layer in contact with the smooth wall.