Fluids in micropores. II. Self-diffusion in a simple classical fluid in a slit pore

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Abstract
Self‐diffusion coefficients D are computed for a model slit pore consisting of a rare‐gas fluid confined between two parallel face‐centered cubic (100) planes (walls) of rigidly fixed rare‐gas atoms. By means of an optimally vectorized molecular‐dynamics program for the CYBER 205, the dependence of D on the thermodynamic state (specified by the chemical potential μ, temperature T, and the pore width h) of the pore fluid has been explored. Diffusion is governed by Fick’s law, even in pores as narrow as 2 or 3 atomic diameters. The diffusion coefficient oscillates as a function of h with fixed μ and T, vanishing at critical values of h, where fluid–solid phase transitions occur. A shift of the pore walls relative to one another in directions parallel with the walls can radically alter the structure of the pore fluid and consequently the magnitude of D. Since the pore fluid forms distinct layers parallel to the walls, a local diffusion coefficient D(i)∥ associated with a given layer i can be defined. D(i)∥ is least for the contact layer, even for pores as wide as 30 atomic diameters (∼100 Å). Moreover, D(i)∥ increases with increasing distance of the fluid layer from the wall and, for pore widths between 16 and 30 atomic diameters, D(i)∥ is larger in the center of the pore than in the bulk fluid that is in equilibrium with the pore fluid. The opposite behavior is observed in corresponding smooth‐wall pores, in which the discrete fluid–wall interactions have been averaged by smearing the wall atoms over the plane of the wall. The temperature dependence of D for fixed h is determined and the nature of melting of a pore solid is examined. It is found that the solid tends to melt first in the middle of the pore. All of the various results are related to the structural properties of the pore fluid, as manifested by the local density and pair correlation functions.