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

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)∥ i...

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