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 ..mu.., 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 ..mu.. 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/sup (//sup i//sup )//sub parallel/ associated with a given layer i can be defined. D/sup (//sup i//sup )//sub parallel/ is least for the contact layer, even for pores as wide as 30 atomic diameters (approx.100 A). Moreover, D/sup (//sup i//sup )//sub parallel/ increases with increasing distance of the fluid layer from the wall and, for pore widths between 16 and 30 atomic diameters, D/sup (//sup i//sup )//sub parallel/ 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.
- Research Organization:
- Departments of Agronomy and Chemistry, Purdue University, West Lafayette, Indiana 47907
- OSTI ID:
- 5646049
- Journal Information:
- J. Chem. Phys.; (United States), Vol. 88:2
- Country of Publication:
- United States
- Language:
- English
Similar Records
Fluids in micropores. V. Effects of thermal motion in the walls of a slit-micropore
Determination of the thermodynamic correction factor of fluids confined in nano-metric slit pores from molecular simulation