MASS DIFFUSION OF ATOMIC FLUIDS IN RANDOM MICROPORE SPACES USING EQUILIBRIUM MOLECULAR-DYNAMICS

An equilibrium-molecular-dynamics study of the diffusion of atomic fluids in random pore systems is reported. The pore space is generated by the use of a simple percolation technique on a tessellation of three-dimensional space with periodic boundary conditions. Simulations are performed on the percolating cluster only. The substrate atoms are explicitly represented and the substrate-fluid and fluid-fluid interactions are modeled using Lennard-Jones potentials. Using this technique, results are presented for an argon-in-graphite system for different porosities at 140 K and a fluid density of 3.5 k gm-3. The results, which include mean-square displacement, velocity-autocorrelation functions and their associated memory kernels, and the total-force autocorrelation functions, clearly indicate that mass diffusion in completely random pore systems is different from that in the bulk phase. Trends shown in the results here are similar to those reported in Monte Carlo, random-walk, and theoretical works elsewhere. Restriction of the technique to the study of atomic fluids and random pore systems is made here for simplicity; it is presently being extended to molecular fluids and mixtures in real micropore spaces. The substrate is assumed to be rigid; however, it is felt that the technique can be extended to include substrate-fluid energy exchange.