Mass diffusion of diatomic fluids in random micropore spaces using equilibrium molecular dynamics

An equilibrium-molecular-dynamics study of diffusion of a heteronuclear diatomic fluid 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 using the rattle algorithm [H. C. Andersen, J. Comput. Phys. 52, 24 (1983)] at constant temperature are performed on the percolating cluster only where 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 a heteronuclear diatomic molecule (CO-like) within a graphitelike system for porosities between the percolation threshold and φ=0.99, and over the temperature range of T=100 K to T=1500 K at atmospheric pressure. The results, which include mean-square-displacement and velocity-autocorrelation functions (VACF's), indicate once again that mass diffusion within porous media is fundamentally different from that in the bulk phase. The degree of anomalous behavior tends to decrease with temperature and porosity. Although this is the case, the mean-square-displacement exponent in the long-time limit for the lower porosities (φ=0.312 and φ=0.4) increases with temperature to a limiting value much less than one in the vicinity of T=300 K. Trends in the VACF's seen here have been reported in past Lorentz gas simulation studies. Effective diffusion coefficients for our gas-in-pore system were calculated from the VACF's-these appear to vary exponentially with porosity and inverse temperature. This temperature variation is similar to that of a liquid in the bulk phase and hence brings into doubt the use of a temperature-independent tortuosity that relates the diffusion coefficient of a gas in the bulk phase to that within porous media.