Driving Force for Molecular Diffusion: Comparision between Theory and Simulation
Jessica R. Whitman, Daniel Matuszak, Gregory L. Aranovich, and Marc D. Donohue. Chemical and Biomolecular Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218
Lattice Density Functional Theory (LDFT) has been used to develop a general property whose gradient is linear with the flux of a species. The path-dependent property, Γn, appears in LDFT equations as the impingement rate of n-molecules onto the malleable constituents in the surroundings and it is proposed that the gradient of Γn is the driving force for diffusion. Malleable constituents include vacancies and molecules of a species whose density gradients can be influenced by diffusion. In this work, Molecular Dynamics simulations are used to study the effect molecule mobility has on the flux of a diffusing species. Simulation results for “color” counter-diffusion (half of the molecules of species n are labeled A and half are labeled B) are compared to the simulation results of A molecules diffusing through static B molecules. These conditions are analogous to comparing the diffusion of species A through an amorphous polymer membrane and through a crystalline polymer membrane. Comparisons between simulation and theory will be discussed.