Abstract :
The experimental and theoretical context of transport phenomena in microporous carbons is briefly reviewed. The theory of transport of fluids in porous materials and of the routes for obtaining transport properties from simulation are discussed. A description is given of the simulation techniques required to study these phenomena, including a recently developed non-equilibrium molecular dynamics (NEMD) method. Single graphite slit pores have been adopted as a simple and tractable model that retains the essentials of the basic physics of the phenomena. Results of simulations in five different pore sizes within the important ultramicropore range are given. These include isotherms, heat curves and the different diffusion coefficients as a function of density for a spherical methane model at 296 K. The system is representative of supercritical fluid in pores in the micropore size range. It is found that the total effective diffusion coefficient, D, measured from NEMD, increases more rapidly with density than transport diffusion coefficients calculated from the self diffusion coefficient, and can be an order of magnitude larger. Two characteristic types of behaviour have been identified: Type A, where a rather steep rise in D is followed by a gradual decline, and type B, where D increases monotonically over a wide range of density. The results can be understood in terms of the equilibrium structure of the adsorbate and the onset of cooperative flow, where transport is favoured by forward scattering over a small range of density. Distributions of molecular density, streaming velocity and local flux support these conclusions.
