Describing binary mixture diffusion in carbon nanotubes with the Maxwell-Stefan equations. An investigation using molecular dynamics simulations

Abstract

Adsorption and diffusion of pure components and binary mixtures containing methane, ethane, propane, n-butane, isobutane, and hydrogen at 300 K in a variety of configurations of carbon nanotubes (CNTs) have been investigated using configurational-bias Monte Carlo (CBMC) simulations and molecular dynamics (MD) simulations. Both self-diffusivities, D_i,self, and the Maxwell-Stefan (MS) diffusivities, Ð_i, were determined for a variety of molecular loadings ϑ, approaching saturation limits. For comparison purposes, self-diffusivities were also determined in pure fluids of varying densities using MD. At low loadings ϑ, the Di,self correspond to the value for low-density gases. With increasing loadings, however, the D_i,self in CNTs are slightly higher than the values in fluids when compared at the same molecular density. In CNTs, the D_i,self is significantly smaller in magnitude than the MS diffusivity Ð_i, signifying strong correlations between molecular jumps along the tube. Consequently, for mixture diffusion, the component self-diffusivities are close together. MD simulations of binary-mixture diffusion demonstrate that the mixture-diffusion characteristics can be estimated with good accuracy from the pure-component diffusion parameters using the MS diffusion formulation. In the estimation procedure, the binary-exchange parameter Ð₁₂ is estimated from the pure-component self-exchange coefficients Ð₁₁ and Ð₂₂ using the interpolation scheme suggested earlier for transport in zeolites (Skoulidas et al. Langmuir 2003, 19, 7977).