A simulation study of the pore size dependence of transport selectivity in cylindrical pores

DYNAMICS OF SIMPLE FLUIDS CONFINED IN CYLINDRICAL PORE: EFFECT OF PORE SIZE

We study the thermodynamic and dynamical properties of Weeks–Chandler–Anderson (WCA) fluids confined in a cylindrical pore by means of a canonical molecular dynamics simulation method. The pore model is an infinitely long cylinder consisted of a thermal wall and mimics a typical carbon nanotube. The thermodynamic properties are obtained for relatively high density fluids over a wide range of pore diameters at a given temperature. The size dependence of the self-diffusion coefficients in the cylindrical pore is also investigated. It is found that, as the pore diameter decreases, the potential energy and axial component of the pressure exhibit a sharp rise and the self-diffusion coefficient decreases. The observed behaviors can be understood in terms of the geometrical confinement and attenuation of transport induced by dispersive fluid-wall interaction. In addition, anomalous diffusion is observed at the pore size corresponding to twice the particle diameter.

Modeling mixture transport at the nanoscale: departure from existing paradigms

We present a novel theory of mixture transport in nanopores, which represents wall effects via a species-specific friction coefficient determined by its low density diffusion coefficient. Onsager coefficients from the theory are in good agreement with those from molecular dynamics simulation, when the nonuniformity of the density distribution is included. It is found that the commonly used assumption of a uniform density in the momentum balance is in serious error, as is also the traditional use of a mixture center of mass based frame of reference.

Adsorption and diffusion of molecular nitrogen in single wall carbon nanotubes

Using molecular simulation, the adsorption and self-diffusion of diatomic nitrogen molecules inside a single wall carbon nanotube have been studied over a range of nanotube diameters (8.61-15.66 A) and loadings at temperatures of 100 and 298 K. Nitrogen adsorption energy is found to increase as the nanotube diameter is reduced toward the molecular diameter of nitrogen. A discrete organization of the nitrogen into adsorbed layers is observed at high loadings that follows a regular progression determined primarily by geometric considerations. The formation of an adsorbate core at the center of the nanotube is found to increase the self-diffusion of nitrogen. A "wormlike" phase is found for the adsorbed nitrogen in the (15, 0) carbon nanotube at high loadings and at 100 K.

Tractable molecular theory of transport of Lennard-Jones fluids in nanopores

We present here a tractable theory of transport of simple fluids in cylindrical nanopores, which is applicable over a wide range of densities and pore sizes. In the Henry law low-density region the theory considers the trajectories of molecules oscillating between diffuse wall collisions, while at higher densities beyond this region the contribution from viscous flow becomes significant and is included through our recent approach utilizing a local average density model. The model is validated by means of equilibrium as well nonequilibrium molecular dynamics simulations of supercritical methane transport in cylindrical silica pores over a wide range of temperature, density, and pore size. The model for the Henry law region is exact and found to yield an excellent match with simulations at all conditions, including the single-file region of very small pore size where it is shown to provide the density-independent collective transport coefficient. It is also shown that in the absence of dispersive interactions the model reduces to the classical Knudsen result, but in the presence of such interactions the latter model drastically overpredicts the transport coefficient. For larger micropores beyond the single-file region the transport coefficient is reduced at high density because of intermolecular interactions and hindrance to particle crossings leading to a large decrease in surface slip that is not well represented by the model. However, for mesopores the transport coefficient increases monotonically with density, over the range studied, and is very well predicted by the theory, though at very high density the contribution from surface slip is slightly overpredicted. It is also seen that the concept of activated diffusion, commonly associated with diffusion in small pores, is fundamentally invalid for smooth pores, and the apparent activation energy is not simply related to the minimum pore potential or the adsorption energy as generally assumed.