Friction based modeling of multicomponent transport at the nanoscale

We present here a novel theory of mixture transport in nanopores, which considers the fluid-wall momentum exchange in the repulsive region of the fluid-solid potential in terms of a species-specific friction coefficient related to the low density transport coefficient of that species. The theory also considers nonuniformity of the density profiles of the different species, while departing from a mixture center of mass frame of reference to one based on the individual species center of mass. The theory is validated against molecular dynamics simulations for single component as well as binary mixture flow of hydrogen and methane in cylindrical nanopores in silica, and it is shown that pure component corrected diffusivities, as well as binary Onsager coefficients are accurately predicted for pore sizes sufficiently large to accommodate more than a monolayer of any of the components. It is also found that the assumption of a uniform density profile can lead to serious errors, particularly at small pore diameter, as also the use of a mixture center of mass frame of reference. The theory demonstrates the existence of an optimum temperature for any fluid, at which the fractional momentum dissipation due to wall friction is a minimum.