Mean Free Path of Gas Molecules in Organic Nanochannels Using Molecular Simulations

Abstract

Summary A computational method using molecular–simulation data is introduced to estimate the average mean–free–path length of multicomponent hydrocarbon molecules in an organic nanochannel. Grand–canonical Monte Carlo (MC) simulation is used first to construct the equilibrium distribution of the gas molecules in the channel. These results show that the smaller the channel is, the denser the gas mixture becomes because of nanoconfinement effects. Capillary condensation occurs in the smaller channels. The fluid composition inside a channel becomes progressively heavier when the bulk–fluid pressure outside the nanopore is reduced and the lighter hydrocarbons leave the channel. The mean–free–path lengths of the confined molecules are computed using the trajectories of the molecules displaced over time in the equilibrium molecular–dynamics (MD) simulation. The average length of the confined molecules is estimated to be an order of magnitude smaller than the theoretical value. Further, the length does not show a strong dependence on the channel width and the pressure. Consequently, the predicted Knudsen–number value does not vary significantly, as anticipated by the kinetic theory of gases and by the molecular simulations of pure fluids. This invariance indicates that compositional change caused by nanoconfinement eliminates transition into other transport regimes where continuum mechanics is no longer valid.