A molecular dynamics investigation of the unusual concentration dependencies of Fick diffusivities in silica mesopores

Molecular Dynamics (MD) simulations were carried out to determine the self-diffusivitiy, Di,self, the Maxwell–Stefan diffusivity, Ði, and the Fick diffusivity, Di, for methane (C1), ethane (C2), propane (C3), n-butane (nC4), n-pentane (nC5), n-hexane (nC6), n-heptane (nC7), and cyclohexane (cC6) in cylindrical silica mesopores for a range of pore concentrations. The MD simulations show that zero-loading diffusivity Ði(0) is consistently lower, by up to a factor of 20, than the values anticipated by the classical Knudsen formula. The concentration dependence of the Fick diffusivity, Di is found to be unusually complex, and displays a strong minimum in some cases; this characteristic can be traced to molecular clustering.

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► For molecules that adsorb strongly at the mesopore walls, the Knudsen diffusivity formula does not hold.
► For diffusion of alkanes molecular clustering clustering occurs when the temperature is lower than the critical temperature.
► Clustering of molecules has a strong influence on the concentration dependence of the diffusivities.