Mutual diffusion in a binary Ar-Kr mixture confined within zeolite NaY

Molecular dynamics investigations of the mutual diffusion coefficients in an Ar-Kr mixture confined in the zeolite NaY are reported. Velocity auto- and cross correlations were computed at two different temperatures (200 and 600 K). The importance of the appropriate choice of reference frame while evaluating the time correlation functions is illustrated for argon in the zeolite NaY. Mutual diffusivities in the mixture were obtained in the barycentric reference frame. Recently, Zhou and Miller showed that the distinct diffusivity ${\mathcal{D}}_d$ is zero for the Ar-Kr mixture in bulk. On confinement, it is seen that at 200 K the ratio $R={\mathcal{D}}_{11}/{\mathcal{D}}_s=0.77,$ where ${\mathcal{D}}_{11}$ is the mutual diffusivity and ${\mathcal{D}}_s$ is the mixture self-diffusivity. However, at 600 K, $R=0.97,$ implying that the contribution from distinct diffusion is only slightly negative. The large negative ${\mathcal{D}}_d$ at 200 K could be attributed to strong localization of Ar and Kr in the physisorption sites within the zeolite cages. Analysis of error bars and an efficient computational algorithm for evaluation of the velocity cross correlation function are also presented. The results have implications in biology, chemistry, and other situations where transport of confined mixtures is encountered.