A very simple yet effective method was described which permitted the identification of individual penetrant jump events, and their quantitative treatment in a statistical sense. The method was applied in equilibrium molecular dynamics simulations for systems of gaseous alkanes, methane through n-butane, including also a mixture of methane and n-butane, dispersed in n-decane or n-eicosane. Equilibration and attainment of a linear diffusion regime was confirmed by means of various criteria, and the jumps detection method was applied to all systems studied. The results obtained clearly showed the existence of distinct jump events in all cases, although the average jump length was reduced with penetrant or liquid alkane molecular weight. The method allowed one to determine the average jump length and the corresponding jumps frequency. On the basis of these results, it was possible to estimate a random walk type diffusion coefficient, As,jumps, of the penetrants, which was found to be substantially lower compared with the overall diffusion coefficient DS,MSD obtained by the mean square displacement method. This led to the assumption that the overall penetrant diffusion in the studied systems involved a combination of longer jumps with a smoother and more gradual displacement; a result that confirmed assumptions suggested in previous studies.

New Effective Method for Quantitative Analysis of Diffusion Jumps, Applied in Molecular Dynamics Simulations of Small Molecules Dispersed in Short Chain Systems. Raptis, T.E., Raptis, V.E., Samios, J.: Journal of Physical Chemistry B, 2007, 111[49], 13683-93