Is Stokes-Einstein relation valid for the description of intra- diffusivity of hydrogen and oxygen in liquid water?

IN Tsimpanogiannis and OA Moultos, FLUID PHASE EQUILIBRIA, 563, 113568 (2022).

DOI: 10.1016/j.fluid.2022.113568

In this study, all available data from experiments and molecular simulations for the intra-diffusivities of H-2 and O-2 in H2O, and for the self-diffusivity of pure H2O are analyzed to examine the validity of the Stokes-Einstein relation. This analysis is motivated by the significant amount of work devoted through the years for improving the predictions of intra-and self-diffusivities in binary and multi- component mixtures relevant to chemical and environmental processes. Here, we calculate the slopes s and t corresponding to the ln(D) vs. ln(T/eta) and ln(D/T) vs. ln(1/eta) plots, respectively, where D is the intra-diffusivity, eta the viscosity, and T the temperature of the T/eta systems. Our results show that s and t deviate from unity no matter if the experimental or simulation data are used. This means that the Stokes-Einstein relation is violated for the binary systems of H-2 and O-2 with H2O, and for pure H2O. Although prior studies mainly focused on re-evaluating the parameter A of the SE-based semi-theoretical/semi- empirical approaches expressed as D = A T/eta, our results indicate that reliable predictions for the intra-and self-diffusivities can be achieved by improving the accuracy of the prediction of slopes s and t.

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