Is Stokes-Einstein relation valid for the description of intra-diffusivity of hydrogen and oxygen in liquid water?
Ioannis N. Tsimpanogiannis, Othonas A. Moultos
Abstract
In this study, all available data from experiments and molecular simulations for the intra-diffusivities of H2 and O2 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(Tη) and ln(DT)vs.ln(1η) plots, respectively, where D is the intra-diffusivity, η the viscosity, and T the temperature of the 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 H2 and O2 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=ATη, 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.