Pure Compound Self-Diffusivity Correlation With Residual Entropy

Abstract

Two models of self-diffusivity in terms residual entropy have been proposed in the literature. A generalized residual entropy model is proposed here by merging aspects of these two models. The generalized model enables inference of interaction and scaling effects by probing the significance of related terms when minimizing deviations of each model relative to experimental data. As a result, the interaction effect between residual entropy and temperature is found to be negligible using a database of 688 experimental measurements for 13 n-alkanes. Similarly, scaling analysis disfavors the “Rosenfeld” scaling. Instead, the scaling proposed by Dzugutov is favored. While implementing these two simplifications, the reduced 3-parameter model is shown to provide improved accuracy relative to the previous 6-parameter model. On the other hand, comparison to a previously proposed 2-parameter self-diffusivity model shows that the 2-parameter model provides practically the equivalent accuracy to the 3-parameter model. The 2-parameter model does not apply the principle of residual entropy. Instead, it applies a corresponding states approach, where a model is fit to the self-diffusivity of the Lennard-Jones model potential and Lennard-Jones parameters are treated as adjustable parameters. By considering accuracy relative to the number of parameters, the 2-parameter and 3-parameter models are deemed equally acceptable. The root-mean-square logarithmic deviation for the 2-parameter model is 7.5% compared to 6.9% for the 3-parameter model and 10.2% for the 6-parameter model. An open-source database of experimental measurements for 34 compounds at 1685 state points is used in all evaluations. The analysis here is limited to nonassociating compounds. The mean average percentage error was less than 5% for all three models.