Systematic Evaluation of Ion Diffusion and Water Exchange

As a fundamental property of allfluids, diffusion plays myriad roles in bothscience and our daily lives. Diffusive properties of many liquids including water have beenextensively studied both experimentally and theoretically, while for transition metal ions,there exist significant experimental data that have not been extensively studiedtheoretically. Hence, high-confidence predictions for challenging systems like radioactiveions that are biohazardous cannot be reliably generated. In this work, a workflow namedISAIAH (Ion Simulation using AMBER for dIffusion Action when Hydrated) wasdesigned to accurately simulate the diffusion coefficients of 15 monoatomic ions withcharges varying from-1 to +3 in four water models. As the results indicate, goodagreement with experimental values was achieved, leading us to select239Pu4+(for which no experimental data are available) as acandidate ion to make a theoretical prediction of its diffusion coefficient in water. Among all the forcefield parameter sets, the onesparametrized using an augmented 12-6-4 Lennard-Jones (LJ) potential showed lower average unsigned errors (AUE) for ions ofvarious radii and electron configurations relative to some 12-6 LJ parameters. This observation agrees well with the fact that diffusionis affected by both the hydration free energy (HFE) and the ion-oxygen distance (IOD) between solute and solvent molecules, bothof which are handled well by the 12-6-4 model.

Automated summary

Diffusion, as a fundamental property of fluids, plays a crucial role in both scientific research and everyday life. While the diffusion properties of many liquids, such as water, have been extensively studied, there has been a lack of theoretical research on transition metal ions. In this study, a workflow called ISAIAH was developed to accurately simulate the diffusion coefficients of 15 monoatomic ions in water. The results showed good agreement with experimental data, which allowed for the theoretical prediction of the diffusion coefficient of 239Pu4+ ion. The use of an augmented 12-6-4 Lennard-Jones potential parameter set showed better results compared to some other parameter sets, indicating that both hydration free energy and ion-oxygen distance play important roles in diffusion.