Structure making and breaking effects of ions on the anomalous diffusion of water revealed by machine learning potentials

The dynamics of water exhibits anomalous behavior in the solvation of ions, and understanding the perturbation that ions make on the hydrogen bond structure of water remains an open question. In this study, we investigate the anomalous diffusion behavior of water and its molecular origins in aqueous NaCl and CsI solutions using deep machine learning potentials developed at the MP2 level of theory. Our simulations reveal a suppression of water diffusion in NaCl solutions and an enhancement of water diffusion in CsI solutions across various salt concentrations, reproducing the experimental observations. We further explore the microscopic origins of this anomalous diffusion behavior by examining the structural changes of the water hydrogen bond network, as well as the vibrational properties of water molecules. Our findings highlight the distinct roles that different ions play in modulating the structure and dynamic behavior of water in salt solutions, and provide key insights into the fundamental mechanisms of structure making and breaking that govern the anomalous diffusion of water.