Understanding Ionic Solutions More Clearly

Solutions that contain electrolytes, like saltwater, are extremely common in nature and industry. Despite their ubiquity, the physics behind these solutions are poorly understood and have been explained by multiple and conflicting theories. Wills et al. systematically tested various simulation methods in previous research, suggesting possible sources of conflicts and guidelines for future research on the structure of electrolytes in water.

By simulating salt solutions using several empirical force fields or ab initio simulations, the authors showed different models can strongly vary in how they handle oppositely charged ions, even how small changes in the model and thermodynamic conditions can lead to drastically different results. For instance, the interactions between oppositely charged ions in water are influenced by entropic and enthalpic contributions.

“Counterintuitively, coulomb screening makes opposite charges repel, and hence, in close proximity, the charges are stabilized by entropy,” coauthor Alec Wills said.

To derive these results, the authors calculated potentials of mean force for saltwater solutions as a function of the average distance between the sodium and chloride ions in the water. By examining these potentials at different ion concentrations, the team could infer insights into the stability and associations between the oppositely charged ions.

Wills said the authors are currently addressing new questions raised by the study. For example, in contrast to a scientific fact taught in classrooms – that like charges repel, not bind to, one another – their study suggests two ions with the same charge could bind, rather than repel, under certain thermodynamic conditions. Such results could lead to further understanding of other systems, such as the association of charges at metal-water interfaces.

Source: “Role of water model on ion dissociation at ambient conditions,” by Alec Wills and Marivi Fernández-Serra, Journal of Chemical Physics (2021). The article can be accessed at https://doi.org/10.1063/5.0046188 .