Understanding solutions of ions in water at the molecular level

What is it about?

Water is the most significant liquid responsible for sustaining life on our planet, and it is important to provide a molecular-level understanding of its macroscopic properties, including its “anomalies” when compared with other liquids. A description in terms of interactions between pairs of molecules is not sufficient. Cooperative effects in water, that is the extra effects of groups of three or more molecules, are very important, and of these the three-molecule effects have been shown to be the most important, with only very small additional contributions from four-molecule interactions and negligible effects from larger groups. The latest generation of interaction potentials for water have been based on this understanding, and contain two-body, three-body and four-body interactions from accurate ab-initio electronic structure calculations to build up extended hydrogen bonded networks, not just in liquid water and ice but also in aqueous ionic solutions. Our work demonstrates that the three-body term in water and aqueous ionic systems can be accurately reproduced by a classical induction model. For water, this model describes the electrostatic interaction between molecules using distributed multipole moments up to hexadecapole, and the response of each molecule to electrostatic fields by polarizabilities up to quadrupole–quadrupole. Monatomic ions are described by point charges and dipole–dipole polarizabilities, while polyatomic ions are described by distributed multipoles up to hexadecapole and distributed polarizabilities up to quadrupole–quadrupole. Although this may seem elaborate, it is easy to construct and fast to evaluate.

Featured Image

Photo by Christoffer Engström on Unsplash

Photo by Christoffer Engström on Unsplash

Why is it important?

The accuracy of the proposed classical model demonstrates that the three-body term for aqueous ionic systems can be accurately modeled classically. This approach provides a fast, efficient, and accurate path toward modeling the three-body term in aqueous ionic systems that is fully transferable across systems with different ions without the need to obtain and fit to tens of thousands of ab initio calculations for each ion to extend existing many-body force fields to interactions between water and ions.

Perspectives