Ions transiting biomembranes might pass readily from water through ion-specific membrane proteins if those protein channels provide environments similar to the aqueous solution hydration environment. Indeed, bulk aqueous solution is an important reference condition for the ion permeation process. Assessment of this hydration mimicry view depends on understanding the hydration structure and free energies of metal ions in water to provide a comparison for the membrane channel environment. To refine these considerations, we review local hydration structures of ions in bulk water, and the molecular quasi-chemical theory that provides hydration free energies. In that process, we note some current views of ion-binding to membrane channels, and suggest new physical chemical calculations and experiments that might further clarify the hydration mimicry view.

Anion hydration is complicated by H-bond donation between neighboring water molecules in addition to H-bond donation to the anion. This situation can lead to competing structures for chemically simple clusters like (H 2 O) n Cl − and to anharmonic vibrational motions. Quasi-chemical theory builds from electronic structure treatment of isolated ion-water clusters, partitions the hydration free energy into inner-shell and outer-shell contributions, and provides a general statistical mechanical framework to study complications of anion hydration. The present study exploits dynamics calculations on isolated (H 2 O) n Cl − clusters to account for anharmonicity, utilizing ADMP (atom-centered basis sets and density-matrix propagation) tools. Comparing singly hydrated F − and Cl − clusters, classic OH-bond donation to the anion occurs for F − , while Cl − clusters exhibit more flexible but dipole-dominated interactions between ligand and ion. The predicted Cl − -F − hydration free energy difference agrees well with experiment, a significant theoretical step for addressing issues like Hofmeister ranking and selectivity in ion channels.

Li+ transport within a solid electrolyte interphase (SEI) in lithium ion batteries has challenged molecular dynamics (MD) studies due to limited compositional control of that layer. In recent years, experiments and ab initio simulations have identified dilithium ethylene dicarbonate (Li2EDC) as the dominant component of SEI layers. Here, we adopt a parameterized, non-polarizable MD force field for Li2EDC to study transport characteristics of Li+ in this model SEI layer at moderate temperatures over long times. The observed correlations are consistent with recent MD results using a polarizable force field, suggesting that this non-polarizable model is effective for our purposes of investigating Li+ dynamics. Mean-squared displacements distinguish three distinct Li+ transport regimes in EDC — ballistic, trapping, and diffusive. Compared to liquid ethylene carbonate (EC), the nanosecond trapping times in EDC are significantly longer and naturally decrease at higher temperatures. New materials developed for fast-charging Li-ion batteries should have a smaller trapping region. The analyses implemented in this paper can be used for testing transport of Li+ ion in novel battery materials. Non-Gaussian features of van Hove self -correlation functions for Li+ in EDC, along with the mean-squared displacements, are consistent in describing EDC as a glassy material compared with liquid EC. Vibrational modes of Li+ ion, identified by MD, characterize the trapping and are further validated by electronic structure calculations. Some of this work appeared in an extended abstract and has been reproduced with permission from ECS Transactions, 77, 1155–1162 (2017). Copyright 2017, Electrochemical Society, INC.

Accurate predictions of the hydration free energy for anions typically have been more challenging than for cations. Hydrogen bond donation to the anion in hydrated clusters such as F(H 2 O) n − can lead to delicate structures. Consequently, the energy landscape contains many local minima, even for small clusters, and these minima present a challenge for computational optimization. Utilization of cluster experimental results for the free energies of gas phase clusters shows that, even though anharmonic effects are interesting, they need not be troublesome magnitudes for careful applications of quasi-chemical theory to ion hydration. Energy-optimized cluster structures for anions can leave the central ion highly exposed and application of implicit solvation models to these structures can incur more serious errors than for metal cations. Utilizing cluster structures sampled from ab initio molecular dynamics simulations substantially fixes those issues.