PNNL

Abstract

Although long recognized, understanding multiple lengthscale correlations in the Xray and neutron pair distribution functions of aqueous electrolytes remains a persistent challenge. This work leverages polyoxoanions to support cation-centric coordination polyhedra in NaNO3(aq) and NaNO2(aq) with characteristic pair-wise correlations interpreted through classical molecular dynamics simulations and graph-theoretical analyses. We demonstrate that the water oxygen Ow · · · Ow correlations associated with Na+-coordination polyhedra (and their oligomers) have two characteristics lengthscales. That between 3.5 - 5.5 Å is associated with Ow coordinated to the same Na+, while the second between 5.5 - 7.5 Å is associated with Ow coordinating different Na+ connected by bridging anions, vertex-, edge-, or face-sharing. The ubiquitous contraction of the PDF between 5.5 - 7.5 Å observed in many electrolytes derives from the loss of the many-body bulk Ow · · · Ow and growth of all combinations of Ow and On correlations that emerge due to the ion network of Na-coordination polyhedra.