Cation Coordination Polyhedra Lead to Multiple Lengthscale Organization in Aqueous Electrolytes

Although long recognized, understanding multiple lengthscale correlations in the X-ray and neutron pair distribution functions of aqueous electrolytes remains a persistent challenge. This work leverages polyoxoanions to support cation-centric coordination polyhedra in NaNO$_{3(aq)}$ and NaNO$_{2(aq)}$ with characteristic pair-wise correlations interpreted through classical molecular dynamics simulations and graph-theoretical analyses. We demonstrate that the water oxygen Ow $\cdots$ Ow correlations associated with Na$^+$-coordination polyhedra (and their oligomers) have two characteristics lengthscales. That between 3.5 - 5.5 \AA\ is associated with Ow coordinated to the same Na$^+$, while the second between 5.5 - 7.5 \AA\ is associated with Ow coordinating different Na$^+$ connected by bridging anions, corner-, edge-, or face-sharing. The ubiquitous contraction of the PDF between 5.5 - 7.5 \AA\ observed in many electrolytes derives from the loss of the many-body bulk Ow $\cdots$ Ow and growth of all combinations of Ow and On correlations that emerge due to the ion network of Na-coordination polyhedra.