Microheterogeneity and Ion Clustering in Aqueous Potassium Dihydrogen Phosphate Solutions

The molecular-scale organization in concentrated aqueous potassium dihydrogen phosphate solutions may play a critical role in their behavior, such as in the structural processes that precede crystal nucleation. In this work, molecular dynamics simulations are used to investigate the structure and dynamics of potassium dihydrogen phosphate solutions over a range of concentrations, where these solutions are analyzed using a combination of structural correlation functions, Kirkwood-Buff analysis, cluster analysis, mobility measurements, and spatial density distribution functions. The results show that increasing concentration promotes hydrogen-bond-mediated association between dihydrogen phosphate ions, leading to the formation of extended ion clusters and ion-rich domains. The extent of clustering can depend on the water model used in the simulation, with weaker hydration environments promoting larger and more interconnected dihydrogen phosphate networks. Mobility analysis further indicates a significant reduction in ion diffusion as clustering increases, while water molecules remain comparatively mobile. Spatial density distributions around representative dihydrogen phosphate ions reveal distinct local environments associated with cluster interiors, cluster surfaces, and bulk-like solvent regions within the same solution. These findings provide clear evidence for microheterogeneity in concentrated potassium dihydrogen phosphate solutions arising from the competition between ion hydration and dihydrogen phosphate-dihydrogen phosphate hydrogen bonding. Such locally heterogeneous environments arising from ion association may promote the formation of prenucleation clusters, potentially influencing nucleation pathways as observed in experimental studies of potassium dihydrogen phosphate crystallization.