January 30, 2025
Journal Article
Ion correlations decrease particle aggregation rate by increasing hydration forces at interfaces
Abstract
olid-liquid interfaces are central to a range of phenomena including catalysis, heterogeneous nucleation, water desalination, and biomolecular assembly. However, the connection between interfacial solution structure, particle forces, and emergent phenomena remains ambiguous, particularly at high ionic strength. In this case study on boehmite aggregation, we used a combination of three-dimensional atomic force microscopy, light scattering, molecular dynamics simulations, and colloidal theory, to establish a connection between molecular structure at the interface, the emerging hydration forces between two approaching particles, and the resulting structure and kinetics of particle aggregation. In contrast to expectations from DLVO theory, we observed a non-monotonic dependence of the aggregation rate on the concentration of sodium chloride, nitrate, or nitrite. The initial aggregation rate increases by two orders of magnitude as ionic strength increases from 0.01 to 1 molal, but then decreases by fifteen-fold in 4 molal solutions. These results cannot be explained by solution viscosity, enhanced surface charge, or increased van der Waals attraction, and were thus ascribed to hydration forces. Validating that conclusion, we measured an increase in repulsive hydration forces coupled to increased spacing of interfacial oscillatory features from 0.27–0.31 nm in 0.01 molal to 0.38–0.52 nm in 2 molal concentrations of the same electrolytes. Moreover, molecular dynamics simulations indicated that these changes corresponded to enhanced ion correlations near the interface and produced loosely-bound aggregates that retain electrolyte between the particles. We anticipate that these results will enable the prediction of particle aggregation, attachment, and assembly, with broad relevance to interfacial phenomena.Published: January 30, 2025