PNNL

Abstract

This study was motivated by the experimental observation that the presence of alkali nitrates enhances boehmite nanoparticle aggregation, and the related more fundamental uncertainty as to what controls short-range forces driving aggregation at small distances. The metadynamics rare event theory simulations presented here allowed us to probe the energy barriers to aggregation with and without pre-sorbed electrolyte ions in the interlayer, as well as how these barriers differed between two crystal faces: basal–basal (010), and edge–edge (001). We found that the presence of NaNO3 and KNO3 lowered barriers to aggregation, and that basal–basal interactions were preferred over edge–edge interactions. Due to the smaller energy barriers, aggregates containing adsorbed ions in the interlayer are predicted to undergo more reversible aggregation/disaggregation. Through analysis of the density of water and ions in the interlayer at aggregated states, we proposed that both stable states and the magnitudes of the energy barriers between them stem from the relative ordering of the interstitial water network at mineral-surface sites. That is, energy minima occur where the mineral surface sites are commensurate with the hydrogen bonding environment for water monolayers, and energy barriers to aggregation derive from the need to perturb that ordered structure. Adsorbed ions promote aggregation by disrupting that interstitial water network, creating a more flexible environment for different aggregation states. Ion-specific effects are thus thought to derive from the differing ion concentrations in the interlayer. With this detailed interfacial structure and energetics information, upscaled studies that couple the molecular-scale results on short-range interactions here with classical “dispersion forces” to predict particle aggregation across a range of solution compositions would be beneficial. This research was supported by Interfacial Dynamics in Radioactive Environments and Materials (IDREAM), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) (FWP 68932).