The pH-Dependent Interplay of Surface Energy and Aggregation State for Nanoparticulate Goethite

The thermodynamic properties of iron oxides in water are known to depend on both particle size and aggregation state. Smaller particles have larger surface areas and, as a result, more surface energy; however, aggregation diminishes this effect. Determining the quantitative relationships among particle size, thermodynamic properties, and aggregation states in aqueous systems is complicated by the fact that the solution chemistry simultaneously influences both the aggregation states and the surface chemistries of hydrated iron oxide particles. Previous thermodynamic studies have implicitly assumed that specific surface energy values are pH-independent, but this assumption is at odds with the knowledge that surface complexation reactions, particularly with protons, alter surface energies. The objective of this study was to systematically quantify how the surface energy of goethite (alpha-FeOOH) in water varies as a function of the pH and particle size in a KCl electrolyte. We synthesized four batches of goethite with different primary particle sizes and morphologies, calculated their surface energies in water using measured reduction potential values and thermodynamic relationships, and approximated their aggregation states using dynamic light scattering and measured reduction potential values. At near-neutral pH values, differences in goethite surface energies could be explained by accounting for variances in reactive surface areas controlled by aggregation. However, at a mildly acidic pH, goethite exhibited a significantly different specific surface energy value, consistent with the change in the goethite surface charge changing. These findings highlight the critical influence of solution pH on surface thermodynamics that must be incorporated into concepts of how iron oxide particle size affects the reactivity and equilibrium relationships in aqueous environments.