Influence of diluents and Hofmeister series on diglycolamide aggregation in solvent extraction

Abstract

The non-ideal behavior of the organic phase, driven by aggregate distributions, is often overlooked in solvent extraction (SX) models, even though it plays a crucial role in shaping speciation. Incorporating aggregate polydispersity is essential for accurately capturing extraction behavior in mesoscopic and chemical engineering-scale models. To address this gap, the present study investigates how diluent properties, Hofmeister-series conjugate bases, and lanthanide co-extraction influence the aggregation behavior of N,N${}^{\prime }$-dimethyl-N,N${}^{\prime }$-di(n-octyl)diglycolamide (DMDODGA), a commonly employed representative of the diglycolamide (DGA) class of solvating extractants. Molecular dynamics simulations were used to examine how aggregate speciation is modulated by diluent polarity, acid protonation states, and conjugate base identity. Solvophobic clustering dominated in low-polarity media, while high-polarity diluents promoted solvent shielding, resulting in dispersed distributions with diffuse, transient clusters. Ion specificity within inner-sphere clusters, governed by hydrogen bonding, was reinforced by hard, monoatomic anions such as Cl${}^{-}$ and disrupted by softer, polytopic anions such as NO${}_3^{-}$ and HSO${}_4^{-}$. Lanthanide inclusion suppressed overall aggregation, with increasing solvent polarity further diminishing ion specificity and favoring smaller aggregates, whereas nonpolar solvents promoted large, compact clustering. These insights facilitate control over aggregate speciation as a strategy to mitigate organic-phase non-ideality, offering a foundation for optimizing extraction process design through data-driven flowsheet modeling.