How do diluent-aggregates interactions affect the structure of colloidal systems in solvent extraction?

The structure and dynamics of extractant-based aggregates strongly influence metal ion extraction and third-phase formation in liquid-liquid systems. Here, classical molecular dynamics simulations were used to study aggregates composed of Eu(NO3)3, water, and the extractants DMDOHEMA or DMDBTDMA in two linear n-alkanes, n-heptane, and n-dodecane. Micelle organization and solvent interactions were analyzed through radial distribution functions and potentials of mean force. Polar and apolar micellar radii were similar for both extractants, but diluent penetration into the apolar region was greater in n-dodecane than in n-heptane. Extractant chains generally wrapped around the polar core, with orientations primarily driven by entropy, except for DMDBTDMA in n-dodecane, where stronger diluent-extractant interactions were observed. Penetrating diluent molecules are oriented roughly perpendicular to the micelle, following the same behavior as the extractant chains. These results reveal that the apolar regions of the aggregates are highly dynamic, with chain motions becoming nearly isotropic beyond the polar core. Solvent molecules readily penetrate the micelle interior, challenging the conventional view of reverse micelles as rigid, star-like structures. Overall, this study demonstrates how the interplay between the extractant and diluent governs aggregate structure and dynamics, offering molecular-level insights into the factors controlling ion extraction and third-phase formation.