Thermodynamic and structural insights into glycolamide-based solvent extraction: unravelling the effect of...

The interplay between macroscopic thermodynamic observables and microscopic structural reorganization is central to advancing targeted separation strategies for lanthanoids and actinoids. In the current study, Isothermal solution calorimetry was utilized to determine the extraction enthalpies (ΔH_extr) of nitric acid and representative metal ions (La³⁺, Ce³⁺, Nd³⁺ and Zr⁴⁺) using three structurally diverse glycolamide extractants across 1 M and 4 M nitric acid regimes. The results revealed that the exothermicity (ΔH_extr) of the diglycolamide (DGA) variants, N,N,N',N'-tetraoctyldiglycolamide (TODGA) and N,N,N',N'-tetradecyldiglycolamide (TDDGA), systematically diminished as a function of increasing aqueous acidity. Complementary dynamic light scattering (DLS) measurements of the metal-loaded organic phases mapped this thermochemical attenuation directly to the growth of meso-scale supramolecular aggregates at elevated acidities. To decouple the energetic penalties of assembly from coordination, comparisons were extended to the monoglycolamide analogue, N,N-dioctyl-2-hydroxyacetamide (DOHyA). Benefiting from a restricted architectural framework, DOHyA exhibited suppressed aggregation tendencies alongside markedly higher extraction enthalpies relative to its DGA counterparts. These insights demonstrate a distinct inverse correlation between extractant self-assembly and enthalpy, emphasizing that competitive supramolecular aggregation can act as a significant energetic sink. This work establishes isothermal calorimetry as an indispensable diagnostic probe for mapping the subtle balance of intermolecular forces dictating phase-transfer energetics under industrially relevant feed conditions.