Hydrophobicity-Driven and Interaction-Corrected Modeling for Gold Ion-Pair Extraction Using Ammonium and Phosphonium...

Solvent extraction (SX) using 1,2-dichloroethane (DCE) solutions of ionic liquids (ILs) allows tunable hydrophobic partitioning for precious metal recovery. Bis(trifluoromethanesulfonyl)amide (TFSA)-based quaternary ammonium and phosphonium ILs (0.5-5 mM in DCE) were synthesized in this study and their Au(III) extraction from 1 M HCl (AuCl₄^-) was evaluated using atomic absorption spectroscopy (AAS) slope analysis at a 1:1 phase ratio, confirming a 1:1 ion-pair stoichiometry (slope 0.62-1.33, R² > 0.99) and yielding apparent extraction equilibrium constants (logK'_ex) of 2.6-4.9. Phosphonium ILs outperformed their ammonium analogs; alkyl and ether substituents increased logK'_ex via enhanced hydrophobic partitioning, whereas cyano substituents reduced logK'_ex due to high hydration energies, as indicated by density functional theory (DFT)-calculated solvation energy differences. COSMO-RS-predicted distribution coefficients (logK_d) correlated strongly with logK'_ex (R = 0.888, R² = 0.789), forming the COSMO-based optimization of metal extraction using thermodynamic parameters (COMET) model. Incorporation of the DFT-derived electron affinities into multiple regressions (MICOMET, molecular interaction-corrected COMET) improved R² to 0.852, quantifying the hydrophobicity (∼75%) versus interaction (∼25%) contributions to logK'_ex. Ultraviolet-visible (UV-Vis) spectroscopy analyses confirmed retention of AuCl₄^- speciation with the absorption maxima unchanged. This integrated framework can guide the data-driven design of IL extractants for DCE-based SX processes.