Synergistic tuning of electronic effect and steric hindrance for selective Li+ extraction and Li/Mg separation in organophosphate–ionic liquid systems

Dual roles of electronic effect and steric hindrance in NOPs are decoupled. • A structure-performance paradox in linear and branched NOPs is revealed. • The intrinsic Li + /Mg 2+ micro-solvation divergence (CIP vs. SSIP) is uncovered. • A dual-parameter model is established to predict extraction capacity and selectivity. This study proposes a synergistic electronic effect and steric hindrance regulation mechanism in neutral organophosphate-extraction systems targeting efficient Li and Mg extraction from salt lake brines. Using tributyl phosphate (TBP), tripentyl phosphate (TAP), and triisoamyl phosphate (TIAP) as model extractants, we decoupled the dual roles of linear-chain elongation and branching in alkyl chain structures. The former enhances the P O electron density and intrinsic Li + affinity via the positive inductive effect, whereas branched structures govern the selectivity via steric hindrance. Combined molecular dynamics (MD) simulations and density functional theory (DFT) calculations revealed that Li + forms a compact four-coordinate contact-ion pair (CIP, [Li·2L·H 2 O·TFSI]), whereas Mg 2+ exists as a bulky six-coordinate solvent-separated ion pair (SSIP, [Mg·4L·2H 2 O] 2+ ). This difference considerably amplifies steric sieving. Specifically, the bulky branched chains of TIAP impose a severe conformational energy penalty (97.5 kJ/mol) on the crowded Mg 2+ shell, strongly suppressing Mg 2+ coordination. Consequently, the extraction efficiency of TIAP is comparatively low in pure Li + systems (60.5%) but surges to 83% in high-Mg backgrounds under the synergistic action of steric Mg 2+ rejection and the Hofmeister effect, achieving a superior separation factor of 215.0. Finally, a quantitative dual-parameter model that accurately explains the extraction capacity (TAP > TBP > TIAP) and selectivity (TIAP > TAP > TBP) trends was established. This study provides deep molecular insights into Li + /Mg 2+ separation along with a robust theoretical framework for rational design of selective ligands for complex brines. Download: Download high-res image (94KB) Download: Download full-size image