Solvent-Controlled Na + /K + Extraction and Transport by a Synthesized Pseudocyclic Ionophore: Insight into the Thermodynamic Sink via DFT

A novel pseudocyclic ionophore, 2-[(2-hydroxyethoxy)carbonyl]benzoic acid, was synthesized and evaluated for selective liquid-liquid extraction and bulk liquid membrane (BLM) transport of Na+ and K+ ions. The study revealed pronounced solvent-dependent behavior: the highest extraction occurred in chloroform (CHCl₃), whereas optimal transport was achieved in 1,2-dichloroethane (DCE), showing significant selectivity for Na+ over K+. The counter-anion also influenced the extraction and transport behavior, with picrate (Pic–) showing better performance than 2,4-dinitrophenolate (DNP–). Density Functional Theory (DFT) calculations at the B3LYP/6-31G(d,p) level with the PCM solvation model provided structural and thermodynamic rationale for these observations. Geometry optimizations indicated that the smaller Na+ radius allows for a more compact and electrostatically favorable coordination arrangement compared with K+. Thermodynamic analysis revealed that the greater theoretical binding stability in CHCl₃ acts as a “thermodynamic sink,” creating an energy barrier that retards ion release and consequently limits transport flux. Conversely, higher complex lability in DCE facilitates efficient ion release at the receiving phase, optimizing carrier-mediated transport. Frontier molecular orbital (FMO) analysis showed a slightly larger HOMO-LUMO gap for the Na+ complex, which was interpreted as a comparative electronic trend consistent with the observed Na+ preference. These results demonstrate that Na+ selectivity and transport efficiency rely on an optimum balance of geometrical fit, complex stability, counter-anion effects, and solvent-modulated thermodynamic lability, providing useful guidelines for designing next-generation ion-selective carriers.