Abstract

Ion selectivity in crown ether extractants arises from the interplay between hydration, molecular structure, and thermodynamics. Here, we present a computational study of ion selectivity in functionalized crown ethers, elucidating the roles of cavity size, functionalization, solvent environment, and partial hydration under liquid-liquid extraction conditions for Li+, Na+, K+, Mg2+, and Ca2+. Solvent effects were assessed by comparing chloroform and octane, identifying chloroform as the more suitable medium for stabilizing hydrated ion-crown complexes. Density functional theory calculations combined with an associative hydration model including explicit counterion competition show that selectivity cannot be rationalized solely by intrinsic ion-ligand complexation energies, but instead arises from the balance between dehydration, structural distortion, and partial hydration retention. C7-crown cavities stabilize hybrid crown-water coordination motifs, while functionalization modulates competition between ion-ligand and ion-water interactions. Phosphine-type crowns promote compact coordination environments and sharper selectivity, whereas benzene-type analogues retain more hydrated complexes. NBO and ALMO-EDA analyses indicate that ion-ligand interactions are dominated by electrostatics, although strong local interactions alone do not guarantee higher selectivity due to competing hydration effects. Overall, K+ emerges as the most selective ion, followed by Mg2+, while Na+ and Li+ show intermediate behavior, and Ca2+ remains less favored. These results indicate that effective crown ether design requires balancing cavity size and donor strength, with larger flexible C7-cavities favoring moderately hydrated ions and strong oxygen-donor functionalization enhancing competition with ion hydration.