Anion Binding in Solution: Beyond the Electrostatic Regime

A fundamental understanding of anion binding by receptors is essential for managing salts during energy, water, and food production. However, the limited understanding of solvent effects in ion recognition leads to a persistent blind spot that prevents effective receptor design. We experimentally discovered an underlying 1/epsilon(r) dependence of anion affinity on solvent dielectric constant (epsilon(r)). We found this relationship by measuring how chloride binds to macrocyclic triazolophane receptors across a wide range of solvents: epsilon(r) = 4.7-56.2. Solvent weakens affinity by screening electrostatics; electrostatics dominates when epsilon(r) < 4.7 (chloroform) and then transits a cross-over region (4.7 < epsilon(r) < 20.5; acetone), after which it no longer governs affinity (acetonitrile and DMSO). Density functional theory helped us understand this dependence. Our theory-backed model accurately predicts Cl- affinity in solvents used in liquid-liquid extractions in the nuclear fuel cycle. This model offers a general foundation for anion recognition and electrostatically driven complexation.