Specific Ion Effect on the “Water Bridge” Interaction at an Interface: A New Understanding into the Extraction and Separation Selectivity Based upon Competitive Diffusion and Adsorption

Understanding the essence of a specific ion effect on the intermolecular interaction at the near-interface boundary layer during solvent extraction is crucial for developing new approaches to achieve enhanced extraction and separation of various valuable metals from complex aqueous solutions. The present work aims to detect the microscopic effect from competitive diffusion and adsorption of various salt cations in the near-interface boundary layer on their interaction with amphiphilic model molecules, 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (P507), at the air–water interface by using the Langmuir–Blodgett (LB) monolayer technique. It was found that the interaction between cations and P507 molecules through a long-range hydrogen bond “water bridge” confined in the near-interface boundary layer plays a crucial role in determining the difference in the diffusion mass transfer rate of ions with different hydration abilities. Conventional understanding about competitive extraction of various metal cations controlled mainly by their thermodynamic difference in the interaction intensity with the P–O and PO groups in P507 molecules cannot explain such a kind of specific ion effect on the mass transfer kinetics via ion hydration. Molecular dynamics simulation revealed that the hydration ability of ions is the key factor determining the mass transfer rate and interaction intensity. In the low salt concentration region, the interaction intensity of salt cations with the P–O and PO groups in P507 molecules is one of the determinants of the competitive adsorption behavior at the interface. However, in a higher salt concentration region, the specific ion salting-out effect via ion hydration becomes significant, causing a decrease in hydration of the target salt cation; therefore, it dominated the diffusion mass transfer rate of ions. This work provides a new insight to understand the kinetic role of competitive diffusion and adsorption of ions in the near-interface boundary layer and their effect on extraction and separation selectivity. It lays the foundation for achieving controllable separation of target metal ions by adjusting the coexisting ion species and their concentrations.