Molecular Recognition and Hydration Energy Mismatch Combine To Inform Ion Binding Selectivity at Aqueous Interfaces

Abstract

There is a critical need for receptors that are designed to enhance anion binding selectivity at aqueous interfaces in light of the growing importance of separation technologies for environmental sustainability. Here, we conducted the first study of anion binding selectivity across a series of prevalent inorganic oxoanions and halides that bind to a positively charged guanidinium receptor anchored to an aqueous interface. Vibrational sum frequency generation spectroscopy and infrared reflection absorption spectroscopy studies at the water–air interface reveal that the guanidinium receptor binds to an oxoanion series in the order SO4 2– > H2PO4 – > NO3 – > NO2 – while harboring very weak interactions with the halides in the order I– > Cl– ≈ Br–. In spite of large dehydration penalties for sulfate and phosphate, the more weakly hydrated guanidinium receptor was selective for these oxoanions in contradiction to predictions made from ion partitioning alone, like the Hofmeister series and Collins’s rules. Instead, sulfate binding is likely favored by the suppression of dielectric screening at the interface that consequently boosts Coulombic attractions, and thus helps offset the costs of anion dehydration. Geometric factors also favor the oxoanions. Furthermore, the unique placement of iodide in our halide series ahead of the stronger hydrogen-bond acceptors (Cl–, Br–) suggests that the binding interaction also depends upon single-ion surface partitioning from bulk water to the interface. Knowledge of the anion binding preferences displayed by a guanidinium receptor sheds light on the receptor architectures needed within designer interfaces to control selectivity.

Ion Pairing Mediates Molecular Organization Across Liquid/Liquid Interfaces

“…These ordered water molecules are probed via the spectrally broad −OH stretches that extend into the −CH stretching region. As we will show, and has been reported elsewhere, − the interfacial field can be screened by the presence of anions at the interface. Thus, the −OH signals describe the extent of charge screening from the perspective of the bulk water molecules and can be used to separate electrostatic and pairing effects when considered in conjunction with tail conformations and MD simulations.…”

Sum-Frequency Vibrational Spectroscopic Study of the Cation−π Interaction: Amine and Guanidine

“…The strong OH signal indicates that the ODG molecules are protonated (p K a ∼ 13.8 for arginine guanidinium), and positive charges of the headgroups outweigh the charge compensation of the adsorbed Cl – counterions. Previous work investigated a surfactant having a guanidine headgroup with different types of salts in the subphase by sum-frequency vibrational spectroscopy . With two alkyl chains per molecule, the surfactant formed a stable monolayer.…”

“…Previous work investigated a surfactant having a guanidine headgroup with different types of salts in the subphase by sum-frequency vibrational spectroscopy. 35 With two alkyl chains per molecule, the surfactant formed a stable monolayer. After adding salt into the solution, the OH signal decreased due to the screening by the counterions, and among different types of anions compared such as oxoanions and halides, SO 4 2− was found to be the most effective.…”

Antagonistic Role of Aqueous Complexation in the Solvent Extraction and Separation of Rare Earth Ions

“…Interface-sensitive experimental methods have begun to probe the structure of interfacial ion–extractant complexes and the role of hydrogen bonding in their formation and stabilization at the aqueous–organic interface. − For instance, a thermal switch was developed to arrest interfacial complexes of lanthanides and water-insoluble extractants formed in the midst of the extraction process, thereby allowing for the characterization of intermediate interfacial states by X-ray scattering. , Model systems of water-insoluble surfactant monolayers at the liquid–vapor interface have also been studied by interface-sensitive methods to explore the interactions between ions and surfactants whose headgroups mimic those of weakly amphiphilic extractants. − Although studies at the liquid–vapor interface cannot model ion transport through the interface or structures that may form uniquely at a liquid–liquid interface, they may be able to model many aspects of the nanoscale variation of density, dielectric properties, and water structuring near a liquid interface that can influence ion–extractant complexation. However, studies of water-insoluble surfactants or extractants cannot address the issue of the location of complexation of water-soluble extractants.…”