The Role of Interfacial Molecular Structure in the Adsorption of Ions at the Liquid-Liquid Interface

One of the most common and important interfaces is the boundary layer between an aqueous phase solution of ions and a hydrophobic medium. Whether the hydrophobic medium is a membrane, a macromolecular assembly, or a simple organic liquid, our molecular-level understanding of how ions in an adjacent aqueous phase approach, alter, and transport across the boundary is still quite deficient, largely due to experimental challenges in making the appropriate measurements. This paper reports some of the first measurements of the behavior of common inorganic ions at the interface between different aqueous phase salt solutions (NaCl, NaBr, NaNO3, and Na2SO4) and a hydrophobic organic liquid (CCl4). The results show that the ions reside within the interfacial region where they affect the water hydrogen bonding in a manner specific to the ion under study. Distinct differences in the behavior of ions at this interface relative to the air-water interface are found and discussed.