Shear Flow Induced Specific Ion Interfacial Effect on Enhanced Difference in Mass Transfer in the Boundary Layer

Abstract

Understanding the nature of specific ion effect on mass transfer in boundary layer of liquid-liquid interface is of crucial importance in modern precise and controllable chemical reactions and separations. In present work, a new phenomenon was noticed that aqueous shear flow near the boundary layer of oil-aqueous interface exhibits a significant influence on mass transfer selectivity of target ions, while the mass transfer rate of target ions in the background aqueous solutions containing strongly hydrated or weakly hydrated ions was obviously different. A thin-layered oil film based laminar flow model experiment was employed to detect the difference in mass transfer of Er 3+ , as the target ions, in the boundary layer near the oil-aqueous interface of dispersed oil droplets or spread oil film. Two typical cations, Mg 2+ and Al 3+ , with obvious difference in interface hydrophilic-hydrophobicity were used as the background ions to explore their effect on mass transfer of Er 3+ ions in the laminar boundary layer. It was revealed that competitive hydration and interfacial distribution of background ions are subject to aqueous shear flow rate. The shear flow induced specific ion effect results in an obvious difference in thinning of the stagnant region within the boundary layer and increases the mass transfer resistance of target ions towards the interface. Thus, the diffusive mass transfer rate of target ions in laminar boundary layer can be controllable based on the difference in hydration behavior of background ions. The present work suggests a novel perspective to understand the specific ion effect on surface of fluids. It is aimed to develop new strategies to achieve selective mass transfer of target ions.

Keywords: shear flow, mass transfer, boundary layer, Interface, specific ion effect, Hydration