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 near liquid–liquid interface is of crucial importance in modern precise and controllable chemical reaction and separation processes. 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. 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 experiment model was employed to detect the specific effect from two typical background cations, Mg2+ and Al3+, on mass transfer of Er3+ ions in laminar boundary layer. It was revealed that competitive hydration and mass transfer of background ions is subject to aqueous shear flow rate and results in the difference in mass transfer rate of Er3+ ions. Aqueous shear flow increases mass transfer resistance of weakly hydrated Mg2+ ions towards the interface, while strongly hydrated Al3+ ions are more likely to be squeezed into the stagnant region in the boundary layer, because of its stronger interaction with the hydration shell around Er3+ ions. As a result, the influence from the interaction of extractant molecules with strongly hydrated Al3+ ions on mass transfer of Er3+ ions are significantly larger than that of weakly hydrated Mg2+ ions. It appears an obvious shear flow induced specific ion effect on enhanced difference in mass transfer of Er3+ ions. This work suggests a novel perspective to understand the essence of specific ion interfacial effect under shear flow. It is aimed to develop new strategies for achieving controllable reaction and separation on the flowing surface of fluids.