Controllable molecular orientation at the interface and coordination with second hydration-sphere to enhance separation of Pt(IV) and Pd(II) in hydrochloric acid solutions: Mass transfer based separation and molecular dynamics

Abstract

In conventional liquid-liquid extraction, Pt(IV) and Pd(II) chloride complex anions PtCl62− and PdCl42− in concentrated hydrochloric acid cannot be separated using simple amine extractants according to the ion association mechanism based on the thermodynamic difference in the interaction of their inner sphere coordinated chloride ions with protonated amine. This study exhibits a novel strategy for enhancing the separation of PtCl62− and PdCl42− anions using thin-layer organic membrane-based laminar flow (TOMLF) extraction by controlling the acidified primary amine N1923 (A-N1923) molecular orientation on the surface of concentrated hydrochloric acid with a laminar shear flow. Experiments revealed that the separation selectivity is primarily determined by the interaction of A-N1923 molecules with the outer sphere-bound water molecules around PtCl62− and PdCl42− anions, and is easily controlled by regulating the A-N1923 molecular orientation on the surface of laminar flow. It is a new phenomenon that has not been previously documented. The evolution of A-N1923 molecular orientation and complexation sites induced by laminar shear flow would result in a controllable spatial geometry recognition, therefore promoting or inhibiting the separation of PtCl62− and PdCl42− anions in concentrated hydrochloric acid. This work is expected to provide new insight into the development of hydration-sphere coordination strategies based upon shear-switch laminar flow for controllable host-guest molecule recognition at a flowing liquid-liquid interface.