Shear laminar flow switch-induced soft molecular recognition at liquid-liquid interface

Understanding the self-assembly behavior of macrocyclic molecules under shear flow and its impact on the mass transfer of target ions near the liquid-liquid interface is of great importance in host-guest chemistry and mordern separation processes. In this work, a new strategy based on soft-film molecular recognition (SF-MRT) was proposed to investigate the effect of aqueous shear flow on specific recognition of octahedral RhCl63-, octahedral PtCl62-, and planar quadrilateral PdCl42- anions with different hydration configurations using 18-crown-6 as an extractant spreaded on the surface of concentrated hydrochloric acid. Remarkably, the SF-MRT process achieved an extraction order of RhCl63- > PdCl42- > PtCl62-, whereas the impregnated resin coated with a layer of oil film containing 18-crown-6 yielded a reverse extraction order of PtCl62- > PdCl42- > RhCl63-. Experimental results demonstrated the functional group (-SO3Na+) present on the surface of the resin particles exhibits a preferential adsorption of H3O+ ions through the cation exchange mechanism, resulting in crown ether molecules only recognizing H3O+ ions, and subsequently facilitating the preferential extraction of PtCl62- anions by 18-crown-6H3O+ through the ionic association mechanism. However, during the SF-MRT process, the hydrophilic and flexible cavity formed by the ether oxygen group (-O-) in crown ether molecules primarily recognizes the water molecules in the hydration shell surrounding platinum metal anions due to the lower concentration of H3O+ dissociated on the surface of concentrated hydrochloric acid. Compared to the sparse water molecules around the hydrated PtCl62- anions, the dense water molecules around the hydrated RhCl63- anions are more easily recognized. The directional and flexible hydrogen bond network formed between the crown ether molecules and platinum metal anions through the "water bridges" interaction is more amenable to be controlled by the laminar shear flow beneath the thin-layer extractant oil film. This discovery lays the foundation for the development of new flexible and controllable molecular recognition methods.