Tracking Molecular Transport Across Oil/Aqueous Interfaces: Insight into Antagonistic Binding in Solvent Extraction

Liquid/liquid (L/L) interfaces play a key, yet poorlyunderstood,role in a range of complex chemical phenomena where time-evolvinginterfacial structures and transient supramolecular assemblies actas gatekeepers to function. Here, we employ surface-specific vibrationalsum frequency generation combined with neutron and X-ray scatteringmethods to track the transport of dioctyl phosphoric acid (DOP) anddi-(2-ethylhexyl) phosphoric acid (DEHPA) ligands used in solventextraction at buried oil/aqueous interfaces away from equilibrium.Our results show evidence for a dynamic interfacial restructuringat low ligand concentrations in contrast to expectation. These time-varyinginterfaces arise from the transport of sparingly soluble interfacialligands into the neighboring aqueous phase. These results supporta proposed antagonistic role of ligand complexationin the aqueous phase that could serve as a holdback mechanism in kineticliquid extractions. These findings provide new insights into interfaciallycontrolled chemical transport at L/L interfaces and how these interfacesvary chemically, structurally, and temporally in a concentration-dependentmanner and present potential avenues to design selective kinetic separations.

Automated summary

Liquid/liquid (L/L) interfaces play a crucial yet poorly understood role in various complex chemical phenomena, acting as gatekeepers to function. By using surface-specific vibrational sum frequency generation combined with neutron and X-ray scattering methods, we track the transport of DOP and DEHPA ligands in solvent extraction at buried oil/aqueous interfaces away from equilibrium. Our results show evidence of dynamic interfacial restructuring at low ligand concentrations, contrary to expectations. These findings provide new insights into interfacially controlled chemical transport at L/L interfaces, presenting potential avenues to design selective kinetic separations.