The selective separation of ions from an aqueous phase using organophosphorus-based solvent extraction reagents has been widely implemented in rare-earth element recovery, environmental remediation, and laboratory-scale sample purifications. However, an understanding of the molecular-level mechanisms mediating solvent extraction is surprisingly lacking, which limits the development of new chemical separation schemes. The major challenge in understanding these processes at a liquid/liquid interface lies in the inherent complexity of the interface, where subtle changes in chemical makeup such as the ligand tail structure and concentration can have a profound influence on packing, coordination, and mass transfer from one bulk phase to another. Herein, we employ vibrational sum frequency generation (SFG) spectroscopy to investigate how the chemical make-up of a ligand affects adsorption thermodynamics, ordering, and the overall stabilization of liquid/liquid interfaces. Specifically, time-resolved SFG measurements were applied to access the interfacial structure and time-evolving variations in this interface that describe relevant mass transfer phenomena and underlying mechanisms on a molecular level. Further, comparison to the SFG data obtained at the air/liquid interface shows that the solvation in the oil phase plays a key but often overlooked role in understanding extraction mechanisms. Our results demonstrate that interactions in both the polar and nonpolar phases are essential for building functional self-assembled supramolecular structures capable of extraction.  This work further provides insights into designing ligands with enhanced functionality through <i>using</i> interfaces and intrinsically anisotropic interactions for targeted extractions.

Understanding the self assembly and stabilization of liquid /liquid interfaces with sum frequency generation spectroscopy

ACS Spring 2022

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