Approximately 10-15% of the world’s energy use is spent on separating chemical species from complex mixtures.  Avenues to improve selectivity and efficiency in these processes could revolutionize sustainable chemistries while allowing for the recovery of increasingly in demand elements. Such advances are, however, hindered by a limited understanding of the interfacial mechanisms that mediate chemical separations. Here, we present new insight into the interfacial chemistry of solvent extraction at buried liquid/liquid interfaces using vibrational sum frequency generation (SFG) spectroscopy. Transient surface specific vibrational spectra describing the interfacial chemistry are measured <i>during</i> the extraction of technologically important Co<sup>2+</sup> ions from a bulk aqueous phase into a neighboring organic phase. Our results show the presence of extended H-bonding networks that are formed at the interface that bridge ligands into aggregate species, which are the entities that ultimately interact with ions in the aqueous phase and participate in the extraction.  The combination of pH dependent static and time-resolved measurements show an evolution of these interfacially assembled structures that are readily tuned by altering the compositions of the aqueous phase. The results of this work point to new design principles to control separations by tuning surface charge and associated H-bonding networks to facilitate organization and extraction.

Revealing the missing mechanistic steps of solvent extraction at buried liquid/liquid interfaces with vibrational sum frequency generation

ACS Spring Meeting 2021

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