posted on 2021-10-25, 13:52authored byPan Sun, Erik A. Binter, Zhu Liang, M. Alex Brown, Artem V. Gelis, Ilan Benjamin, Mrinal K. Bera, Binhua Lin, Wei Bu, Mark L. Schlossman
Solvent extraction
is used widely for chemical separations and
environmental remediation. Although the kinetics and efficiency of
this process rely upon the formation of ion–extractant complexes,
it has proven challenging to identify the location of ion–extractant
complexation within the solution and its impact on the separation.
Here, we use tensiometry and X-ray scattering to characterize the
surface of aqueous solutions of lanthanide chlorides and the water-soluble
extractant bis(2-ethylhexyl) phosphoric acid (HDEHP), in the absence
of a coexisting organic solvent. These studies restrict ion–extractant
interactions to the aqueous phase and its liquid–vapor interface,
allowing us to explore the consequences that one or the other is the
location of ion–extractant complexation. Unexpectedly, we find
that light lanthanides preferentially occupy the liquid–vapor
interface. This contradicts our expectation that heavy lanthanides
should have a higher interfacial density since they are preferentially
extracted by HDEHP in solvent extraction processes. These results
reveal the antagonistic role played by ion–extractant complexation
within the aqueous phase and clarify the advantages of complexation
at the interface. Extractants in common use are often soluble in water,
in addition to their organic phase solubility, and similar effects
to those described here are expected to be relevant to a variety of
separations processes.