Ultrafast lasers probe elusive chemistry at the liquid-liquid interface


Date:
August 4, 2020
Source:
DOE/Oak Ridge National Laboratory
Summary:
Real-time measurements provide missing insight into chemical
separations to recover cobalt, a critical raw material used to
make batteries and magnets for modern technologies. Results track
the dynamics of molecules designed to grab cobalt from solutions
containing a mixture of similar species.



FULL STORY ========================================================================== Real-time measurements captured by researchers at the Department of
Energy's Oak Ridge National Laboratory provide missing insight into
chemical separations to recover cobalt, a critical raw material used to
make batteries and magnets for modern technologies.


========================================================================== Results published in ACS Applied Materials and Interfaces, track the
dynamics of molecules designed to grab cobalt from solutions containing
a mixture of similar species.

"Understanding the molecular events that make it possible to separate
elements is key to optimizing or creating new, tailored approaches for
broad areas of materials recovery," said Ben Doughty of ORNL's Chemical Sciences Division.

The study investigates the fundamental chemistry underlying solvent
extraction, a method of separating elements using two liquids that do
not dissolve into one another, namely oil and water.

When agitated, oil-and-water solutions will self-separate into distinct
layers.

The phenomenon can be used to transfer targeted materials dissolved in
one liquid phase to another, allowing specific elements like cobalt to
be separated from everything else in the mix.

"The catch is that you need to have molecules at the interface between
these liquid layers that are poised to bind selectively with the
materials you want to extract," said Doughty. "But the complex chemistry happening at the surface has not been well understood." Insight into
the chemical reactions that enable cobalt and other separations has
eluded researchers for decades, owing to the challenges of probing the liquid-liquid interface where oil and water meet. The molecularly thin
surface is akin to a needle in a haystack, tending to be obscured by the
bulk solution when traditional spectroscopic methods are used. Adding
to the difficulty are competing time scales of activity, ranging from femtoseconds -- one quadrillionth of a second -- to minutes, that
conventional static measurements do not capture.



========================================================================== "This interface is essentially the gatekeeper between oil and water
layers, where chemical bonds that facilitate extractions are made or
broken. To fine- tune the separation process, you need to understand
what is happening at this interface in real time," Doughty said.

ORNL is one of a few groups specializing in techniques to probe a
functioning liquid-liquid interface.

Building from previous work on polymers, the team looked at the ligand
di-(2- ethylhexyl) phosphoric acid, or DEHPA, an industry-standard
extractant that selectively binds with cobalt ions over similar metals
such as nickel that often naturally accompany cobalt in solution.

DEHPA dissolved in oil was introduced to water-based solutions with and
without cobalt and probed using vibrational sum frequency generation,
an ultrafast pulsed laser technique that allowed researchers to home in
on reactions taking place at the liquid-liquid interface.

What sets this technique apart from other experimental methods is the capability to track kinetics at the interface, or the changes taking
place at the surface during a chemical reaction.



========================================================================== "Solvent extraction is designed to work within specific conditions for a
given target, and pH is a commonly adjusted variable. So, our experiment
was set up to observe the influence of pH ranges on DEHPA and understand
what gives rise to the sweet spot for cobalt extraction," Doughty said.

The oil-based ligand interacts with water to form aggregates, or groups of molecules that play an important role in extractions. Their job is to bind
and transport cobalt, but they need to be the right size and structure
to work effectively. The team discovered that hydrogen bonds influence
the arrangement of these aggregates and are sensitive to pH changes.

"Our findings highlight the essential role hydrogen bonding plays in
developing new extraction methodologies," said Doughty. "Moreover, we
observed that the pH of the bulk solution impacts hydrogen bonding and
could potentially be adjusted to tune the liquid-liquid interface for
peak performance." Understanding the design rules for extraction opens
avenues for reducing the energy and environmental costs of processing
cobalt and, in turn, securing ethically sourced supply chains.

Cobalt recovery is just one example of how fundamental insight into
chemical separations could be beneficial. Informed strategies could be
applied to broad areas of critical materials recovery and nuclear waste
cleanup where solvent extraction methods are widely employed.


========================================================================== Story Source: Materials provided by
DOE/Oak_Ridge_National_Laboratory. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Azhad U. Chowdhury, Lu Lin, Benjamin Doughty. Hydrogen-Bond-Driven
Chemical Separations: Elucidating the Interfacial Steps of
Self-Assembly in Solvent Extraction. ACS Applied Materials &
Interfaces, 2020; 12 (28): 32119 DOI: 10.1021/acsami.0c06176 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200804165114.htm

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