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submitted on 03.03.2020 and posted on 04.03.2020by Azhad U. Chowdhury, Lu Lin, Benjamin Doughty
Chemical separations, particularly liquid extractions,
are pervasive in academic and industrial laboratories, yet a mechanistic
understanding of the events governing their function are obscured by
interfacial phenomena that are notoriously difficult to measure. In this work, we
investigate the fundamental steps of ligand self-assembly as driven by changes
in the interfacial H-bonding network using vibrational sum frequency
generation. Our results show how the bulk pH modulates the interfacial structure
of extractants at the buried oil/aqueous interface via the formation of unique
H-bonding networks that order and bridge ligands to produce self-assembled aggregates.
These extended H-bonded structures are key to the subsequent extraction of Co2+
from the aqueous phase in promoting micelle formation and subsequent ejection
of said micelle into the oil phase. The combination of static and time resolved
measurements reveals the mechanisms underlying complexities of liquid extractions
at high [Co2+]:[DEHPA] ratios by showing an evolution of interfacially
assembled structures that are readily tuned on a chemical basis by altering the
compositions of the aqueous phase. The results of this work point to new mechanistic
principles to design separations
through the manipulation of surface charge, electrostatic screening, and the
associated H-bonding networks that arise at the interface to facilitate
organization and subsequent extraction