Surfactant-enhanced heterogeneity of the aqueous interface drives water extraction into organic solvents

Industrial liquid/liquid extraction (LLE) is the principal means of metal recovery and purification applied to a wide variety of solutions, from the dissolved ores in mining, to spent fuel from nuclear power plants. This approach is one of the most industrially relevant separation techniques, and is based at a fundamental level upon the variable solubility of solutes (or their complexes) between two immiscible solvents. While there is a natural energy driving force for a solute to partition into one solvent or another, the specific mechanism of transport across the phase boundary can be the rate limiting step in LLE. Further, it may be essential to the formation of metastable species that exist in the interfacial region, distinct reactivity, and be deleterious to post-processing.

One of the best practical examples is the formation and extraction of water–extractant adducts observed in the ternary water/organic/extractant systems. Unfortunately, despite its implications for LLE, a detailed description of the structural and dynamic mechanisms by which such adducts are formed at the interface is yet to be established. Describing that process, more so, necessitates connecting the evolving interfacial molecular organization in the presence of surfactants to dynamic surface fluctuations and interfacial heterogeneity.

Recently, Washington State University scientists Dr. Michael Servis and Professor Aurora Clark, from the Department of Chemistry employed molecular dynamics simulations combined with state-of-the-art network theory analysis to reveal features of interfacial structures and their relationship to the extraction of water in the water/n-hexane/ tri-n-butyl phosphate (TBP) system. Their goal was to present a holistic understanding of how interfacial heterogeneity and spatial fluctuations become amplified in the presence of surfactants, enabling water extraction into the organic phase. Their work is currently published in the research journal, Physical Chemistry Chemical Physics.

While a pure water/hexane interface exhibits molecular-scale roughness and long-range oscillations, known as capillary waves, the authors observed that the surfactant adsorption substantially increases interfacial roughness that is correlated with key changes in the interfacial hydrogen bonding. As expected, the surfactant TBP has strong interactions with interfacial water, forming hydrogen bonds, but unexpectedly, TBP decreases the number of hydrogen bonds that waters exactly at the interface with hexane have with the waters beneath them in the aqueous phase. By decreasing the water-water interactions, new interfacial species are formed, including the water-bridged TBP dimer. The capillary fluctuations at the interface become more extreme, such that protrusions of water are formed into the hexane phase. It is these protrusions that are the primary mechanism by which water is extracted into hexane.

In summary, Servis-Clark study presented an in-depth assessment of the adsorption of TBP at the water/n-hexane interface via MD simulation, in a bid to understand the effects of surfactants on the structural and dynamic properties of the interface and mechanisms of water extraction into the organic phase. Generally, their study enabled further exploration of how solution conditions can control interfacial behavior to create more efficient solvent extraction systems. Altogether, the results presented will inform future interfacial mass transport modeling and design of efficient LLE systems that can now utilize capillary wave behavior as a design principle.