Mesoscopic Aspects of Phase Transitions in a Solvent Extraction System

Abstract

In liquid-liquid extraction, organic phase splitting arises when high concentrations of polar solutes (acids/metal ions) are extracted. Herein, we investigate the mesoscopic roots that underpin phase splitting in alkane phases containing mixed amphiphiles, of contemporary interest in solvent extraction separation systems, by extracting various oxoacids. The oxoacids exhibited individual macroscopic (extractive and physical) behaviors, inducing phase splitting into heavy and light domains under markedly different conditions. Using small-angle X-ray scattering (SAXS) data analyzed using the generalized indirect Fourier transform (GIFT) method, we showed that, in all cases, acid extraction drove the self-assembly of reverse micelles into rods. These grew with increased acid extraction until reaching a critical length of 20 nm, at which point interactions produced interconnected cylinders or lamellar sheets that prelude phase splitting into heavy and light domains. In all cases, the heavy phase contained the same surfactant ratio-TBP (tri-n-butyl phosphate) and CMPO (octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide)-even though the concentrations of acid, water, and amphiphiles were markedly different. The remarkable similarities in structure and amphiphile stoichiometries underpinning phase splitting across the macroscopically different acid extraction series allude to the mesoscopic roots of organic phase behavior in solvent extraction. Our studies show that the structures underpinning phase splitting in solvent extraction systems are more complex than previously thought and are reminiscent of phase transitions in soft matter.

Solvent extraction: the coordination chemistry behind extractive metallurgy

“…Recent work on the extraction of lanthanides by malonamides provides important information on the composition and structures of the reverse micelles formed. 47 An understanding of the supramolecular chemistry involved in forming such large assemblies is key to defining the origin of the chemical potential which favours the transfer of the metal-containing species from a homogenous aqueous phase into the water pool in the reverse micelle. 48…”

Ion-Specific Clustering of Metal-Amphiphile Complexes in Rare Earth Separations

“…Although the exact role of these long-range interactions is yet to be clarified, this work adds to the growing body of evidence showing their significance in SX. 28,30,61 Further investigations of these mesoscale phenomena can help elucidate the different extraction trends from NO3versus SCNmedia, and the role of the co-extracted anions in general. Water content in the organic phase also needs to be monitored as it influences the morphology of the aggregates and is correlated with lanthanide extraction.…”

“…Small angle X-ray and neutron scattering (SAXS and SANS, respectively) have been the principal techniques in this effort and they show the link between aggregation structures on extraction efficiency and third phase formation (an undesired fluid-fluid phase transition and splitting due to high metal loading) in different extraction systems. [25][26][27][28][29][30][31][32][33] A significant portion of these studies used Baxter's sticky hard sphere model to describe the interactions between reverse micellar (RM) aggregates. In Baxter model, spherical aggregates attract each other when they are very close and show hard sphere repulsion when the distance between their centers is less than or equal to their hard sphere diameter (See results and discussion for details).…”

Reversed micelle synergistic extraction from phosphonium ionic liquid extractants in diluent for rare earth

“…When the extractant is dissolved in a nonpolar diluent, it represents a surfactant-in-oil system. 29 In some cases, the principal role of extractant is not as a ligand but as a surfactant, stabilizing reverse micelle in which the metal is found in the "water pool." 30 When the reversed micelles dominate an extraction reaction, the rather higher slope may be obtained via slope analysis.…”