Competitive Interactions at Electrolyte/Octanol Interfaces: A Molecular Perspective

Abstract

Much is understood about electrolyte liquid/liquid interfaces, yet the relationships between ion solvation, adsorption, and the instantaneous surface have not been the topic of significant study. The thermally corrugated capillary wave characteristics of the instantaneous aqueous surface contribute to heterogeneous interfacial structural and dynamic properties. Those properties are sensitive the nature of the immiscible nonpolar solvent. In this work, we examine the role of interfacial heterogeneity upon ion behavior and further, how this is influenced by a partially polar solvent relative to a vapor phase analog. We compare and contrast ion solvation in electrolyte/vapor and electrolyte/octanol biphasic systems, focusing upon the changes to interfacial heterogeneity in the presence of the octanol solvent and the variations of ion concentration at different interfacial regions. The interplay between competing forces introduced by strong octanol water interactions at the interface is examined, with a new understanding of how such competition may lead to tailored interfacial properties.

Unexpected inverse correlations and cooperativity in ion-pair phase transfer

“…Yet the distribution of Li + is into the crest regions is in sharp contrast to prior observations in the absence of a strongly interacting surfactant and it is well-known that loss of solvating H 2 O is thermodynamically unfavorable for the Li + . 10,64,65 In combination, these data provide indirect evidence of strong Li + •••TBP interactions within the crest region of the instantaneous surface, where TBP replaces one H 2 O in the first coordination shell (Figure S18). Direct analysis of the density of Li(NO 3 ) m (TBP) -m+1 n species along the µ axis reveals a predominance of Li(NO 3 )(TBP) 2 that systematically grows from the trough to the crest region of the instantaneous surface (Figure 5A).…”

“…ITIM analysis is then performed on the molecules and ions that comprise the bulk electrolyte network for the identification of truly interfacial molecules/ions using a probe sphere of radius 1.5 Å. 10,50 Using the mean of the density of the waters within the instantaneous surface the z-axis is modified such that the mean is set to µ = 0 (Figure 1). Molecules within the instantaneous surface with negative µ are considered to be in extending into the organic phase and within the "crest" region of the capillary wave front, while those with positive µ values extend into the aqueous phase and are part of the "trough" region.…”

“…Understanding the changes to metal ion speciation within the interfacial region, the distribution of ions, and organizational features of the instantaneous water/oil interface are an active and vibrant ongoing area of research. [5][6][7][8][9][10] Yet the specific mechanisms of transport (particularly role within LLE and are often employed to increase the distribution ratio of metal ions of interest, [21][22][23] though the mechanism of how this occurs is not understood. Interestingly, these electrolytes are often co-extracted alongside other unwanted species, like HNO 3 and H 2 O.…”

Uranyl Speciation in the Presence of Specific Ion Gradients at the Electrolyte/Organic Interface

“…78 Prior work has demonstrated that Li + sheds solvating H 2 O within the instantaneous surface, which disfavors residence therein. 61 When combined with observation of relatively consistent NO − 3 concentration in the instantaneous surface and subjacent layers, the negative charge density in the region directly contacting the organic phase is presumed to be an outcome of Li + cation depletion in the instantaneous surface rather than anionic excess.…”

“…The speciation, ion concentrations, residence times, and other properties were calculated in each slab and then compared to the analogous metrics of species present the instantaneous surface of the water. The Identification of Truly Interfacial Molecules (ITIM) algorithm 60,61 was employed to define the instantaneous surface of water and ions directly in contact with the organic phase, and for the comparison of the speciation and dynamic properties of the ions in the slabs vs. the instantaneous surface. The density of molecules in the instantaneous surface is fitted to a Gaussian function to obtain the position along z of the mean µ 0 of the distribution.…”