Influence of aqueous ionic strength upon liquid:liquid interfacial structure and microsolvation

Abstract

Ionic strength of an aqueous solution is often used to alter the efficacy of industrial processes that rely upon liquid:liquid phase boundaries (e.g. solvent extraction), however there is little understanding of how this condition may alter the properties of the phase boundary itself. The current work examines the interfacial structure and processes within the biphasic system (NaNO 3) aq :n-hexane from 0-10 M NaNO 3 at 298 K and 1 atm pressure. The extent of ion-pairing was quantified, alongside the primary solvation environments of both water and ions. Ion concentration was found to be depressed in the interfacial region relative to the bulk, and as such there is less likelihood of the formation of contact ion-pairs or ion-clusters at the interface. This in turn, leads to the interesting observation of more hydrogen bonds per water at the interface than in the bulk. At 10 M NaNO 3 enough ions are present at the interface to severely perturb the mesoscopic interfacial properties of width and tension (the latter doubling in size relative to the neat water interface with n-hexane). Chemical theories, like the Kelvin equation and its analogues for liquid:vapor interfaces, would intuit that such a large change in interfacial tension should influence the microsolvation processes of the immiscible solvents (microsolvation being the rare event where water can penetrate n-hexane and n-hexane may penetrate and be fully solvated by water). However, there is no statistical difference between the concentrations of water and n-hexane in their respective co-solvents as a function of interfacial tension in these non-ideal solutions. Based upon these data, the ability of electrolyte concentration to alter transport across the interface does not appear to be related to the perturbation imparted by the electrolyte upon the interfacial properties themselves.

Surface Energies and Structure of Salt–Brine Interfaces

“…This is consistent with the trigonal bipyramid structure reported for Na + from density functional theory (DFT). 70−72 Bond angle distributions (BADs) provide a method of further probing the structure of the dissolved ions, which can vary with concentration 20,73 and type of ion. 72,74−79 The O−Na + −O BAD identifies that, due to the high molar concentration of NaCl in solution, the structure of the hydration shell is distorted and does not strictly adhere to the coordination geometry.…”

Competitive Interactions at Electrolyte/Octanol Interfaces: A Molecular Perspective

“…3,4,51,[56][57][58][59] Specific ion effects are also observed (in this work and others), where small cations like Na + decrease d (and increase interfacial tension) more strongly than larger ions like Cs + . 60,61 Although these observations are well-known for electrolyte/vapor and interfaces with non-polar solvents, the outcome in the presence of a polar immiscible solvent is not well studied.…”

Considerations on film reactivity in the aqueous biphasic hydroformylation

“…During the last decades the spectroscopy technology has been improved significantly. This enables scientist to discover interfaces using IR and Raman spectroscopy with nm resolution . A probe is immersed in the first liquid and is moved via piezoelectric motor until it reaches the second liquids surface.…”