Shuttling Mechanism of Ion Transfer at the Interface between Two Immiscible Liquids

Abstract

The transfers of hydrophilic ions between aqueous and organic phases are ubiquitous in biological and technological systems. These energetically unfavorable processes can be facilitated either by small molecules (ionophores) or by ion-transport proteins. In absence of a facilitating agent, ion-transfer reactions are assumed to be "simple", one-step processes. Our experiments at the nanometer-sized interfaces between water and neat organic solvents showed that the generally accepted one-step mechanism cannot explain important features of transfer processes for a wide class of ions including metal cations, protons, and hydrophilic anions. The proposed new mechanism of ion transfer involves transient interfacial ion paring and shuttling of a hydrophilic ion across the mixed-solvent layer.

Void‐Assisted Ion‐Paired Proton Transfer at Water–Ionic Liquid Interfaces

“…Mirkin and co-workers proposed af acilitated transfer, from water to low permittivity solvents,o fh ydrophilic ions by hydrophobic counter ions. [17] This process involves atransient ion-pairing that facilitates the transfer across the interface, which agrees with the phenomenon observed here,although it fails to explain the exclusion of alkali metal cations.…”

Mechanism of the Transfer of AuCl4– and TOA+ Ions Across the Liquid/Liquid Interface

“…Such a phenomenon is similar to the transfer of hydrophilic cations from an aqueous phase into a DCE phase containing tiny amounts of hydrophobic counterions. 20 The only main difference is that a small amount of simple AuCl 4 − can transfer into the neat DCE phase (see the purple curve in Figure 1), but hydrophilic cations cannot transfer into the neat DCE phase. 20 This indicates that the mechanism of the transfer of AuCl 4 − is a combination of simple AuCl 4 − ion transfer and the shuttling of AuCl 4 − across the ITIES by TOA + in the interface region (mixed-solvent layer region).…”

“…20 The only main difference is that a small amount of simple AuCl 4 − can transfer into the neat DCE phase (see the purple curve in Figure 1), but hydrophilic cations cannot transfer into the neat DCE phase. 20 This indicates that the mechanism of the transfer of AuCl 4 − is a combination of simple AuCl 4 − ion transfer and the shuttling of AuCl 4 − across the ITIES by TOA + in the interface region (mixed-solvent layer region). As described in ref 20, the shuttling mechanism works in such a way: under the influence of the electric field, AuCl 4 − and TOA + will move toward the mixed-solvent layer and form a short-lived ion pair in the layer and the ion pair then diffuses across the mixed-solvent layer toward the organic phase and dissociates at the boundary of the mixed-solvent layer and the organic bulk phase; after dissociation, TOA + will diffuse into the mixed-solvent layer to facilitate the transfer of the next AuCl 4 − ion and the AuCl 4 − will enter the organic bulk phase; this process is also driven by the electric field.…”

Chronoamperometric response at nanoscale liquid–liquid interface arrays

“…To date, nano-scale ITIES (nanoITIES) have been prepared in two ways: (a) those supported at the tip of a single [7][8][9] or dual [9,10] nanopipette (producing single or double nanoITIES) and (b) those produced by placing nanoporous materials containing geometrically irregular or regular pore arrays at the ITIES [11].…”