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

Abstract

At the water–trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate ([P14,6,6,6][FAP]) ionic liquid interface, the unusual electrochemical transfer behavior of protons (H+) and deuterium ions (D+) was identified. Alkali metal cations (such as Li+, Na+, K+) did not undergo this transfer. H+/D+ transfers were assisted by the hydrophobic counter anion of the ionic liquid, [FAP]−, resulting in the formation of a mixed capacitive layer from the filling of the latent voids within the anisotropic ionic liquid structure. This phenomenon could impact areas such as proton-coupled electron transfers, fuel cells, and hydrogen storage where ionic liquids are used as aprotic solvents.

Development and Application of Supported Ionic Liquid Membranes in Microbial Fuel Cell Technology: A Concise Overview

“…On the one hand, at immiscible water/IL interfaces, it seems that H + transfers into the IL via so-called void-assisted ion-paired proton transfer, which means that proton transport is facilitated by pairing with hydrophobic anions and filling the voids as a capacitive layer in the interfacial IL phase, as it was found for highly hydrophobic IL with a [P 66614 ] + cation and [FAP] − (tris(-pentafluoroethyl)trifluorophosphate) anion [43]. On the other hand, alkali metal cations did not follow such facilitated transfer, as their hydrated ionic radii exceeded the estimated size of the voids (being a consequence of the anisotropic nature of ILs) [43]. In the IL bulk phase, although proton transport becomes hindered by the non-polar alkyl groups of the cations, protons have a higher mobility than other diffusing cationic species.…”

“…In general, ILs with hydrophobic anions are applied for bioelectrochemical applications in the form of a membrane (SILM, in most cases), in order to minimize the water uptake of the liquid membrane and the leakage of IL from the supporting layer. Although the ILs commonly used for such a purpose (prepared by using [ [42][43][44][45]. Moreover, it is known that in addition to the positive effect of a longer alkyl chain length of the cation on the hydrophobicity of IL, mainly the anion defines the hydrophobicity and the extent of miscibility with water [43,44,46,47].…”

“…Although the ILs commonly used for such a purpose (prepared by using [ [42][43][44][45]. Moreover, it is known that in addition to the positive effect of a longer alkyl chain length of the cation on the hydrophobicity of IL, mainly the anion defines the hydrophobicity and the extent of miscibility with water [43,44,46,47]. Therefore, the hygroscopic character and sensitivity towards hydrolysis of [PF 6 ] − may cause considerable water solubility and simultaneously, the dissolution of IL in the aqueous phase (the latter aspect is minor, but still, the contamination of aqueous phase by IL is best avoided) [34,45].…”

Investigating the Proton and Ion Transfer Properties of Supported Ionic Liquid Membranes Prepared for Bioelectrochemical Applications Using Hydrophobic Imidazolium-Type Ionic Liquids

“…Based on this, although the question of the exact proton transfer mechanism through [bmim][NTf 2 ] still remains open, it can at least be stated that the role of [NTf 2 ] − anion could be important in that matter. It was already proposed that the ion transfer in aprotic ILs proceeds via vehicle mechanism, and that the anisotropic cation-anion structure in [bmim][NTf 2 ] results in a wider free space for anion motion, which could play a role in proton transfer [ 51 , 52 , 53 ]. However, many of the conclusions are based solely on computational data, and in order to understand the mechanism, further research is needed.…”

Interfacial doping of carbon nanotubes at the polarisable organic/water interface: a liquid/liquid pseudo-capacitor

“…31,32 Such interfaces can have an impact in areas such as proton-coupled electron transfer, fuel cells, and hydrogen storage, where ionic liquids are used as aprotic solvents. 33 In our laboratory, the reduction of aqueous metal salts by a lipophilic electron donor, with the heterogeneous electron transfer proceeding via SWCNTs/graphene adsorbed at the ITIES, has been employed to functionalise these materials with Pt, Pd and Au metal nanoparticles (NPs). [34][35][36] In combination with an ex situ metal deposition step, prior to assembly at the ITIES, this methodology was used to asymmetrically decorate both sides of a chemical vapour deposition (CVD) grown graphene sheet with Au and Pd NPs.…”