Gibbs Transfer Energies of Ions from a Mixed Solvent of 2H,3H-Decafluoropentane and 1,2-Dichloroethane to Water

Electrochemistry in bicontinuous microemulsions derived from two immiscible electrolyte solutions for a membrane-free redox flow battery

HYPOTHESES

Bicontinuous microemulsions (BMEs) have attracted attention as unique heterogeneous mixture for electrochemistry. An interface between two immiscible electrolyte solutions (ITIES) is an electrochemical system that straddles the interface between a saline and an organic solvent with a lipophilic electrolyte. Although most BMEs have been reported with nonpolar oils, such as toluene and fatty acids, it should be possible to construct a sponge-like three-dimensionally expanded ITIES comprising a BME phase.

EXPERIMENTS

Dichloromethane (DCM)-water microemulsions stabilized by a surfactant were investigated in terms of the concentrations of co-surfactants and hydrophilic/lipophilic salts. A Winsor III microemulsion three-layer system, consisting of an upper saline phase, a middle BME phase, and a lower DCM phase, was prepared, and electrochemistry was conducted in each phase.

FINDINGS

We found the conditions for ITIES-BME phases. Regardless of where the three electrodes were placed in the macroscopically heterogeneous three-layer system, electrochemistry was possible, as in a homogeneous electrolyte solution. This indicates that the anodic and cathodic reactions can be divided into two immiscible solution phases. A redox flow battery comprising a three-layer system with a BME as the middle phase was demonstrated, paving the way for applications such as electrolysis synthesis and secondary batteries.

Light-induced electron transfer/phase migration of a redox mediator for photocatalytic C-C coupling in a biphasic solution

Photocatalytic molecular conversions that lead to value-added chemicals are of considerable interest. To achieve highly efficient photocatalytic reactions, it is equally important as it is challenging to construct systems that enable effective charge separation. Here, we demonstrate that the rational construction of a biphasic solution system with a ferrocenium/ferrocene (Fc⁺/Fc) redox couple enables efficient photocatalysis by spatial charge separation using the liquid-liquid interface. In a single-phase system, exposure of a 1,2-dichloroethane (DCE) solution containing a Ru(II)- or Ir(III)-based photosensitizer, Fc, and benzyl bromide (Bn-Br) to visible-light irradiation failed to generate any product. However, the photolysis in a H₂O/DCE biphasic solution, where the compounds are initially distributed in the DCE phase, facilitated the reductive coupling of Bn-Br to dibenzyl (Bn₂) using Fc as an electron donor. The key result of this study is that Fc⁺, generated by photooxidation of Fc in the DCE phase, migrates to the aqueous phase due to the drastic change in its partition coefficient compared to that of Fc. This liquid-liquid phase migration of the mediator is essential for facilitating the reduction of Bn-Br in the DCE phase as it suppresses backward charge recombination. The co-existence of anions can further modify the driving force of phase migration of Fc⁺ depending on their hydrophilicity; the best photocatalytic activity was obtained with a turnover frequency of 79.5 h^-1 and a quantum efficiency of 0.2% for the formation of Bn₂ by adding NBu₄⁺Br^- to the biphasic solution. This study showcases a potential approach for rectifying electron transfer with suppressed charge recombination to achieve efficient photocatalysis.

Fluorination Effect on the Gibbs Transfer Energy for Methylene Group from 1,2-Dichloroethane or 1,1,1,2,3,4,4,5,5,5-​Decafluoropentane to Water

The ion-transfer reactions of alkyl and perfluoroalkyl carboxylate ions (CH₃(CH₂)_n-2COO^- and CF₃(CF₂)_n-2COO^-) at 1,2-dichloroethane (DCE) | water (W) and 1,1,1,2,3,4,4,5,5,5-decafluoropentane (DFP) | W interfaces were investigated. These ions gave reversible or quasi-reversible voltammetric waves due to their ion transfer across the interfaces, and the formal potentials and the formal Gibbs transfer energies from a non-aqueous solvent to water, ΔG°^'_tr,α→W (α = DCE and DFP), were determined. The ΔG°^'_tr,α→W for CH₃(CH₂)_n-2COO^- and CF₃(CF₂)_n-2COO^- linearly increased with n, allowing for estimating ΔG°^'_tr,α→W for methylene groups. The estimated value of ΔG°^'_tr,DCE→W for the -CH₂- group was higher than that of ΔG°^'_tr,DCE→W for the -CH₂- group, whereas the ΔG°^'_tr,DCE→W for the -CF₂- group was lower than that of ΔG°^'_tr,DCE→W for the -CF₂- group, indicating that the -CH₂- (or -CF₂-) group is more favorably (or unfavorably) solvated in the DCE phase compared to the DFP phase. From the estimated values, the fluorination effect of alkyl chains on partitioning the alkyl group between the biphase media has also been discussed.

A Theoretical Approach to the Fluorophilicity of Ions via the Gibbs Energy of Ion Transfer at the Fluorous Solvent/Water Interface

The non-Bornian solvation model has been applied to a theoretical consideration of the Gibbs free energy for the transfer of fluorinated anions, non-fluorinated cations, and non-fluorinated anions at the 2H,3H-decafluoropentane (DFP)/water (W) and 1,2-dichloroethane (DCE)/W interfaces. According to our previous experimental results, the fluorinated anions are more stable in DFP than DCE, while the non-fluorinated cations and anions are less stable in DFP. To understand this characteristic feature of DFP, energy decomposition analyses have been performed for the hypothetical transfer of ions at the DFP/DCE interface. In conclusion, the characteristics of DFP as a fluorous solvent should be explained in terms of the higher repulsive interaction of the solvent molecule with ions, particularly with non-fluorinated ions.

Ion-Transfer Voltammetry at Fluorous Ether | Water Interfaces

This paper describes that fluorous ethers, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, can be used for a non-aqueous medium in electrochemistry at a liquid | liquid interface. These solvents dissolved a high concentration of tetraalkylammonium salts with highly-fluorinated anions, such as bis(nonafluorobutanesulfonyl)imide and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ions, and the solution had a high conductivity. The fluorous ether | water interfaces exhibited a substantial polarizable potential window, and various ions, including tetraphenylarsonium and tetraphenylborate ions, gave a reversible voltammetric wave due to their ion transfer across the interfaces. Using the tetraphenylarsonium-tetraphenylborate assumption, the formal potentials for the ion transfer and the formal Gibbs energies of ion transfer from the ethers to water were estimated. The Gibbs energies were close to those from a previously reported fluorous solvent, 1,1,1,2,3,4,4,5,5,5-decafluoropentane; the fluorous ethers also exhibited a higher affinity for fluorinated ions. Because of a lower volatility, the fluorous ethers would be more advantageously used in two-phase electrochemistry, particularly concerning analytical purposes.