Ion Distributions at Electrified Water-Organic Interfaces: PB-PMF Calculations and Impedance Spectroscopy Measurements

Abstract

The interface between two immiscible electrolyte solutions consisting of alkali chlorides in water and the organic electrolyte BTPPATPFB in 1,2-dichloroethane is characterized with X-ray reflectivity, interfacial tension and impedance spectroscopy measurements over a range of applied voltage between the bulk solutions. X-ray reflectivity probes the interfacial ion distribution on the sub-nanometer length scale, whereas interfacial tension and impedance spectroscopy characterize quantities such as interfacial excess charge and differential capacitance that represent integrations over the interfacial ion distribution. Predictions of interfacial ion distributions by the recently introduced PB-PMF method, which combines Poisson's equation with ion potentials of mean force, provide excellent agreement, within one to two experimental standard deviations, with both X-ray reflectivity and interfacial tension measurements. However, the agreement with the differential capacitance measured by impedance spectroscopy, and modeled by the Randles equivalent circuit, is not as good. Values of measured and calculated differential capacitance can deviate by as much as 20% for applied electric potential differences larger than approximately ±100 mV. These comparisons indicate that our understanding of the ion distributions that underlie these measurements is adequate, but that further understanding of the modeling of impedance spectroscopy data is required for quantitative agreement at larger applied electric potential differences. Ion distributions at interfaces underlie many electrochemical and biological processes, including electron and ion transfer across biomembranes and liquid interfaces, phase transfer catalysis and solvent extraction. The interface between two immiscible electrolyte solutions (ITIES) has been used as a model system to study ion and electron transfer across liquid-liquid interfaces by electrochemical techniques.1,2 These techniques characterize the interfacial ion distribution in terms of integrated properties such as interfacial excess charge or differential capacitance, which can be calculated by integrating the ion distribution over the spatial coordinate perpendicular to the interface.The differential capacitance of ITIES has been widely investigated.3-5 Impedance spectroscopy data for ITIES are often modeled by the Randles equivalent circuit to yield the interfacial differential capacitance. 6 Although this model is convenient in many circumstances, its limitations have been discussed in the literature. For example, Samec and co-authors reported limited agreement between the results from interfacial tension and impedance spectroscopy measurements of the interface between aqueous and 1,2-dichloroethane (DCE) electrolyte solutions and suggested that the Randles equivalent circuit might be at fault at high electric potential differences. 7,8 Impedance spectroscopy studies of ITIES have measured interfacial differential capacitance that depends on the nature of the ions. 4,[9][10][11][12][13][14] Interfac...

On the non-ideal behaviour of polarised liquid-liquid interfaces

“…The high frequency behaviour observed in EIS spectra at a polarised ITIES has been attributed to stray capacitance and resistance associated with the parasitic coupling of the reference and counter electrodes, as well as to the resistance of the bulk solution [24,30]. Wiles et al [15] suggested that the high frequency behaviour comes from artefacts due to both the cell and reference arm resistances of the organic phase, and stray capacitances of the potentiostat and leads.…”

Role of ion hydration for the differential capacitance of an electric double layer

“…Mobile ions rearrange so as to screen the charge at the electrode surface, thus forming an electrical double layer (EDL) in this process. The EDL has great importance in biological, colloidal and polyelectrolyte sciences, [1][2][3][4][5] surface conductivity, 6 renewable energy systems, 7,8 new methods for oil recovery, 9 and in electrical double layer capacitors, a device that stores electrochemical energy through the EDL. 10,11 The electrode is often approximately described by a perfectly planar charged surface, but other geometries have also been taken into account.…”