Electrochemical polarization phenomena at the interface of two immiscible electrolyte solutions—III. Progress since 1983

Versatile Tools for Understanding Electrosynthetic Mechanisms

Electrosynthesis is a popular, green alternative to traditional organic methods. Understanding the mechanisms is not trivial yet is necessary to optimize reaction processes. To this end, a multitude of analytical tools is available to identify and quantitate reaction products and intermediates. The first portion of this review serves as a guide that underscores electrosynthesis fundamentals, including instrumentation, electrode selection, impacts of electrolyte and solvent, cell configuration, and methods of electrosynthesis. Next, the broad base of analytical techniques that aid in mechanism elucidation are covered in detail. These methods are divided into electrochemical, spectroscopic, chromatographic, microscopic, and computational. Technique selection is dependent on predicted reaction pathways and electrogenerated intermediates. Often, a combination of techniques must be utilized to ensure accuracy of the proposed model. To conclude, future prospects that aim to enhance the field are discussed.

Phospholipid layer stabilization via Yb(III) on ITIES and facilitated K(I) transport

The stabilization of lecithin layer physically adsorbed on the interface between two immiscible electrolyte solutions (ITIES) using addition of Yb3+ to the aqueous phase was studied using cyclic voltammetry. Stability of this layer was evaluated according to the time of the layer formation; the optimal time amounted to 180 s. The stability of the layer as a function of the potential applied on the system during the layer formation was investigated. The behavior of the monolayer during the layer polarization was studied by the facilitated K+ transport using dibenzo-18-crown-6. The stoichiometry of Yb/anion/lecithin complexes was investigated using electrospray ionization mass spectrometry (ESI-MS).

Simple Ion Transfer at Liquid|Liquid Interfaces

The main aspects related to the charge transfer reactions occurring at the interface between two immiscible electrolyte solutions (ITIES) are described. The particular topics to be discussed involve simple ion transfer. Focus is given on theoretical approaches, numerical simulations, and experimental methodologies. Concerning the theoretical procedures, different computational simulations related to simple ion transfer are reviewed. The main conclusions drawn from the most accepted models are described and analyzed in regard to their relevance for explaining different aspects of ion transfer. We describe numerical simulations implementing different approaches for solving the differential equations associated with the mass transport and charge transfer. These numerical simulations are correlated with selected experimental results; their usefulness in designing new experiments is summarized. Finally, many practical applications can be envisaged regarding the determination of physicochemical properties, electroanalysis, drug lipophilicity, and phase-transfer catalysis.

Individual Extraction Constants of Some Univalent Organic Cations in the Two-Phase Water–Phenyltrifluoromethyl Sulfone System

From extraction experiments and γ-activity measurements, the extraction constants corresponding to the general equilibrium C+(aq) + I−(aq) ⇔ C+(org) + I−(org) taking place in the two-phase water–phenyltrifluoromethyl sulfone (abbrev. FS 13) system (C+ = organic cation; aq = aqueous phase, org = FS 13 phase) were evaluated. Furthermore, the individual extraction constants of 8 univalent organic cations in the mentioned two-phase system were calculated.

Self-organization of water in lithium/nitrobenzene system

The effect of lithium ion on the ordering of water in water-saturated nitrobenzene has been probed by 2H NMR, diffusion ordered spectroscopy and neutron scattering. It was shown that increased water concentration in LiClO4/wet nitrobenzene results in the formation of a metastable solvatomer with mixed water and nitrobenzene character, Li+(W/NB). This species is shown to decay over hours to two solvatomers, one dominated by nitrobenzene Li+(NB) and the other dominated by water Li+(W). To confirm the assignment of these solvation states, diffusion ordered deuterium NMR spectroscopy has been used to elucidate the hydrodynamic radii of these solvatomers. Neutron scattering yields vibrational spectroscopy information that shows how addition of lithium to the nitrobenzene/water system results in relatively slow self-organization of the water environment (hours).

Ultraviolet laser photo-modulation voltammetry of tetraphenylborate at a liquid/liquid interface

Photo-modulation voltammetry was applied to detecting the photolysis of tetraphenylborate (TPhB) at a water/1,2-dichloroethane (DCE) interface by using a He-Cd laser emitting a beam with a major 325-nm line and minor lines of shorter wavelengths. When the interface was irradiated from the water-phase side, a new wave appeared in the photomodulation voltammogram, suggesting that TPhB was photolyzed and the anionic product was transferred across the interface. The concentration dependence of the photocurrents was successfully explained by a theory based on the photolytic process at the interface.

Extraction Distribution of Bromophenol Blue and Bromocresol Green in the Two-Phase Water-1,2-Dichloroethane System

The equilibrium distribution constants of electroneutral bromophenol blue and bromocresol green between the 1,2-dichloroethane and aqueous phases were determined from extraction measurements. By using known thermodynamic parameters and applying general relations, the dissociation constants of these two organic dyes in 1,2-dichloroethane saturated with water were calculated.

Extraction Distribution of 2-Nitroso-1-Naphthol in the Two-Phase Water-Nitrobenzene System

General relations among the thermodynamic parameters, which characterize dissociation equilibria of an acid HA in both phases of a two-phase extraction system and ion transfers across the interface of this system, was formulated. Moreover, dissociation constant of 2-nitroso-1-naphthol in nitrobenzene saturated with water was calculated as pK d nb(HA) = −logK d nb(HA) = 16.48.

Selectivity behavior and multianalyte detection capability of voltammetric ionophore-based plasticized polymeric membrane sensors

The current response features ofvoltammetric ion-selective polymeric membranes doped with neutral ionophores in view of practical sensor development are elucidated. The membranes are designed to extract ions only under applied external potentials and interrogated by normal-pulse voltammetry and pulsed amperometry. They contain two polarizable interfaces to avoid loss of lipophilic ions at the sample side and to maximize the available potential window. A simple theoretical model is developed that describes the observed current at the end of an uptake pulse to the applied membrane potential, which is the sum of both boundary potentials (at the sample and inner electrolyte side) and the membrane internal iR drop. The results describe how the selectivity of the resulting sensor must be dependent on the applied potential. Evidently, the role of the applied potential is akin to incorporating lipophilic cationic and anionic sites with potentiometric ionophore-based membranes, which are well known to considerably affect membrane selectivity and to define the charge type of the assessed ions. This has important implications for sensor design, as the applied cell potential can be used to tune sensor selectivity. Theory also explains the role of the inner electrolyte on sensor behavior. A maximum measuring range is expected with ions in the inner electrolyte that are difficult to extract into the membrane. This corresponds to Kihara's experimental results and contrasts to common ion-selective electrode practice, where a salt of the analyte ion is normally present in the inner electrolyte. Separate and mixed solution experiments with membranes containing the sodium-selective ionophore tert-butyl calix[4]arene tetramethyl ester and the lithium ionophore ETH 1810 agree very well with theoretical expectations. Multianalyte detection capability with a single sensing membrane is demonstrated in a selectivity-modifying pulsed amperometric detection mode, where each applied voltage yields a different practical selectivity of the sensor. The sensor is altered from being sodium to potassium selective as the magnitude of the applied potential is repetitively varied within the pulse sequence. The sensors show high long-term stability under continuous measuring conditions over 15 h.

Transport studies of beta-lactam antibiotics and their degradation products across electrified water/oil interface

An electrochemical method for quantifying beta-lactam antibiotics (cephalexin and ampicillin) and their hydrolysis products is described. Cyclic voltammetry at the water/nitrobenzene interface in a four-electrode system was used. The zwitterionic compounds were ionized to the necessary electrochemical form by pH adjustment. The pH change, however, resulted also in hydrolysis of the antibiotics. Hydrolysis products were characterized across UV-vis spectrum. The various hydrolysis products as well as the ionized antibiotics were studied in voltammetric transfer from water to nitrobenzene using the method of the interface between two immiscible electrolyte solutions (ITIES). It was concluded that this electrochemical method is suitable for the quantification of beta-lactam antibiotics and their hydrolysis products.