Voltammetry of Ion Transfer across the Electrochemically Polarized Micro Liquid-Liquid Interface between Water and a Room-temperature Ionic Liquid, Tetrahexylammonium Bis(trifluoromethylsulfonyl)imide, Using a Glass Capillary Micropipette

Amperometric Ion Sensing Approaches at Liquid/Liquid Interfaces for Inorganic, Organic and Biological Ions

Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) has become an invaluable tool for the selective and sensitive detection of cationic and anionic species, including charged drug molecules and proteins. In addition, neutral molecules can also be detected at the ITIES via enzymatic reactions. This chapter highlights recent developments towards creating a wide spectrum of sensing platforms involving ion transfer across the ITIES. As well as outlining the basic principles needed for performing these sensing applications, the development of ITIES-based detection strategies for inorganic, organic, and biological ions is discussed.

Improved thin-layer electrolysis cell for ion transfer at the liquid|liquid interface using a conducting polymer-coated electrode

This article reviews the development of a thin-layer electrolysis cell with both a thin aqueous phase and a thin organic phase for ion transfer at the liquid|liquid interface for the absolute determination of a redox-inactive ion. In particular, an improvement of the cell performance by using a conducting polymer-coated electrode in the organic phase is discussed. The applicability of the thin-layer electrolysis cell to the absolute determination of redox-inactive ions using the flow-injection method or stripping method is also described.

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.

Nanopipet voltammetry of common ions across the liquid-liquid interface. Theory and limitations in kinetic analysis of nanoelectrode voltammograms

Finite element simulations of ion transfer (IT) reactions at the nanopipet-supported interface between two immiscible electrolyte solutions (ITIES) were carried out, and the numerical results were generalized in the form of an analytical approximation. The developed theory is the basis of a new approach to kinetic analysis of steady-state voltammograms of rapid IT reactions. Unlike the conventional voltammetric protocol, our approach requires the initial addition of a transferable ion to both liquid phases, i.e., to the filling solution inside a nanopipet and the external solution. The resulting steady-state IT voltammogram comprises two waves corresponding to the ingress of the common ion into the pipet and its egress into the external solution. We demonstrate that both ingress and egress waves are required for characterization of pipet geometry and precise determination of thermodynamic and kinetic parameters for rapid IT reactions. In this way, one can eliminate large uncertainties in kinetic parameters, which are inherent in the previously reported approaches to analysis of nearly reversible steady-state voltammograms of either IT at pipet-supported ITIES or electron transfer at solid electrodes. Numerical simulations also suggest that higher current density at the edge of the nanoscale ITIES increases the significance of electrostatic effects exerted by the charged inner surface of a pipet on IT processes.

Kinetic study of rapid transfer of tetraethylammonium at the 1,2-dichloroethane/water interface by nanopipet voltammetry of common ions

Steady-state voltammetry at the pipet-supported liquid/liquid interface has previously been used to measure kinetics of simple and facilitated ion transfer (IT) processes. Recently, we showed that the conventional experimental protocol and data analysis produce large uncertainties in kinetic parameters of rapid IT processes extracted from pipet voltammograms. Here, we used a new mode of nanopipet voltammetry, in which a transferable ion is initially present as a common ion in both liquid phases, and improved methodology for silanization of the outer pipet wall to investigate the kinetics of the rapid transfer of tetraethylammonium (TEA(+)) at the 1,2-dichloroethane/water interface. This reaction was often employed as a model system to check the IT theory. The determined standard rate constant and transfer coefficient of the TEA(+) transfer are compared with previously reported values to demonstrate limitations of conventional nanopipet voltammetry with a transferrable ion present only in one liquid phase.

Ionic liquids in analytical chemistry

The role of ionic liquids (ILs) in analytical chemistry is increasing substantially every year. A decade ago there were but a handful of papers in this area of research that were considered curiosities at best. Today, those publications are recognized as seminal articles that gave rise to one of the most rapidly expanding areas of research in chemical analysis. In this review, we briefly highlight early work involving ILs and discuss the most recent advances in separations, mass spectrometry, spectroscopy, and electroanalytical chemistry. Many of the most important advances in these fields depend on the development of new, often unique ILs and multifunctional ILs. A better understanding of the chemical and physical properties of ILs is also essential.

Cyclic voltammetry at micropipet electrodes for the study of ion-transfer kinetics at liquid/liquid interfaces

Cyclic voltammetry at micropipet electrodes is applied to the kinetic study of ion transfer at liquid/liquid interfaces. Simple and facilitated transfer of an ion that is initially present outside a tapered pipet was simulated by the finite element method, enabling complete analysis of the resulting transient cyclic voltammogram (CV) with a sigmoidal forward wave followed by a peak-shaped reverse wave. Without serious effects of uncompensated ohmic resistance and capacitive current, more parameters can be determined from a transient CV than from the steady-state counterpart obtained with a smaller pipet or at a slower scan rate. A single transient CV under kinetic limitation gives all parameters in a Butler-Volmer-type model, i.e., the formal potential, the transfer coefficient, the standard ion-transfer rate constant, k(0), and the charge of a transferring ion as well as its diffusion coefficients in both phases. Advantages of the transient approach are demonstrated experimentally for reversible, quasi-reversible and irreversible cases. With a multistep transfer mechanism, an irreversible transient CV of facilitated protamine transfer gives an apparent k(0) value of 3.5 x 10(-5) cm/s, which is the smallest k(0) value reported so far. With the largest reliable k(0) value of approximately 1 cm/s reported in the literature, an intrinsic rate of the interfacial ion transfer varies by at least 5 orders of magnitude.

Facilitated Transfer of Alkali-Metal Cations by Dibenzo-18-crown-6 across the Electrochemically Polarized Interface between an Aqueous Solution and a Hydrophobic Room-Temperature Ionic Liquid

The facilitated transfer of alkali metal cations (Li+, Na+, K+, Rb+, Cs+) by dibenzo-18-crown-6 (DB18C6) across the electrochemically polarizable interface between an aqueous solution (W) and a hydrophobic ionic liquid, N-octadecylisoquinolinium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([C18Iq][TFPB]), has been studied using cyclic voltammetry at the interface formed at the tip of a micropipet. In cyclic voltammograms (CVs), the current due to the facilitated transfer of the cations by DB18C6 from W to [C18Iq][TFPB] can be measured within the polarized potential window of the [C(18)Iq][TFPB]|W interface. The stoichiometry of the complexes in [C18Iq][TFPB] for Li+, Na+, K+, and Rb+ are found to be 1:1 while for the Cs+ transfer both 1:1 and 1:2 complexes are likely to be formed. The formation constants of the 1:1 complexes for Li+, Na+, K+, and Rb+ in [C18Iq][TFPB], , evaluated from CVs are log = 5.0, 7.0, 8.2, and 7.3, respectively. The value for K+ is 1 order of magnitude greater than that for Na+. This higher selectivity of DB18C6 to K+ over Na+ in [C18Iq][TFPB] compared with that in molecular solvents suggests that the RTIL provides a unique solvation environment for the complexations of DB18C6 with the ions.