Liquid ∣liquid microinterface. Localization of the phase boundary by voltammetry and chronoamperometry; influence of the microchannel dimensions on diffusion

Electroanalytical applications of ITIES - A review

Over the last decades, the interface between two immiscible electrolyte solutions (ITIES) attracted considerable attention of the scientific community due to their vast applications, such as extraction, catalysis, partition studies and sensing. The aim of this Review is to highlight the potential of electrochemistry at the ITIES for analytical purposes, focusing on ITIES-based sensors for detection and quantification of chemically and biologically relevant (bio)molecules. We start by addressing the evolution of ITIES in terms of number of publications over the years along with an overview of their main applications (Chapter 1). Then, we provide a general historical perspective about pioneer voltammetric studies at water/oil systems (Chapter 2). After that, we discuss the most impacting improvements on ITIES sensing systems from both perspectives, set-up design (interface stabilization and miniaturization, selection of the organic solvent, etc.) and optimization of experimental conditions to improve selectivity and sensitivity (Chapter 3). In Chapter 4, we discuss the analytical applications of ITIES for electrochemical sensing of several types of analytes, including drugs, pesticides, proteins, among others. Finally, we highlight the present achievements of ITIES as analytical tool and provide future challenges and perspectives for this technology (Chapter 5).

Comparative electrochemical study of veterinary drug danofloxacin at glassy carbon electrode and electrified liquid-liquid interface

This work compares the electroanalytical performance of two electroanalytical systems based on (1) the glassy carbon electrode (GCE), and (2) the electrified liquid-liquid interface (eLLI), for the detection of fluoroquinolone antibiotic-danofloxacin (DANO). Our aim was to define the optimal conditions to detect the chosen analyte with two employed systems, extract a number of electroanalytical parameters, study the mechanism of the charge transfer reactions (oxidation at GCE and ion transfer across the eLLI), and to provide physicochemical constants for DANO. Detection of the chosen analyte was also performed in the spiked milk samples. To the best of our knowledge, this is the first work that directly compares the electroanalytical parameters obtained with solid electrode (in this case GCE) and eLLI. We have found that for DANO the latter provides better electroanalytical parameters (lower LOD and LOQ) as well as good selectivity when the milk was analyzed.

How to polarise an interface with ions: the discrete Helmholtz model

The distribution of electrolytes in an electric field usually relies on theories based on the Poisson-Boltzmann formalism. These models predict that, in the case of a metallic electrode, ionic charges screen the electrode potential, leading to concentration-dependent ion distributions. This theoretical framework was first applied at solid-liquid interfaces and then transposed to soft interfaces. However, in this latter case, the potential in which the electrolytes evolve is not homogeneous, which is less amenable to a mean-field description. In this report, we show that at polarised soft interfaces the potential difference takes place between two closely interacting ionic monolayers. In this configuration, ions of opposite charges directly neutralise each other leading to an absence of diffuse layers and charge screening by surrounding ions. Thus, independently of the electrolyte concentrations, the surface charge density is a linear function of the potential difference, which results in a constant capacitance.

Electrochemical Behavior and Detection of Diclofenac at a Microporous Si3N4 Membrane Modified Water–1,6-dichlorohexane Interface System

The electrochemical behavior when the liquid–liquid interface was modified by commercially available, microporous silicon nitride membrane, was achieved using cyclic voltammetry with tetramethyl ammonium. The transfer characteristics of the ionizable drug diclofenac ( DCF − ), as an anti-inflammatory, anti-rheumatic, antipyretic, and analgesic treatment in common use in biomedical applications, were also investigated across microporous silicon nitride-modified liquid interface. Thus, some thermodynamic variables for DCF − , such as the standard Gibbs energy of transfer, the standard transfer potential and lipophilicity were estimated. Furthermore, the influence of possible interfering substances (ascorbic acid, sugar, amino acid, urea, and metal ions) on the detection of DCF − was investigated. An electrochemical DCF sensor is investigated using differential pulse voltammetry (DPV) as the quantification technique, a linear range of 8–56 µM and a limit of detection of 1.5 µM was possible due to the miniaturized interfaces formed within silicon nitride.

Interaction of inorganic anions with iron-mineral adsorbents in aqueous media--a review

A number of inorganic anions (e.g., nitrate, fluoride, bromate, phosphate, and perchlorate) have been reported in alarming concentrations in numerous drinking water sources around the world. Their presence even in very low concentrations may cause serious environmental and health related problems. Due to the presence and significance of iron minerals in the natural aquatic environment and increasing application of iron in water treatment, the knowledge of the structure of iron and iron minerals and their interactions with aquatic pollutants, especially inorganic anions in water are of great importance. Iron minerals have been known since long as potential adsorbents for the removal of inorganic anions from aqueous phase. The chemistry of iron and iron minerals reactions in water is complex. The adsorption ability of iron and iron minerals towards inorganic anions is influenced by several factors such as, surface characteristics of the adsorbent (surface area, density, pore volume, porosity, pore size distribution, pHpzc, purity), pH of the solution, and ionic strength. Furthermore, the physico-chemical properties of inorganic anions (pore size, ionic radius, bulk diffusion coefficient) also significantly influence the adsorption process. The aim of this paper is to provide an overview of the properties of iron and iron minerals and their reactivity with some important inorganic anionic contaminants present in water. It also summarizes the usage of iron and iron minerals in water treatment technology.

Ion amperometry at the interface between two immiscible electrolyte solutions in view of realizing the amperometric ion-selective electrode

This article reviews the development in ion amperometry at the interface between two immiscible electrolyte solutions (ITIES) in view of realizing the amperometric ion-selective electrode (ISE). The concept of polarizability of ITIES in a multi-ion system is outlined. Principle aspects of ion amperometry at ITIES are discussed including the use of amperometry as a tool for the clarification of the ion sensing mechanism, and for determining the concentrations of ions in the solution. The reference is made to recent amperometric measurements at the supported liquid membrane (SLM) and polymer composite liquid membranes (PCLM), which, together with the micro-hole supported ITIES, appear to be particularly suitable for realization of the amperometric ISE.

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.

Ion-transfer voltammetry at silicon membrane-based arrays of micro-liquid-liquid interfaces

Microporous silicon membranes, fabricated by lithographic patterning and wet and dry silicon etching processes, were used to create arrays of micro-scale interfaces between two immiscible electrolyte solutions (muITIES) for ion-transfer voltammetry. These membranes served the dual functions of interface stabilization and enhancement of the rate of mass-transport to the interface. The pore radii were 6.5 microm, 12.8 microm and 26.6 microm; the pore-pore separations were ca. 20- to 40-times the pore radii and the membrane thickness was 100 microm. Deep reactive ion etching (DRIE) was used for pore drilling through the silicon, which had been previously selectively thinned by potassium hydroxide etching. DRIE produces hydrophobic fluorocarbon-coated internal pore walls. The small pore sizes and large pore-pore separations used resulted in steady-state voltammograms for the transfer of tetramethylammonium cation (TMA(+)) from the aqueous to the organic phase, whereas the reverse voltammetric sweeps were peak-shaped. These asymmetric voltammograms are consistent with the location of the ITIES at the aqueous side of the silicon membrane such that the organic phase fills the micropores. Comparison of the experimental currents to calculated currents for an inlaid disc micro-interface revealed that the interfaces were slightly recessed, up to 10 microm (or 10% of the pore length) in one case. Facilitated ion transfer, with an organic-phase ionophore, confirmed the location of the organic phase within the pores. These microporous silicon membranes offer opportunities for various analytical operations, including enhancing the rate of mass transport to ITIES-based sensing devices and stabilization of the ITIES for hydrodynamic applications.

Multiphase flow in microfluidic systems --control and applications of droplets and interfaces

Micro- and nanotechnology can provide us with many tools for the production, study and detection of colloidal and interfacial systems. In multiphase flow in micro- and nanochannels several immiscible fluids will be separated from each other by flexible fluidic interfaces. The multiphase coexistence and the small-volume confinement provide many attractive characteristics. Multiphase flow in microfluidic systems shows a complicated behavior but has many practical uses compared to a single-phase flow. In this paper, we discuss the methods of controlling multiphase flow to generate either micro- or nano-droplets (or bubbles) or stable stratified interfaces between fluidic phases. Furthermore, applications of the droplets and interfaces in microchannels are summarized.

Chronoamperometry at micropipet electrodes for determination of diffusion coefficients and transferred charges at liquid/liquid interfaces

Chronoamperometry was carried out at liquid/liquid interfaces supported at the tip of micropipet electrodes for direct determination of the diffusion coefficient of a species in the outer solution. The diffusion coefficient was used for subsequent determination of the transferred charges per species from the diffusion-limited steady-state current. A large tip resistance of the micropipets causes prolonged charging current so that the faradic current can be measured accurately only at a long-time regime (typically t > 5 ms). At the same time, the long-time current response at the interfaces surrounded by a thin glass wall of the pipets is enhanced by diffusion of the species from behind the pipet tip. Therefore, numerical simulations of the long-time chronoamperometric response were carried out using the finite element method for accurate determination of diffusion coefficients. Validity of the simulation results was confirmed by studying simple transfer of tetraethylammonium ion. The technique was applied for transfer/adsorption reactions of the natural polypeptide protamine and also for Ca2+ and Mg2+ transfers facilitated by ionophore ETH 129. With the diffusion coefficient of protamine determined to be (1.2 +/- 0.1) x 10(-6) cm(2)/s, the ionic charge transferred by each protamine molecule was obtained as +20 +/- 1, which is close to the excess positive charge of protamine. Also, the diffusion coefficient of ETH 129 was determined to demonstrate that each ionophore molecule transfers +0.67 and +1 charge per Ca2+ and Mg2+ transfer, respectively, which corresponds to formation of 1:3 and 1:2 complexes with the respective ions.

Modifying the liquid/liquid interface: pores, particles and deposition

The modification of the liquid/liquid interface with solid phases is discussed in this article. Modified interfaces can be formed with molecular assemblies, but here attention is focussed on solid materials such as mesoscopic particles, or microporous and mesoporous membranes. Charge transfer across the modified liquid/liquid interface is considered in particular. The most obvious consequence of the introduction of such modifying components is their effect on the transport to, and the transfer of material across, the liquid/liquid interface, as measured voltammetrically for example. One particularly interesting reaction is interfacial metal deposition, which can also be studied under electrochemical control: the initial formation of metal nuclei at the interface transforms it from the bare, pristine state to a modified state with very different reactivity. Deposition at interfaces between liquids is compared and contrasted with the cases of metal deposition in bulk solution and conventional heterogeneous deposition on conducting solid surfaces. Comparison is also made with work on the assembly of pre-formed micron and nanometre scale solids at the liquid/liquid interface.