Potentiometric investigation of protonation reactions at aqueous-aqueous boundaries within a dual-stream microfluidic structure

The laminar flow regime prevailing in pressure-driven flow through a Y-shaped microfluidic channel was utilized to create a stable boundary between two aqueous liquids. Transverse transport of ions between these two liquids gave rise to a diffusion potential, which was monitored by measurement of the open circuit potential. In this report, the influence on the cross-channel potential distribution of protonation reactions occurring in the boundary zone between the two co-flowing liquids is presented. The proton source was present in one of the co-flowing streams, and an uncharged proton acceptor was present in the other aqueous stream. The time-dependent transport equation for diffusion and migration was augmented by chemical reaction terms and was solved for all species present in both streams as a theoretical basis for the analysis. Within this model, the system was assumed to be homogeneous along the channel height, and effects of nonuniform velocity profiles were neglected. A reduction in potential by several millivolts was predicted for a protonation reaction occurring close to the boundary between the two aqueous streams, provided that the mobility of the protonated species was lower than the mobility of the co-cation in the background electrolyte (alkali metal cation in this case). The magnitude of the decrease in the potential was greater for protonated molecules with lower mobility or if the mobility of the background electrolyte cation was increased. Experimental results are presented for imidazole and D-histidine as proton acceptors present in 10 mM KCl, 10 mM NaCl, or 10 mM CsCl solution and co-flowing with a stream of 10 mM hydrochloric acid, which served as the proton source. Decreases in measured potential, in line with the predicted diminished potential, were obtained.     

Concentrated Ionic Fluids: Is There a Difference Between Chloride-Based Brines and Deep Eutectic Solvents?

Deep Eutectic Solvents (DESs) have been lauded as novel solvents, but is there really a difference between them and concentrated aqueous brines? They provide a method of adjusting the activity of water and chloride ions which can affect mass transport, speciation and reactivity. This study proposes a continuum of properties across concentrated ionic fluids and uses metal processing as an example. Charge transport is shown to be governed by fluidity and there is no discontinuity between molar conductivity and fluidity irrespective of cation, charge density or ionic radius. Diffusion coefficients of iron(III) and copper(II) chloride in numerous concentrated ionic fluids show the same linear correlation between diffusion coefficient and fluidity. These oxidising agents were used to etch copper, silver and nickel and while the etching rate increased with fluidity for copper, etching of silver and nickel only occurred at high chloride and low water activity as passivation occurred when water activity increased. Overall, brines provide a high chloride content at a lower viscosity than DESs, but unlike DESs, brines are unable to prevent passivation due to their high water content. The results show how selective etching of mixed metal waste streams can be achieved by tuning chloride and water activity.     

Protein diffusion across the interface in aqueous two-phase systems

We present a detailed study of the diffusive transport of proteins across a fluid phase boundary within aqueous two-phase systems. The aim of the work is to investigate whether local effects at the phase boundary cause a retardation of the diffusive transport between the phases. Possible modifications of interfacial mass transfer could be due to protein adsorption at the phase boundary or local electric fields from electric double layers. Experiments with a microfluidic system have been performed in which protein diffusion (bovine serum albumin and ovalbumin) within a bilaminated configuration of two phases containing polyethylene glycol and dextran is analyzed. A one-dimensional model incorporating phase-specific diffusion constants and the difference in chemical potential between the phases has been formulated. A comparison of experimental and simulation data shows a good overall agreement and suggests that a potential local influence of the phase boundary on protein transport is insignificant for the systems under investigation.     

Design, simulation and application of a new micromixing device for time resolved infrared spectroscopy of chemical reactions in solution

We present a novel micromachined fast diffusion based mixing unit for the study of rapid chemical reactions in solution with stopped-flow time resolved Fourier transform infrared spectroscopy (TR-FTIR). The presented approach is based on a chip for achieving lamination of two liquid sheets of 10 microm thickness and approximately 1 mm width on top of each other and operation in the stopped-flow mode. The microstructure is made on infrared transmitting calcium fluoride discs and built up with two epoxy negative photoresist layers and one silver layer in between. Due to the highly laminar flow conditions and the short residence time in the mixer there is hardly any mixing when the two liquid streamlines pass through the mixing unit, which allows one to record a mid-IR transmission spectrum of the analytes prior to reaction. When the flow is stopped, the reactant streams are arrested in the flow-cell and rapidly mixed by diffusion due to the reduced interstream distances and the reaction can be directly followed with hardly any dead time. On the basis of two model reactions-neutralisation of acetic acid with sodium hydroxide as well as saponification of methyl monochloroacetate-the performance of the mixing device was tested revealing proper functioning of the device with a time for complete mixing of less than 100 ms. The experimental results were supported by numerical simulations using computational fluid dynamics (CFD), which allowed a reliable, quantitative analysis of concentration, pressure and flow profiles in the course of the mixing process.     

Electrochemical diffusion potential in the gas phase

Potentiometric based electrochemical measurement of diffusion potential at a junction between two flowing flame plasma gases is described. A flame electrochemical cell was constructed using a specially designed burner, which supports two individual flames, each fed by separate premixed methane/oxygen/nitrogen streams. The two flames were in intimate contact, creating a flowing fluid gaseous junction. By aspirating metal salt solutions into these premixed feed gases, the concentration gradient at the interface between the two flames may be controlled. A measurable electrochemical diffusion potential was formed at this junction, the magnitude of which was dependent on the concentration ratio of charged species with different mobilities. In our flame electrolyte, the dominant charged species were atomic or molecular cations and electrons, which have a difference in mobilities of approximately three orders of magnitude. A two-electrode system, in conjunction with a high impedance electrometer was used to measure the potential difference across the flame electrochemical cell. The measured potential difference was analysed using theory developed for the liquid junction potentials by the Henderson equation.     

Electrochemical Determination of Nanoparticle Size: Combined Theoretical and Experimental Study for Matrixless Silver Nanoparticles

A chronoamperometric procedure for the preparation of silver nanoparticles (AgNPs) in aqueous systems with no extra added stabilizing agents is presented. The uniqueness of the prepared nanoparticle systems was explored by theoretical considerations. The proposed theoretical model predicts the structural parameters of the obtained nanoparticle system. The parameters required for the calculations (the zeta potential, conductivity, and effective diffusion coefficient of ionic silver) are available from independently performed measurements. Chronoamperometry at a microelectrode was employed for the evaluation of the effective diffusion coefficient of ionic silver present in the AgNP solution. The values of AgNP radii predicted by the theoretical model for the selected samples were compared to those obtained by Transmission Electron Microscopy (TEM) and Dynamic Light Scattering (DLS) methods. Because of the high polydispersity of the prepared nanoparticle samples, DLS results were overestimated in comparison to both: the TEM results and some theoretical predictions. By correcting the theoretical predictions by the Debye length, the calculated nanoparticle sizes become comparable (within their expanded uncertainties) to those measured in TEM images, especially for the nanosystems at early stages of their formation via the electrosynthesis process.     

Diffusion coefficient measurement in a microfluidic analyzer using dual-beam microscale-refractive index gradient detection. Application to on-chip molecular size determination

We report a microchip-based detection scheme to determine the diffusion coefficient and molecular mass (to the extent correlated to molecular size) of analytes of interest. The device works by simultaneously measuring the refractive index gradient (RIG) between adjacent laminar flows at two different positions along a microchannel. The device, referred to as a microscale molecular mass sensor (micro-MMS), takes advantage of laminar flow conditions where the mixing of two streams occurs essentially by diffusion across the boundary between the two streams. Two flows merge on the microchip, one containing solvent only, referred to as the mobile phase stream and one which contains the analyte(s) of interest in the solvent, i.e. the sample stream. As these two streams merge and flow parallel to each other down the microchannel a RIG is created by the concentration gradient. The RIG is further influenced by analyte diffusion from the sample stream into the mobile phase stream. Measuring the RIG at a position close to the merging point (upstream signal) and simultaneously a selected distance further down the microchannel (downstream signal) provides real-time data related to the extent a given analyte has diffused, which can be readily correlated to analyte molecular mass by taking the ratio of the downstream-to-upstream signals. For the dual-beam RIG measurements, a diode laser output is coupled to a single mode fiber optic splitter with two output fibers. Light from each fiber passes through a graded refractive index (GRIN) lens forming a collimated beam that then passes through the microchannel and then on to a position sensitive detector (PSD). The RIG at both detection positions deflects the two collimated probe beams. The deflection angle of each beam is then measured on two separate PSDs. The micro-MMS was evaluated using polyethylene glycols (PEGs), sugars, and as a detector for size-exclusion chromatography (SEC). Peak purity can be readily identified using the micro-MMS with SEC. The limit of detection was 0.9 ppm (PEG at 11 840 g/mol) at the upstream detection position corresponding to a RI limit of detection (LOD) (3sigma) of 7-10(-8) RI. The pathlength for the RIG measurement was 200 microm and the angular LOD was 0.23 micro(rad) with a detection volume of 8 nl at both positions. The average molecular mass resolution was 9% (relative standard deviation) for a series of PEGs ranging in molecular mass from 106 to 22 800 g/mol. With this excellent mass resolution, small molecules such as monosaccharides, disaccharides, and so on, are readily distinguished. The sensor is demonstrated to readily determine unknown diffusion coefficients.     

Ionic conductivity of superionic conducting glasses in the pseudobinary system AgIAg2MoO4

The ionic conductivity was measured in the temperature range 250–300 K as a function of composition of superionic conducting glasses in the pseudobinary system AgIAg₂MoO₄. The conductivity, ranging from 10^?2 to 10^?4 Ω^?1 cm^?1 at room temperature, increases linearly in logarithmic scale with increasing AgI content, while the total silver ion concentration remains nearly constant in the whole glass-forming region of the present system. Such a composition dependence of conductivity is considered to be evidence that only a fraction of the silver ions in glass contributes to the ionic conduction. The conductivity and the activation energy for conduction differ slightly between bulk glasses and pressed pellets of pulverized glasses. The close agreement in bulk glasses and pellets suggests that bulk rather than grain boundary or surface diffusion dominates the conduction process in the present glasses.     

Electrostatics by Brownian dynamics: solving the Poisson equation near dielectric interfaces

The isomorphism between electrostatics and diffusion is discussed and utilized to develop a Brownian dynamics algorithm for solving the Poisson equation near dielectric interfaces. The electrostatic potential behaves as if carried by noninteracting, randomly moving pseudo-particles whose residence time in a given region of space is proportional to the electrostatic potential there. By applying random numbers from the exact solution for diffusion near a planar discontinuity, the Brownian motion of these particles can be propagated for large time steps, independent of spatial grids or artificial boundary conditions. The applicability of the Brownian algorithm is demonstrated in simple illustrative calculations.     

A simple currentless method of determination of ion fluxes to and within electroactive ion-exchange membranes

A simple protocol applicable for the determination of ion fluxes from a solution to an ion-exchange membrane and within it was proposed. Advantages of this method include the application of a simple typical potentiometric set-up, as the method uses open circuit potential measurements; thus, the membrane state is not biased by external polarization. The proposed approach is based on the analysis of potential vs. time dependences in the course of building up or disappearance of a diffusion layer in a solution, controlled by solution stirring. Using equations describing diffusion processes, both ion fluxes from a solution to a membrane and within the membrane can be calculated. Experimental studies were carried out on examples of electrodes coated by two different kinds of membranes: (i) poly(vinyl chloride)-based silver-selective membrane and (ii) polypyrrole film doped by poly(4-styrenesulfonate) ions, in solutions of silver ions. The results obtained for non-saturated silver-selective membranes highlight the role of membrane thickness and conditioning time as well as confirm the role of diffusion in the membrane as a rate-determining step. For polypyrrole layers, the ion flux results from silver deposition in the surface part of a conducting polymer film, and the flux value was found consistent with silver deposition rate determined earlier.     

Calculation of diffusion impedance in the terms of locally nonequilibrium diffusion

An equation for diffusion impedance is derived in the terms of the lattice diffusion model based on the assumption as to the local nonequilibrium distribution of diffusing particles across the sites of different types. This equation is valid at low lattice occupation. Unlike the multiple trapping model, all site types are interpreted symmetrically. In the boundary condition, it is assumed that there is a unique relationship between the electric potential variation, on one side of the interface and variation of the generalized particle activity on the other side.     

Electrochemical reactivity at redox-molecule-based nanoelectrode ensembles

A model describing electrochemical reactivity at nanoelectrode ensembles consisting of redox-molecule-based active sites immobilized on otherwise passivated electrode surfaces is presented. A mathematical treatment in terms of hemispherical diffusion of redox-active solutes to a layer of independent molecule-based nanoelectrode sites is shown to be equivalent to one in terms of a bimolecular diffusion-limited reaction between a layer of immobilized redox molecules and a reservoir of redox-active solutes. This equivalence derives from the fact that in both cases the mass-transfer problem is essentially that of hemispherical diffusion. The model is further developed to consider rate limitation by both the bimolecular redox reaction between the active-site molecule and redox molecules in solution and the heterogeneous redox reaction between the electrode and the active-site molecule. Analytical expressions are derived for the current–voltage relation corresponding to catalyzed electron transfer at an ensemble of redox-molecule-based nanoelectrode sites, and the expressions are used to interpret preliminary data for ultrasensitive electrochemical detection in flow streams via an electrochemical amplification process that is thought to involve redox mediation by individual analyte molecules adsorbed onto monolayer-coated electrodes.     

The effect of rotational and translational energy exchange on tracer diffusion in rough hard sphere fluids

A study is presented of tracer diffusion in a rough hard sphere fluid. Unlike smooth hard spheres, collisions between rough hard spheres can exchange rotational and translational energy and momentum. It is expected that as tracer particles become larger, their diffusion constants will tend toward the Stokes-Einstein hydrodynamic result. It has already been shown that in this limit, smooth hard spheres adopt "slip" boundary conditions. The current results show that rough hard spheres adopt boundary conditions proportional to the degree of translational-rotational energy exchange. Spheres for which this exchange is the largest adopt "stick" boundary conditions while those with more intermediate exchange adopt values between the "slip" and "stick" limits. This dependence is found to be almost linear. As well, changes in the diffusion constants as a function of this exchange are examined and it is found that the dependence is stronger than that suggested by the low-density, Boltzmann result. Compared with smooth hard spheres, real molecules undergo inelastic collisions and have attractive wells. Rough hard spheres model the effect of inelasticity and show that even without the presence of attractive forces, the boundary conditions for large particles can deviate from "slip" and approach "stick."     

Study of the potential response of solid-state chloride electrodes at low concentration ranges

Ion-selective electrodes based on silver chloride precipitates have been investigated in the low concentration range, by use of a specially designed cell of small volume. Electrode potential measurements and silver determinations in the corresponding solutions by atomic-absorption spectrometry were made. The results prove that the potential response of these ion-selective electrodes in the low concentration ranges is governed by inequality of the ion concentrations in the boundary zone of the test solution contacting the electrode membrane. This is a result of adsorption-desorption processes, a dissolution process followed by recrystallization of the silver chloride at the electrode membrane surface, and photoreduction of silver ions at the electrode surface.     

Synthesis of silver nanoparticles using dl-alanine for ESR dosimetry applications

The potential use of alanine for the production of nanoparticles is presented here for the first time. Silver nanoparticles were synthesized using a simple green method, namely the thermal treatment of silver nitrate aqueous solutions with dl-alanine. The latter compound was employed both as a reducing and a capping agent. Particles with average size equal to 7.5 nm, face-centered cubic crystalline structure, narrow size distribution, and spherical shape were obtained. Interaction between the silver ions present on the surface of the nanoparticles and the amine group of the dl-alanine molecule seems to be responsible for reduction of the silver ions and for the stability of the colloid. The bio-hybrid nano-composite was used as an ESR dosimeter. The amount of silver nanoparticles in the nanocomposite was not sufficient to cause considerable loss of tissue equivalency. Moreover, the samples containing nanoparticles presented increased sensitivity and reduced energetic dependence as compared with pure dl-alanine, contributing to the construction of small-sized dosimeters.     

Changes in the growth mode of electrodeposited silver layers: Surface rearrangements induced by the presence of lead adatoms

The surface characteristics of Ag electrodeposits prepared on polyfaceted Pt(sc) electrodes have been followed through complete Pb upd/stripping voltammetry. The growth mode of Ag overlayers at constant temperature depends on both the cathodic electrodeposition overvoltage and the silver ion concentration in solution. Ag surface rearrangements can be induced by either holding the potential or Pb upd/stripping cycling in the potential range where the degree of surface coverage by Pb adatoms is between 0 and 1. The voltammograms of Ag overlayers at the monolayer level show considerable changes compared with those obtained for thicker Ag overlayers. The present results allow us to establish a correlation between the development of rough electrodeposits and the surface diffusion properties of metal atoms.     

Exact solution of mass diffusion into a finite volume

A new theoretical based model of mass diffusion from an external constant source, through a membrane, into an enclosure and at a constant temperature is developed. Exact solution is presented of one dimensional transient Fickian diffusion equation with initial and boundary conditions appropriate for the mass diffusion into an enclosure. The present model has many practical applications. It may be used as a working tool in diffusion experiments in selecting membrane materials, saving experimental time and cost, predicting the shelf-life of pre-packaged food, drugs and other vulnerable materials to external effects. A good agreement of the model is shown with existing data. The economic advantage of the present model is illustrated by offering shorter experimental time by an order of magnitude. The use of theoretical results minimizes the exposure time to dangerous concentrations of hazardous materials during the experiments. Analysis of the model provides simple linear relationships between various parameters of the mass diffusion into an enclosure.     

Anion-selective properties of alkali metal-free lead phosphate glasses containing silver chloride and their application in an ion-selective electrode

Alkali-free lead phosphate glasses containing silver chloride have been developed for anion responsive sensors. From measurements of the final glass compositions by electron probe microanalysis, it became clear that some of chloride ions in the glass bulk were not volatilized during the glass melting process. Compared with phosphate glasses containing silver oxide, the new glass electrodes containing silver chloride could respond more rapidly, although the response behaviour for anionic species were similar. From the electrode potential vs. time curve for the anionic species, the potential rapidly reached equilibrium when these concentrations varied from 10^?5 to 10^?2 M. The response times, t₉₅, to thiocyanate of the new glass electrode and the phosphate glass electrode containing silver oxide were 30 and 110 s, respectively. Moreover, the response time required for an initial potential change with a concentration jump of thiocyanate with the new glass electrode was found to be independent of the membrane thickness within about 2 mm and of the measuring temperature between 15 and 40°C. It is concluded that the diffusion process of species such as silver ion in the glass bulk does not take part in the initial part of the response behaviour.     

Mathematical modeling of silver extraction by an emulsion liquid membrane process

A general physical model of a typical batch extraction system employing an emulsion liquid membrane process for the extraction of silver has been developed. The model takes into account the extraction reaction between the silver ion and the carrier molecules at the external interface, the diffusion of the complex in the membrane phase, the stripping reaction at the internal interface and the reaction of silver ion with the reagent, HCL, in the internal phase to yield silver chloride incapable of permeating through the membrane phase. In addition, the leakage of the internal aqueous phase to the external aqueous phase due to membrane breakage has been incorporated in this model. The batch extraction of silver using D2EHPA as a carrier has been carried out under various experimental conditions. The experimental data can be well explained by the present model.