Online monitoring of emulsion polymerization using electrical impedance spectroscopy

A frequency‐dependent electrical monitoring technique was developed for characterizing emulsion polymerization online. High resolution electrical impedance spectroscopy (EIS) was conducted to study emulsion polymerization of styrene in terms of the key process parameters. Relationships between EIS data and process and product attributes such as monomer conversion and particle size were investigated. Electrical impedance values were measured at frequencies ranging from 1 Hz to 100 kHz. EIS parameters revealed changes in emulsion properties with time for both aqueous and organic phases during polymerization. At relatively higher frequencies (≥1 kHz) EIS data were found to be dominated by events in the continuous phase showing transitions in polymerization rates and intervals, while changes in monomer droplet and polymer particle sizes were dominant at lower frequencies (1 Hz). The EIS data were found to correlate well with polymerization rate, monomer conversion and particle size profiles at the appropriate frequencies in real time. © 2014 Society of Chemical Industry     

Electrochemical studies of self-assembled monolayers using impedance spectroscopy

Self-assembled monolayers (SAMs) using various organic molecules were constructed on thin thermal silicon dioxide on silicon structures. These structures have potential bio-applications. The monolayers are part of a capacitor sensor based on the electrolyte-insulator-silicon (EIS) structure. The main goal of this work is to investigate the ability of SAMs for acting as an intermediate layer between aqueous solutions and silicon based biosensors. This work presents monolayers based on trimethoxy silanes with various functional groups such as: (a) (3-aminopropyl)-trimethoxysilane, (b) octadecyltrimethoxysilane and (c) trimethoxy(3,3,3-trifluoropropyl)silane. The biosensor is a differential sensor with a built in reference based on the structure of electrolyte-insulator-silicon or in ion-sensitive-field-effect-transistors (ISFETs). While the methyl and fluorine groups can act as the reference part, which is passive to the bio-molecules detection, the capacitor containing the amine groups has the sensing capabilities to the biological molecules. The amine (NH₂) tail groups can be active for those attachments via glutaraldehyde.The film's study and characterization was made by spectroscopic ellipsometry, contact angle method (CA) and FT-IR spectroscopy. The electrical methods such as capacitance-voltage (C-V) and electrochemical impedance spectroscopy (IS) were used to investigate the whole electrolyte/SAM/SiO₂/Si structure. We will show the detection capabilities of these 2D structures, its electrical equivalent model through a simulation fitting and will discuss the application of those structures to bio-sensitive metal/oxide/silicon (MOS) devices.     

Model parameterisation of nonlinear devices using impedance spectroscopy

Spatially resolved simulation models are valuable tools to predict the behaviour (electrical, thermal, ageing) of energy storage systems. However, parameterisation of the electrical impedance-based model is often difficult because local measurements are hardly possible. For homogeneous conditions, e.g. homogeneous current, state of charge and temperature distribution, the measured overall impedance of a cell or electrode can be divided equally over the volume elements of the spatially resolved model. Homogeneous conditions are though very rare, so all larger devices show at least a current distribution because of non-zero ohmic resistances in current collectors, active masses and electrolyte.In this paper, a method is presented and validated for calculating the local impedance of nonlinear inhomogeneous devices from the overall impedance of the device. The method consists of iteration between impedance parameter identification and calculation of current distribution.     

Online monitoring of synthesis and curing of phenol-formaldehyde resol resins by Raman spectroscopy

The synthesis and curing of phenol-formaldehyde resol resins were monitored online by Raman spectroscopy. The synthesis of the resins (F/P 2.0, alkalinity 4.5 wt%) was studied at rising temperature (40-90 °C) for 90 min and at constant temperatures (80 °C, 90 °C, 100 °C, and 110 °C) for 120 min. The progress of the curing was investigated isothermally (80 °C, 90 °C, 100 °C, and 110 °C) for 120 min for three resins with different degrees of condensation. The synthesis and curing of the resins were started in the reactor and the advancement of the methylolation and condensation reactions was followed through the window of the reactor in the wave number region of 2000-400 cm⁻¹ with use of a fiber optic probe for the data collection. The Raman spectra of six model compounds (formaldehyde, phenol, 2-hydroxybenzyl alcohol, 4-hydroxybenzyl alcohol, 2-benzylphenol, and 4-benzylphenol) were analyzed to facilitate the interpretation of the spectra of the resins. The consumptions of free phenol and free formaldehyde, as well as the progress of the methylolation and condensation reactions were easily monitored by following the changes in intensity of the characteristic Raman bands. The results for the cured resins obtained by Raman spectroscopy were in good agreement with the structures and residual reactivities studied by CP/MAS ¹³C NMR spectroscopy and differential scanning calorimetry (DSC), respectively. The results of the study show Raman spectroscopy to be a promising tool for the online monitoring and control of phenol-formaldehyde resol resin synthesis and curing; in addition, Raman spectroscopy offers an effective and fast method for structural study of the solid state resins.     

Impedance spectroscopy: Over 35 years of electrochemical sensor optimization

There is considerable interest in the development of electroanalytical sensors (i.e., potentiometric, amperometric, electrochemical biosensors) for the detection of a wide range of analytes. The success of many of these sensors is governed by the condition and stability of the membrane/electrode surface. In fact, the response mechanism is dictated primarily by the surface structure and a considerable amount of work has been undertaken to characterize the interfacial region. Consequently, electrochemical impedance spectroscopy (EIS) has played a pivotal role in the characterization of many types of sensors. EIS has been used to provide information on various fundamental processes (i.e., adsorption/film formation, rate of charge transfer, ion exchange, diffusion, etc.) that occur at the electrode–electrolyte interface. Understanding and manipulating these interfacial processes has assisted in the development of membranes/electrodes with new and improved response characteristics. This paper reviews some of the work that has been undertaken using EIS over the past 35 years. More importantly, it evaluates the power of EIS in characterizing a wide range of electrochemical sensor systems.     

Non-destructive evaluation of thermally grown oxides in thermal barrier coatings using impedance spectroscopy

The growth of thermally grown oxide (TGO) layers in thermal barrier coatings (TBCs) due to the oxidation of the bondcoat alloy is a critical factor affecting the durability of TBCs. In the present study, diverse TBC specimens were subjected to long-term oxidation at various temperatures. The TGO growth mechanism was investigated according to cross-sectional images of the oxidized specimens. Impedance spectroscopy (IS) was performed to measure the electrical properties of the integrated TBCs non-destructively. Considering the influence of the TGO composition, the derived TGO electrical capacitance was found to have a good correspondence to the observed TGO thickness over a wide range 0–18.3 μm, regardless of the diverse specimens and oxidation conditions. The error was less than ±2.0 μm. With a certain design of the electrode size, IS is generalized and is recommended as an accurate and practical non-destructive evaluation method for the determination of TGO thickness within a very wide range in TBC systems under real operating conditions.     

Development of a new experimental method to determine critical pigment–volume–concentrations using impedance spectroscopy

The critical pigment–volume–concentration (CPVC) of a solvent-based epoxy coating with various pigment–volume–concentrations (PVC) was analyzed. It was shown that electrochemical impedance spectroscopy (EIS) is a suitable method to detect the CPVC of coatings. The CPVC can be derived from EIS Bode plots or by fitting the EIS-curves using simple equivalent circuit diagrams. The parameters pore resistance R_P, capacitance C of the constant phase element (CPE), the relative dielectric constant _R derived from C, and the exponent n of the CPE allow the determination of the CPVC.     

Influence of the migration of chloride ions on the electrochemical impedance spectroscopy of mortar paste

During the last decade, electrochemical impedance spectroscopy (EIS) started to be effectively exploited to investigate the electrical properties of hydrated cement pastes.This paper is devoted to the study of the high- and medium-frequency regions of the impedance spectrum and focused on determining the relationship between chloride migration and impedance spectroscopy in accelerated diffusion tests carried out on samples of mortar.After introducing an experimental protocol based on a four-electrode arrangement, we present the results of a parametric study which is related to the thickness and water-cement ratio (W/C) of the samples.Firstly, all our measurements show that the presence of chloride ions modifies the impedance response of mortar and reveals a small loop as of modification of the composition of the upstream solution. This loop is probably due to the development of interfacial phenomena between material and the solution.Secondly, as the migration of chloride process progresses, an increase followed by a decrease of the bulk electrical resistance is observed, whereas the second loop, due to the presence of chlorides, remains constant.An equivalent electrical circuit is then proposed to fit the different experimental data.     

Exploring equivalence of information from dielectric and calorimetric measurements of thermoset cure—a model for the relationship between curing temperature, degree of cure and electrical impedance

The paper presents a phenomenological model, which accounts for the observed similarity between change in the imaginary electrical impedance maximum (IIM) during the cure of a commercial epoxy system, RTM6, and the cure reaction rate data collected by differential scanning calorimetry (DSC) experiments on the same resin. The model is developed by analysing the information content of dielectric cure monitoring signals and calorimetric cure monitoring signals under both isothermal and non-isothermal cure conditions. The absolute values of the coefficients in the model equation are shown to be indicative of the relative effects of the polymerisation reaction and of the temperature on the dielectric signal. The information presented here strengthens the suggestion that on-line dielectric measurements may be used to quantify the degree of cure in thermosetting resins in real time.     

Online monitoring of the thermal degradation of POM during melt extrusion

Because of its high stiffness, chemical resistance, and low viscosity, Poly (oxymethylene) (POM) is of high relevance for technical applications. The thermal degradation of POM during processing affects its final properties and decreases the long‐term stability. The degradation is indicated by the emission of formaldehyde (FA) gas. The aim of this study is to monitor the thermal degradation of POM online, during the melt extrusion in a co‐rotating twin screw extruder (TSE). The effect of the processing conditions on the thermal stabilisation of the POM is observed by FA emission and online viscosity measurements. The effect of processing conditions on the compounding of POM with two different FA scavengers is also studied. Fourier transform infrared (FTIR) spectroscopy is used for the online measurement of FA gas and acetyl acetone colour measurement for the offline characterisation. The online viscosity is measured by passing the melt through a slit die at constant volume flow rate. An enhanced thermal degradation is found with decreasing throughput and increasing screw speed. A good correlation between the online viscosity and offline FA measurement is observed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

A theoretical approach of impedance spectroscopy during the passivation of steel in alkaline media

A theoretical impedance function is deduced for a proposed mechanism of passive film formation of steel in contact with alkaline aqueous media involving two reaction intermediates: mixed oxide with similar stoichiometry to magnetite and Fe(III)-oxides. The reduction reaction of dissolved oxygen is considered as the only cathodic reaction compensating the anodic current induced by the formation of iron oxides at open circuit potential. The iron dissolution takes place through a chemical dissolution of ferric oxide. A two-layered passive film with 3D structure is considered. A satisfactory agreement between the digital simulations on the basis of the theoretical impedance function and experimental spectra validates the proposed model.     

The effect of second phase La0.67TiO2.87 on the phase structure and impedance spectroscopy of La2Ti2(1 + x)O7 piezoelectric ceramics

The Ti excess La₂Ti_{2 (1+x)} O₇ (x = 0, 0.005, 0.01, 0.02, 0.05, 0.1) piezoelectric ceramics have been prepared by sol-gel technology and solid state synthesis method. Through refinement analysis, the phase structure of the ceramics varies with Ti content. Most monoclinic phase (∼93%) and a handful of orthogonal phase (∼7%) coexist in La₂Ti_{2 (1+0)} O₇ ceramics. Pure monoclinic phase La₂Ti₂O₇ with space group P2₁ appears in La₂Ti_{2 (1+0.005)} O₇ and La₂Ti_{2 (1+0.01)} O₇ ceramics. Monoclinic phase La₂Ti₂ O₇ and a certain proportion of tetragonal phase La_0.67TiO_2.87 coexist in La₂Ti_{2 (1+0.02)} O₇, La₂Ti_{2 (1+0.05)} O₇ and Ti_{2 (1+0.1)} O₇ ceramics. With the excess of Ti content, the monoclinic phase ratio and distortion angles in a-b projection plane of the ceramics increase first and then decrease, which is consistent with the variation tendency of piezoelectric constant d₃₃. The excellent piezoelectric constant for Ti_{2 (1+0.01)} O₇ ceramics is 2.8 pC/N.Impedance analysis shows that the conductive mechanisms of all samples include both grain and grain boundary conductivity at temperature range T ≥ 500 °C. The formation of tetragonal phase La_0.67TiO_2.87 derives from Ti excess in pure monoclinic phase La₂Ti₂O₇. The existence of tetragonal phase La_0.67TiO_2.87 can obviously increase the capacitance of ceramics at x ≥ 0.05. All prepared piezoelectric La₂Ti_{2 (1+x)} O₇ ceramics have highly frequency stability and are candidates for ultrahigh temperature piezoelectric application.     

Voltammetric and electrochemical impedance spectroscopy characterization of a cathodic and anodic pre-treated boron doped diamond electrode

The effect of boron doped diamond (BDD) surface termination, immediately after cathodic and anodic electrochemical pre-treatments, on the electrochemical response of a BDD electrode in aqueous media and the influence of the different supporting electrolytes utilized in these pre-treatments on the final surface termination was investigated with [Fe(CN)₆]^4−/3−, as redox probe, by cyclic and differential pulse voltammetry and electrochemical impedance spectroscopy. The cyclic voltammetry results indicate that the electrochemical behavior for the redox couple [Fe(CN)₆]^4−/3− is very dependent on the state of the BDD surface, and a reversible response was observed after the cathodic electrochemical pre-treatment, whereas a quasi-reversible response occurred after anodic electrochemical pre-treatment. Differential pulse voltammetry in acetate buffer also showed that the potential window is very much influenced by the electrochemical pre-treatment of the BDD surface. Electroactivity of non-diamond carbon surface species (sp² inclusions) incorporated into the diamond structure was observed after cathodic and anodic pre-treatments. Electrochemical impedance spectroscopy confirmed the cyclic voltammetry results and indicates that the BDD surface resistance and capacitance vary significantly with the electrolyte and with the electrochemical pre-treatment, caused by different surface terminations of the BDD electrode surface.     

Kinetic analysis of electrosorption using fast Fourier transform electrochemical impedance spectroscopy: underpotential deposition of Bi in the presence of coadsorbing ClO4 on gold

We demonstrate a relatively simple method for studying various kinetic aspects of metal electrodeposition in the presence of coadsorbing anions on an electrode surface. This method combines cyclic voltammetry (CV) with fast Fourier transform electrochemical impedance spectroscopy (FFT-EIS), where both steady state and transient behaviors of the interface are probed simultaneously. As a model surface reaction, we use electrodeposition of Bi³⁺ in the presence of ClO₄⁻ adsorption from an aqueous solution onto a gold electrode. The voltage range for underpotential deposition (UPD) of Bi³⁺ is determined with potential step (PS) experiments. The voltage dependent UPD coverage of Bi on Au is determined by analyzing the CV data. The relevant kinetic parameters of both Bi³⁺ and ClO₄⁻ deposition reactions under the conditions of CV are measured with FFT-EIS, and analyzed using a complex nonlinear least square method. The differences and similarities between the electrosorption characteristics of Bi³⁺ and ClO₄⁻ on Au are discussed in terms of these kinetic parameters.     

Effect of hot-pressing sintering on thermal and electrical properties of AlN ceramics with impedance spectroscopy and dielectric relaxations analysis

The effects of hot-pressing sintering on the phase composition, microstructure, thermal and electrical properties of AlN ceramics with CeO₂–CeF₃ additives were studied. During hot-pressing sintering, high pressure reduced the grain boundary phase CeAlO₃ and decreased the concentration of oxygen in AlN ceramics. The hot-pressing sintered AlN samples had a much higher thermal conductivity of 191.9 W/m·K than pressureless sintered ones because of the great reduction of grain boundary phases and oxygen impurities in AlN ceramic. As the carbon content in hot-pressing sintered sample was very high, carbon contamination led to the decrease in electrical resistivity and changes in polarization mechanisms for AlN ceramics. The relaxation peak in the dielectric temperature spectrum with an activation energy of 0.64 eV for hot-pressing sintered samples was caused by electrons from free carbon at low temperature. Overall, hot-pressing sintering can effectively increase the thermal conductivity and change the electrical properties of AlN ceramics.     

A comparison of attenuated total reflectance-FTIR spectroscopy and GPC for monitoring biodiesel production

Gel permeation chromatography (GPC) and attenuated total reflectance (ATR)-FTIR spectroscopy were used to monitor the products of transesterification of waste frying oil in methanol to FAME or biodiesel. To evaluate the reliability and reproducibility of each method, quantitative analyses of mixtures of standards (TG, DG, MG, FAME, and glycerol) as well as lipid products of transesterification were carried out. The reproducibility of each method was found to be within ±1–5%. The differences between the results of the two methods were less than ±2%. The GPC method showed good separation of the intermediate products MG and glycerol from the TG starting material and FAME, but DG were not completely separated from TG, GPC gave good quantitative results for MG and FAME, but the TG and DG analyses required correction, depending on the mole ratio of TG/DG. In contrast, ATR-FTIR spectroscopy could only give quantitative data for the sum of TG+DG+MG.     

Development of a new experimental method to determine critical pigment-volume-concentrations using impedance spectroscopy : Part II: Solvent based coatings with components typical for commercial organic anticorrosion coatings or with nanoparticles

Electrochemical impedance spectroscopy (EIS), which has been previously demonstrated to be a suitable method to determine the CPVC coatings [R.E. Lobnig, W. Villalba, K. Goll, J. Vogelsang, I. Winkels, R. Schmidt, P. Zanger, J. Soetemann, Development of a new experimental method to determine critical pigment-volume-concentrations using impedance spectroscopy, Prog. Org. Coat. 55 (2006) 363–374.], was applied to four series of coatings with components typical for solvent based commercial organic anticorrosion coatings, and to three series of coatings with nanopigments. For all coatings, the CPVC determined using EIS correlated very well with the CPVC determined from cross-sections with electron microscopy (SEM). The EIS method was also useful for the coatings with nanopigments, for which pores were too small to be detected by SEM-analysis of cross-sections.     

In‐line monitoring of hydrogen peroxide in two‐phase reactions using raman spectroscopy

Hydrogen peroxide is an environment‐friendly oxidizer, which is used in several chemical processes. However, safety necessitates the determination and control of the concentration of hydrogen peroxide during oxidation reactions. We propose a methodology to monitor hydrogen peroxide in disperse two‐phase reaction mixtures based on in‐line Raman spectroscopy. We compare indirect hard modeling (IHM), peak integration (PI), and partial least squares (PLS). Building predictive PLS and PI calibration models is challenging, whereas the IHM calibration is easy to develop. These methods show good accuracy for known samples (root mean square error of cross validation [RMSECV] of 0.3–0.7 wt %) compared to the classic titration method (RMSECV of 0.4 wt %). After calibration, inline monitoring during reaction is performed demonstrating that the concentration of hydrogen peroxide can be successfully monitored in a fast and reliable way by Raman spectroscopy. The IHM seems to give slightly better inline predictions. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3994–4002, 2017     

Time resolved impedance spectroscopy as a probe of electrochemical kinetics: The ferro/ferricyanide redox reaction in the presence of anion adsorption on thin film gold

In this work, we use the time resolved technique of Fourier transform electrochemical impedance spectroscopy (FT-EIS) for quantitative kinetic analyses of Faradaic and nonfardaic, as well as heterogeneous and adsorption reactions. As a model system, we investigate the Fe(CN)₆^3−/4− redox reaction on a thin film Au electrode in the presence of nonfaradaically adsorbing ClO₄⁻ or Cl⁻ in neutral electrolytes. The redox process indicates a predominantly Faradaic outer sphere reaction, with nearly negligible adsorption of reactants on Au. dc voltage dependent kinetic parameters for both the anion and redox reactions are obtained by FT-EIS in real time during cyclic voltammetry. Equilibrium values of reaction rate constants, adsorption resistances and pseudocapacitances for the redox couple are also obtained. The theoretical background and working principles of these measurements are discussed, and the experimental findings are compared with previously published data.