Electrolyte Ionic Strength Research Articles

A precision structured smart hydrogel for sensing applications

We report on a macroinitiator based smart hydrogel film applied on a microcantilever for sensing applications. The studied hydrogel features a comparatively wide dynamic range for changes in the electrolyte's ionic strength. Furthermore, it offers a simple spin coating process for thin film deposition as well as the capability to obtain high aspect ratio microstructures by reactive ion etching. This makes the hydrogel compatible to microelectromechanical system integration. As a proof of concept, we study the response of hydrogel functionalized cantilevers in aqueous sodium chloride solutions of varying ionic strength. In contrast to the majority of hydrogel materials reported in the literature, we found that our hydrogel still responds in high ionic strength environments. This may be of future interest for sensing e.g., in sea water or physiological environments like urine.

Understanding Effective Electronic Conductivity of Slurry Electrodes Used in Electrochemical Flow Capacitors and All-Iron Redox Flow Batteries

Slurry electrodes are being considered in electrochemical systems1-5 for decoupling power and energy in electrochemical flow capacitors (EFCs) and for redox flow batteries (RFBs) involving electrodeposition reactions. In our all-iron slurry flow batteries (IFBs)3,4 utilizing very low-cost electrolyte materials, the slurry electrodes are used in the negative side involving iron plating/de-plating. During charging, iron is plated onto the slurry particles and then the particles carry the iron metal out of the electrochemical cell to be stored in the external electrolyte reservoirs. The slurry electrodes are synthesized by dispersing conductive carbon particles into electrolyte contained dissolved iron species. The effective electronic conductivity of slurry electrodes affects current density distributions, over-potential boundary layers, utilizations of slurry and electrochemical performance of EFCs and IFBs4. Therefore, fundamental studies of slurry electronic conductivity are needed. Possible factors affecting the effective electronic conductivity of a slurry include carbon loading, mean linear velocity or shear rate, electrolyte ionic strength, particle size, particle shape, surface chemistry, etc. In this work, we studied the effect of mean linear velocity through the slurry electrode channel ranging from stagnant to 22 cm/s on slurry effective electronic conductivity for various loadings of 660R carbon black dispersed in deionized water. A DC measurement using a chronoamperometry technique was made in a 3.5 cm2 electrode cell design. Figure 1 shows the variations of effective electronic conductivity for three different loadings, 5% w.t., 10% w.t. and 15% w.t. of 660R carbon black slurry under mean linear velocity increasing from 0 to 22 cm/s. The Figure 1 presents evidence that shear rates induced by different mean linear velocities/flow rates have certain effects on effective electronic conductivity of slurry electrodes. This is may be due to changes in the agglomeration patterns5of slurry particles. The motivation of this work is to explore possible mechanisms that lead to a better understanding of slurry electrodes. The ultimate goal is to improve the effective electronic conductivity of slurry electrodes and optimize the electrochemical performance of EFCs and IFBs. Acknowledgement This work is supported in-part by the all-iron flow battery project (award number: DE-AR0000352) funded by Department of Energy (DOE) of the United States.

Concentration-Gradient Stabilization with Segregated Counter- and Co-Ion Paths: A Quasistationary Depletion Front for Robust Molecular Isolation or Concentration.

We study the spatiotemporal dynamics of a microfluidic system with a nonselective microfluidic channel gated by an ion-selective membrane which separates the ion flux paths of cations and anions. To preserve electroneutrality, the ionic concentration in the system is shown to converge to a specific inhomogeneous distribution with robust constant current fluxes. A circuit scaling theory that collapses measured asymptotic currents verifies that this is a generic and robust mechanism insensitive to channel geometry, ion selectivity, and electrolyte ionic strength. This first temporally stationary but spatially inhomogeneous depletion front can be used for modulating ionic current and for isotachophoretic isolation of low-mobility molecules and exosomes on small diagnostic chips for various medical applications that require robust high-throughput and integrated platforms.

The Jones-Ray effect reinterpreted: Surface tension minima of low ionic strength electrolyte solutions are caused by electric field induced water-water correlations

The surface tension of electrolyte solutions exhibits a minimum at millimolar electrolyte concentrations and then rises with increasing concentration. This minimum, known as the Jones-Ray effect, has been hotly debated over the past ∼80years. If not considered as an artifact, it is typically ascribed to a phenomenological rare binding site for ions or ion pairs. Here, we propose an alternative underlying mechanism, namely that the hydrogen bond network of water responds to the collective electrostatic field of ions by increasing its orientational order, supported by recent surface tension measurements of NaCl solutions in H2O and D2O, and second harmonic scattering experiments in combination with ion resonant second harmonic reflection experiments. Recent thermodynamic and purely electrostatic treatments of the surface tension provide support for this interpretation. In addition, concerns related to possible artifacts influencing the measurements are quantified experimentally.

Protein Sensing Beyond the Debye Length Using Graphene Field-Effect Transistors

Sensing biomolecules in electrolytes of high ionic strength has been a difficult challenge for field-effect transistor-based sensors. Here, we present a graphene-based transistor sensor that is capable of detection of antibodies against protein p53 in electrolytes of physiological ionic strength without dilution. As these molecules are much larger than the Debye screening length at physiological ionic strengths, this paper proves the concept of detection beyond the Debye length. The measured signal associated with the expected specific binding of the antibodies to p53 is concluded to result from resistance changes at the graphene-electrolyte interface, since a sensor responding to resistance changes rather than charge variations is not limited by Debye screening. The conclusion with changes in interface resistance as the underlying phenomena that lead to the observed signal is validated by impedance spectroscopy, which indeed shows an increase of the total impedance in proportion to the amounts of bound antibodies. This finding opens up a new route for electrical detection of large-size and even neutral biomolecules for biomedical detection applications with miniaturized sensors.

Investigating the effect of ionic strength on the suppression of dendrite formation during metal electrodeposition.

The effect of ionic strength on the electrodeposition of silver has been investigated in acetonitrile (MeCN) containing TBAPF6 or in the ionic liquid [EMIm][OTf]. The use of an ionic liquid allows a greater ionic strength to be investigated as the solubility limits of supporting electrolytes in organic solvents can be overcome using neat ionic liquid. The SEM and XRD data show that polycrystalline silver is deposited in a fcc structure and that dendrite formation is retarded at high ionic strength. Electrochemical measurements undertaken in electrolytes of low ionic strength indicate that the deposition and growth of a few nuclei is preferred and leads to dendrite formation. However, at higher ionic strength, the deposition and growth of significantly more nuclei is observed and therefore dendrite growth rates and tip currents are lower leading to the deposition of spherical particulates. Crucially, the data shows that if the ionic strength of the electrolyte is controlled there are no differences between ionic liquids and molecular solvents for the electrodeposition of silver.

All-printed planar photoelectrochemical cells with digitated cathodes for the oxidation of diluted aqueous pollutants.

A novel outline of a planar photoelectrochemical cell consisting of a semiconductor layer topped by subsequent layers of a digitated insulator and counter electrode is introduced. The use of vertically separated electrodes represents a major development in reducing the footprint (inactive areas) of planar electrochemical cells. The cells, consisting of a nanoparticular titania photoanode and a digitated, metallic cathode, were fabricated by a strictly additive process employing material printing as the exclusive deposition and patterning tool. Transparent conductive oxide-coated glass and polyethyleneterepthalate sheets were used as substrates; nanocrystalline titania dispersion bonded by a novel organosilica binder was used for the fabrication of the photoanode and gold or carbon inks for the fabrication of the digitated cathodes. Due to the digitated shaping of the cathode, photoelectrochemical response was not suffering from iR drop down to low electrolyte ionic strengths. The printed cells were used for electroassisted photocatalytic degradation experiments with aqueous solutions of coumarin. Considerable acceleration of the coumarin degradation rate compared to the plain photocatalytic mode was observed.

Quantifying the Influence of the Crowded Cytoplasm on Small Molecule Diffusion.

Cytosolic crowding can influence the thermodynamics and kinetics of in vivo chemical reactions. Most significantly, proteins and nucleic acid crowders reduce the accessible volume fraction, ϕ, available to a diffusing substrate, thereby reducing its effective diffusion rate, Deff, relative to its rate in bulk solution. However, Deff can be further hindered or even enhanced, when long-range crowder/diffuser interactions are significant. To probe these effects, we numerically estimated Deff values for small, charged molecules in representative, cytosolic protein lattices up to 0.1 × 0.1 × 0.1 μm(3) in volume via the homogenized Smoluchowski electro-diffusion equation. We further validated our predictions against Deff estimates from ϕ-dependent analytical relationships, such as the Maxwell-Garnett (MG) bound, as well as explicit solutions of the time-dependent electro-diffusion equation. We find that in typical, moderately crowded cell cytoplasm (ϕ ≈ 0.8), Deff is primarily determined by ϕ; in other words, diverse protein shapes and heterogeneous distributions only modestly impact Deff. However, electrostatic interactions between diffusers and crowders, particularly at low electrolyte ionic strengths, can substantially modulate Deff. These findings help delineate the extent that cytoplasmic crowders influence small molecule diffusion, which ultimately may shape the efficiency and timing of intracellular signaling pathways. More generally, the quantitative agreement between computationally expensive solutions of the time-dependent electro-diffusion equation and its comparatively cheaper homogenized form suggest that the latter is a broadly effective model for diffusion in wide-ranging, crowded biological media.

On the electrophoretic mobility of succinoglycan modelled as a spherical polyelectrolyte: From Hermans-Fujita theory to charge regulation in multi-component electrolytes

Literature interpretations of the electrophoretic mobility of spherical polyelectrolytes are revisited using the capillary-electrophoresis data of Duval et al. (2006) for the extracellular polysaccharide succinoglycan as an example. Subtle changes in the polyelectrolyte mobility have recently been attributed to new electrokinetic theories that feature multi-component electrolytes, charge regulation, and the so-called polarization and relaxation phenomena. However, these calculations exhibit several unusual trends that have yet to be explained, and so the conclusions drawn from them are controversial. Here, independent computations strengthen conclusions drawn from the original model of Duval et al., i.e., the discrepancies between experiments and all the presently available electrokinetic theories reflect changes in the conformation of succinoglycan arising from changes in the electrolyte pH and ionic strength.

Size distribution of FEBEX bentonite colloids upon fast disaggregation in low-ionic strength water

AbstractBentonite colloids generated from the backfill barrier in nuclear waste repositories may act as radionuclide carriers, if they are stable and mobile. Repository scenarios with highly saline groundwater inhibit colloid stability as particles tend to aggregate but, in the time frame of repositories, groundwater conditions may evolve, promoting particle disaggregation and stabilization. The disaggregation of FEBEX bentonite colloids by fast dilution to lower ionic strength was analysed in this study. Time-resolved dynamic light-scattering experiments were carried out to evaluate the kinetics of bentonite colloid aggregation and disaggregation processes in Na+ and Na+-Ca2+ mixed electrolytes of low ionic strength. Attachment and detachment efficiencies were determined.Aggregation is promoted by increasing ionic strength, being more efficient in the presence of divalent cations. Once bentonite colloids are aggregated, a decrease in ionic strength facilitates disaggregation, but the process is not fully reversible as the initial size of the stable bentonite colloids at low ionic strength is not fully recovered. Particle-size distribution and concentration in suspension were analysed on disaggregated samples by single particle-counting measurements. Small colloids were measured in the disaggregated samples but their population was smaller than in the initial stable sample, especially in the presence of Ca2+.

EOR Potential of Mixed Alkylbenzenesulfonate Surfactant at Low Salinity and the Effect of Calcium on “Optimal Ionic Strength”

The enhanced oil recovery (EOR) potential of different alkylbenzenesulfonate surfactants was investigated in a combined study of crude oil–water phase behavior and interfacial tension (IFT) and macroscopic oil displacement studies. In the presence of small amounts of calcium ions (calcium/sodium = 4 mol %), ultralow oil–brine IFT (<0.001 mN/m) was observed at ionic strengths 10 times lower than “optimal salinity” with sodium chloride only; this suggested an application in low salinity surfactant (LSS) EOR. “Optimal ionic strength” was determined for different calcium/sodium ratios in the brine, which further allowed prediction of phase behavior and IFT range for a given surfactant and a given crude oil at different brine compositions. In laboratory core floods, crude oil displacement by LSS with an optimal ionic strength electrolyte was around 9% higher than at “non-optimal” conditions.

Growth and structure of flocs following electrocoagulation

Abstract The growth and structure of iron precipitate flocs produced in salt water from a bench-scale electrocoagulation (EC) system was investigated. During floc growth, changes in the scattering exponent, an indicator of a flocs’ degree of compaction, were reflected by changes in the particle size (diameter of a volume equivalent sphere). Flocs initially formed loose, open structures that spanned broad size ranges, suggesting low repulsion between particles. The initial aggregates then broke and reformed into more compact structures. Comparing plots of scattering exponent against time, it was found that operating at higher currents caused this process to occur more quickly; however, the plots were all similar in shape, suggesting that the structural progression of the EC flocs was not affected. The final floc structures had an average scattering exponent of 2.34 (standard deviation 0.02), which is consistent with literature for flocs produced from iron-based chemical coagulants despite differences in electrolyte ionic strength. Analysis via transmission electron microscopy (TEM) suggested that EC flocs also exhibited amorphous, fractal structures. Operating at higher currents (providing larger iron concentrations) resulted in larger flocs. The average steady-state floc sizes when operating at 0.087, 0.174, and 0.261 A were 93, 147, and 191 μm, respectively. Floc size distributions also reached steady-state more quickly due to the higher frequency of particle collisions. Using a stainless steel (SS) cathode rather than an aluminum one resulted in 15% larger flocs (154 μm and 134 μm, respectively). However, given that experimental conditions (mixing and current) were otherwise equivalent, more research is required to determine the cause of this difference. The results of this work were consistent with literature regarding chemical coagulation (CC) and may have implications for the design of downstream clarification processes.

Inkjet Printed Interdigitated Conductivity Cells with Low Cell Constant

A novel approach to the fabrication of gold conductive patterns on alumina, silica, glass and polyimide (Kapton) substrates and physical properties of printed interdigitated cells for measuring electrolyte conductance are reported. The process is based on the direct pattering of a gold resinate dissolved in organic solvent by inkjet printing and firing the prints at 400–800°C. The resulting interdigitated electrode devices (16 mm × 52 mm exposed area) consisted of 52 to 9 finger pairs of 240 to 1500 μm wide fingers and spaces, respectively. The sheet resistance of the printed Au layers was around 0.27 Ω on alumina (annealed at 800°C), and 0.86 and 1.18 Ω on glass and Kapton, respectively (annealed at 400°C). The conductivity cell constants of 0.008 to 0.08 cm−1 of the interdigitated devices were found close to theoretically predicted values. Cells with the lowest cell constants are useful for the measurement of electrolytes of low ionic strength, including ultrapure water. Of the three substrates used, alumina has the advantage of withstanding high temperature curing, whereas Kapton features flexibility, and glass transparency.

Corona charge regulation in nanoparticle electrophoresis

Nanoparticle (NP) size and charge play key roles in bioconjugation chemistry, imaging and drug delivery. Although the electrophoretic mobility and hydrodynamic size are routinely measured, interpreting these data can be extremely difficult. Here, the challenge is addressed via an electrokinetic model for spheres bearing a soft amphoteric corona, the charge of which is regulated by a multi-component electrolyte. The model is applied to NPs with a metallic core to which are grafted poly(ethylene glycol) chains with either weak acid or amphiprotic end groups. The results elucidate the separate roles of electrolyte pH and ionic strength on the electrophoretic mobility and diffusion coefficient. In this study, the forces were evaluated directly, rather than from the Stokeslet velocity disturbances. While the second-order convergence was demonstrated by both methods, the direct approach, which uses only the inner part of the global solution, furnished superior accuracy and robustness. This may benefit future attempts to model the dielectric and electroacoustic properties of these complex nanoparticulates.

A Membrane-Based Electro-Separation Method (MBES) for Sample Clean-Up and Norovirus Concentration.

Noroviruses are the leading cause of acute gastroenteritis and foodborne illnesses in the United States. Enhanced methods for detecting noroviruses in food matrices are needed as current methods are complex, labor intensive and insensitive, often resulting in inhibition of downstream molecular detection and inefficient recovery. Membrane-based electro-separation (MBES) is a technique to exchange charged particles through a size-specific dialysis membrane from one solution to another using electric current as the driving force. Norovirus has a net negative surface charge in a neutrally buffered environment, so when placed in an electric field, it moves towards the anode. It can then be separated from the cathodic compartment where the sample is placed and then collected in the anodic compartment for downstream detection. In this study, a MBES-based system was designed, developed and evaluated for concentrating and recovering murine norovirus (MNV-1) from phosphate buffer. As high as 30.8% MNV-1 migrated from the 3.5 ml sample chamber to the 1.5 ml collection chamber across a 1 μm separation membrane when 20 V was applied for 30 min using 20 mM sodium phosphate with 0.01% SDS (pH 7.5) as the electrolyte. In optimization of the method, weak applied voltage (20 V), moderate duration (30 min), and low ionic strength electrolytes with SDS addition were needed to increase virus movement efficacy. The electric field strength of the system was the key factor to enhance virus movement, which could only be improved by shortening the electrodes distance, instead of increasing system applied voltage because of virus stability. This study successfully demonstrated the norovirus mobility in an electric field and migration across a size-specific membrane barrier in sodium phosphate electrolyte. With further modification and validation in food matrixes, a novel, quick, and cost-effective sample clean-up technique might be developed to separate norovirus particles from food matrices by electric force.

Inkjet-printed interdigitated cells for photoelectrochemical oxidation of diluted aqueous pollutants

Planar, interdigitated photoelectrochemical cells were made by inkjet printing. The electrode fingers had widths ranging from 200 to 1500 µm and were revealed by printing a positive protective polymer mask on fluorine-doped tin oxide-coated glass slides and subsequent etching. One finger family was overprinted by an ink-jettable sol–gel composition based on titanium propoxide which was then converted into TiO2 by annealing in air. An incident photon to current conversion efficiency of 0.19 was obtained at 360 nm for 200-nm-thick films. The influence of electrode geometry and titania thickness on the electrochemical properties of resulting cells is discussed in detail. Due to the interdigitated layout, photoelectrochemical response was not suffering from iR drop down to low electrolyte ionic strengths. The printed cells were used in photoelectrocatalytic degradation experiments of aqueous solutions of benzoic acid by broadband ultraviolet irradiation (UVA) and electric bias of 1 V and delivered considerable acceleration of the degradation process compared to the plain photocatalytic mode.

Factors influencing the stability of the polysucrose/alumina system

The influence of such factors as polymer concentration (10–800 ppm), pH of the solution (3–9), and ionic strength of the background electrolyte (0.001–0.1) and the presence of surfactants (SDS, CTAB, TX-100) on the stability of the colloidal suspensions alumina/polysucrose were measured spectrophotometrically. The obtained results, discussed together with the adsorption, the surface charge density, and the surface tension data, showed that the measured suspensions are unstable and flocculate quite easily. Although due to the fact that polysucrose (PS) considered to be neutral proved to be slightly negatively charged, there is a chance to control the stability using the abovementioned factors. The most effective way to obtain stable Al2O3/PS suspensions was the addition of SDS to the system. Because of the competitive adsorption between PS and sodium dodecyl sulfate (SDS) at the alumina surface, the larger number of free macromolecules of PS is present in the bulk solution, what leads to the depletion stabilization process. The other examined factors such as the polymer concentration, pH of the solution, and the ionic strength of the electrolyte have minor influence on the stability of the Al2O3/PS suspensions. The obtained results create new possibilities in the fields of functionalized materials such as adsorbents and catalysts.

Electroanalytical Determination of Benzoic, Oxalic and Glyoxylic Acids Using Platinum Electrode

Due to the importance of Benzoic, Oxalic and Glyoxylic acids, a voltammetric technique using platinum electrode was developed to identify the mentioned acids. The electrochemical behavior of Benzoic, Oxalic and Glyoxylic acids was studied using cyclic voltammetric technique at platinum wire electrode in sodium nitrate as a supporting electrolyte. Benzoic acid gave an oxidation peak at +0.096 V while oxalic acid showed a well single oxidation peak at +1.05 V (vs. AgǀAgCl). Glyoxalic acid showed a reduction peak in the presence of hexacynoferrite K4[Fe2(CN)6] at +0.126V. The effect of supporting electrolyte kind, pH solution, ionic strength and scan rate on the oxidation peak of Benzoic and oxalic acids or the reduction peak of glyoxylic acid were investigated. Linear sweep voltammetry was used for the electrochemical determination of these acids under the optimum experimental conditions . A concentration of 8x10-5 mol.dm-3 lower detection limit was obtained for benzoic acid and a value of 9x10-5 mol.dm-3 was detected for Oxalic and Glyoxylic acid. The effect of some metal ions, amino acids, urea and ascorbic acid as interferences was examined. The results showed that the effect ranged between -5.0 to+14.0 % on peak response. Benzoic, Oxalic and Glyoxylic acids were successfully determined in industrial waste water. Keywords: Benzoic acid, electrochemical behavior, glyoxylic acid, interferences, linear sweep, oxalic acid, platinum electrode, voltammetry.

Batch Studies for the Investigation of the Mechanism of Pb Sorption in Selected Acid Soils of China

The experiment focuses on mechanisms of Pb retention on acid soils. A batch experiment was conducted to investigate the effect of solution pH and ionic strength of electrolytes which will show the mechanisms of Pb retention on the soils. Result show that sorption of lead was affected strongly by solution pH and ionic strength of electrolytes. Retention of lead increased with increase in solution pH and decreased with increase in ionic strength of electrolytes. This suggests that surface complexation and ion exchange are the mechanisms of Pb retention on these acid soils. At pH above 6 there was precipitation of lead. SEM studies visualized the formation of white layers of Pb over the soil surface. Scanning electron microscopy (SEM) revealed that the adsorption of lead ions made the surface of the soil particles rougher than those without lead. This morphological change points to the formation of a surface coating on the soil particles.

Frequency response analysis of potential-modulated orientation changes of a DNA self assembled layer using spatially resolved fluorescence measurements.

Characterization of a gold electrode modified with a mixed self-assembled monolayer composed of ssDNA and mercaptohexanol is described using in-situ fluorescence microscopy with particular focus on the study on the homogeneity of the surface in terms of its response to potential perturbation. A change in DNA orientation is driven by the charge on the gold surface, and the extent and the rate of orientation change can be determined using fluorophore modified DNA and fluorescence. Characterization of the surface is typically performed using techniques that determine the average characteristics of the surface. We show that significant differences in the response of various regions of the electrode surface can be observed. These differences can characterized by potential step-imaging measurements or by frequency response analysis of the fluorescence - potential transfer function. Important for high frequency measurements in these low ionic strength electrolytes is the use of a fourth electrode and shunt capacitor which was found to influence the electrochemical impedance results as well as the optical results. The magnitude of the transfer function was found to vary significantly for different regions on the electrode. We demonstrate this for two 95 μm regions that are on either side of a grain boundary. The phase of the transfer function was found to be more sensitive to variations in the rate of reorientation between these two regions compared to considering only the magnitude. The variation in the response over the electrode surface is an important consideration for the DNA-based biosensing devices based on modified electrode interfaces.