Bulk Electrolyte Solution Research Articles

Tangential flow streaming potential measurements: Hydrodynamic cell characterization and zeta potentials of carboxylated polysulfone membranes

Computational fluid dynamics calculations were carried out to ensure that a self-made tangential flow mode streaming potential measurement cell meets the hydrodynamic stipulations of laminar, steady and established electrolyte flow necessary for reproducible electrokinetic measurements. The calculations show that the cell design meets all of these conditions. Six carboxylated polysulfones with a range of different degrees of substitution (DS) from 0.26 to 1.74 carboxyl groups per polymer repeat unit were synthesized in a two-stage process of lithiation and carboxylation. Ultrafiltration membranes were made from both the unmodified polysulfone and these hydrophilic materials. The zeta potentials of these membrane surfaces were determined in 0.001 M KCl solution as a function of pH. The curves show the theoretically expected profiles for non-ionic and weakly acidic materials. The growing influence of the COOH dissociation on the surface charge formation is indicated by the flattening of the curves at low pH values. The magnitude of the negative zeta potentials plateau values ranged from −52 to −20 mV. While unmodified PSU has a plateau value of −52 mV this value decreases continuously with increasing DS to −20 mV for the PSU-COOH 1.74 material. It is suggested that this arises from a shift of the electrokinetic shear plane into the bulk electrolyte solution due to an extended swelling layer reflecting the enhanced hydrophilicity of these membrane surfaces.

Ion-ion correlations in electric double layers from Monte Carlo simulations and integral equation calculations Part 2. Case of added salt

Anisotropic ion–ion distribution functions gij (R 1, R 2), where R 1 and R 2 are the positions of ions of species i and j respectively, and various other properties of electric double layer systems have been calculated by simulation and integral equation methods. The system is composed of a 1 : 1 or 2 : 1 electrolyte solution between two planar walls with rather high surface charge density (0·267 Cm-2) and in equilibrium with a bulk electrolyte solution with a certain concentration (1·0 M or 2·0 M). In the integral equation calculations, the hypernetted chain (HNC) or the reference hypernetted chain (RHNC) closure has been applied for the ion-ion distribution functions in the inhomogeneous electrolyte (the so-called anisotropic HNC or RHNC approximations). The Widom test particle technique has been used to calculate the ion–ion distribution functions in the simulations. The results of the anisotropic RHNC calculations and the simulations are in almost perfect agreement throughout (they virtually coincide). The HNC approximation is almost as good as the RHNC approximation for 2 : 1 electrolytes, but it is not as good for 1 : 1 electrolytes due to the higher packing density of the monovalent counterions near the surfaces. The excellent results for the pair distributions ensures that thermodynamic properties calculated in the RHNC approximation will be very good. The double layer interaction pressure, which is sensitive to details of the distributions, is indeed in excellent agreement with the simulations. The results also show that the Widom technique is successful for calculating pair distributions for inhomogeneous electrolytes by simulation.

Use of the Poisson−Boltzmann Equation To Estimate The Electrostatic Free Energy Barrier for Dielectric Models of Biological Ion Channels

We have obtained the electrostatic free energy profile of a univalent positive ion as it is moved from a bulk electrolyte solution into a biological ion channel of various model forms. The form of the results was unaffected by the method of calculation. We used a fully consistent nonlinearized Poisson−Boltzmann scheme, a fully consistent linearized Poisson−Boltzmann scheme, and a hybrid scheme. The channel models we used were designed to (crudely) mimic the shape and imbedded charge and dipolar distributions for biological ion-selective channels (such as the voltage-gated K+ “shaker” channel). The results obtained were accounted for on simple physical grounds.

Hemoglobin modified bilayer lipid membranes (BLMs) biosensor for carbon dioxide detection

The present paper describes the modification of self-assembled bilayer lipid membranes (s-BLMs) on a metal support with hemoglobin to develop a novel electrochemical minisensor for the rapid detection of carbon dioxide. Modification was achieved by the introduction of hemoglobin into the bulk electrolyte solution and ion conductivity of BLMs increased with additions of NaHCO 3 in solution. The detection limit of CO 2 detection was 0.375 μM using BLMs containing 15% DPPA. The reversibility of the phenomenon of carbon dioxide binding to hemoglobin could be investigated by the use of s-BLMs. The present carbon dioxide sensor can be fabricated at low cost, with fast response times (about 10 s) and the capability of analyzing small volumes of samples. The long-term stability of the hemoglobin modified BLM-based biosensor is routinely over 48 h.

Aggregation phenomena occurring on an electrode surface in the presence or absence of aggregation in the bulk electrolyte solution

Abstract One single-step and two multi-step models for aggregation processes occurring on an electrode surface in the presence or absence of aggregation phenomena in the bulk solution are used to study the properties of an adsorbed monolayer when the adsorbate monolayer when the adsorbate molecules are able to form aggregates. It is shown that, despite the fact that the three models predict considerable different size distributions of the intermediate aggregates, they give almost the same picture about the composition of an adsorbed monolayer and its capacitance characteristics. According to these models, the only indication of the occurrence of a surface aggregation process in the capacitance plots is the appearance of asymmetric peaks with one vertical side. Bulk aggregates do not affect the shape of the capacitance curves. If they are large, they stabilize the composition of the adsorbed layer and the shape of the capacitance curves remains unchanged with the increase in the surfactant concentration. If they are small, i.e. oligomers, their effect is to reduce the adsorbate concentration in the bulk solution. Finally, the discrepancies between theory and experiment are indicated and the possibility that adsorbed bulk aggregates disintegrate very slowly on the electrode surface is discussed.

Conductivity and Dielectric Dispersion of Gram-Positive Bacterial Cells

The conductivity of bacterial cell suspensions has been studied over a wide range of ionic strengths and is interpreted in terms of their cell wall properties. The experimental data have been analyzed after improving the high κadouble-layer theory of Fixman, by accounting for ionic mobility in the hydrodynamically stagnant layer, i.e., in the bacterial wall. Static conductivity and dielectric dispersion measurements both show that the counterions in the porous gel-like cell wall give rise to a considerable surface conductance. From a comparison of the mobile charge with the total cell wall charge it is inferred that the mobilities of the ions in the bacterial wall are of the same order but somewhat lower than those in the bulk electrolyte solution. The occurrence of surface conductance reduces the electrophoretic mobility in electrophoresis studies. If this effect is not taken into account, the ζ-potential will be underestimated, especially at low electrolyte concentrations.

Flotation of Inherently Hydrophobic Particles in Aqueous Solutions of Inorganic Electrolytes

The flotation of graphite particles in aqueous solutions of inorganic electrolytes was shown to depend on both the nature of the cation/anion pair and the range of the bubble/particle electrostatic interaction. For several electrolytes, as the reduction in the Debye length of the solution approached the decay length of the hydrophobic attraction, then flotation began to occur. Also using earlier reported data, it was possible to relate the flotation to surface tension/electrolyte concentration gradients and bubble coalescence behavior of the different electrolyte solutions. Higher flotation recoveries were attributed to an increased collision probability between the graphite particles, a higher concentration of small noncoalescing bubbles, and an increased stability of the froth. Furthermore, it has also been shown from previous studies that increasing electrolyte concentration causes a decrease in gas solubility. In fact, gas solubility has been shown to be dependent on the hydration entropy of the cation. This phenomenon was explained in terms of competitive utilization of water molecules in the hydration of cations and a consequent loss or gain in gas solubility. Overall, it was shown that a reduction in the electrostatic interactions between particle and bubble assisted flotation. However, in addition, an increase in flotation performance resulted from the inhibition of coalescence of bubbles, which is also linked with dissolved gas concentration gradients (structural differences at the air/solution interfacial region relative to the the bulk electrolyte solution).

Ion Partitioning between Charged Micropores and Bulk Electrolyte Solution. Mixtures of Mono- and Divalent Counterions and Monovalent Co-Ions

Ion Partitioning between Charged Micropores and Bulk Electrolyte Solution. Mixtures of Mono- and Divalent Counterions and Monovalent Co-Ions

Temperature dependence of conductivity in electrolyte solutions and ionic channels of biological membranes

Temperature is a key parameter in the description of any physical system. Experimental study of the temperature dependence of conductivity is very valuable in building and testing theoretical models. This fact does not appear to be fully appreciated and exploited in the study of ionic channels of biological membranes owing in part to the lack of an adequate theory for the temperature dependence of conductivity in electrolyte solutions. To redress this imbalance, and to encourage further temperature-dependence studies in ionic channels, we first give explicit expressions for the conductivity of ions in electrolyte solutions in terms of the microscopic parameters of the liquid. We then propose that the dynamics of ion transport in membrane channels are similar to that in bulk electrolyte solutions, except that ions permeating the pore need to surmount a potential barrier, the height of which can be deduced experimentally. Finally, we use our model to analyze the conductance-temperature relationships obtained in two types of single ionic channels.

Dressed-ion theory for electrolyte solutions: A Debye–Hückel-like reformulation of the exact theory for the primitive model

A detailed derivation of the dressed-ion theory—a formally exact theory for primitive model Coulomb fluids—is presented for the case of bulk electrolyte solutions. It is shown that the exact average electrostatic potential, ψ av(r), in the ion atmosphere around each ion satisfies a linear Poisson–Boltzmann (PB) equation for ‘‘dressed ions,’’ each of which consists of a central ion together with a specific part of the surrounding ion cloud. The dressed-ion charge distribution—a renormalized charge for each ion—takes the role that the bare ionic charge has in the usual PB equation. Apart from this, virtually the only difference between the exact dressed-ion and the approximate Debye–Hückel (DH) theories for the pair distribution function is that the former theory is nonlocal; the spread-out nature of the dressed-ion charge distribution gives rise to a nonlocal polarization response to the average potential. The linear response function relating the polarization and the average potential is investigated in the general case and is found to be intimately related to the dressed-ion charge distributions. A close relationship is also demonstrated between these charge distributions and the electrostatic susceptibility of the electrolyte solution. The theory gives a rigorous definition of the concept of effective point charges for ions. Except at high coupling, the long-range asymptotic behavior of the pair distribution functions is of the Debye–Hückel form, but with effective values of the ionic charges (qi*) and a decay length (κ−1) different from the Debye length (κ−1D). A simple formula relating qi* and κ is derived. Explicit formulae for qi* and κ in terms of κD are given in the limit of low electrolyte concentrations. The long range asymptotic behaviors of the bridge function and the short range parts of the direct and total correlation functions are analyzed in some detail.

Direct electrochemical transduction of an immunological reaction by bilayer lipid membranes

This work reports the electrochemical transduction of an immunological interaction by use of bilayer lipid membranes (BLMs)which were prepared from mixtures of egg phosphatidylcholine (PC) and dipalmitoyl phosphatidic acid (DPPA). Thyroxin (T4)/anti-rabbit T4 was used as a representative immunological reaction for these studies. Antibody-antigen complexation caused transient ion current signals due to dynamic changes of the electrostatic fields at the surface of such membranes. The mechanism of signal generation is based on the perturbation of the electrical double layer and surface structure of the BLMs. The transient charging signals occurred as singular or multiple events which lasted for a period on the order of seconds. The magnitude of these transient ion current signals was directly related to the concentration of the antigen in bulk solution, which could be determined over a range of nM to mM levels in a period of seconds to minutes. Investigation of the effects of lipid composition of BLMs, pH and the presence of Ca 2+_ in bulk electrolyte solution indicated that the response could be optimized for sensitivity and speed.

Role of dissolution of Mn(iii) species in discharge and recharge of chemically-modified MnO2 battery cathode materials

Various experimental techniques, including cyclic-voltammetry, constant current discharge and recharge, with a detecting electrode for determination of soluble Mn(iii) intermediate species, were used to study the reaction mechanism involved in the reduction of a chemical modified form of MnO2 that is multiply rechargeable. The results show how dissolution of Mn(iii) species plays an important role in the overall path of the reduction reaction and also in the rechargeability of the material. Mainly an heterogeneous reduction mechanism is involved; substantial loss of capacity can arise if diffusion of the soluble Mn(iii) species into the bulk electrolyte solution is allowed to take place. In practical cells, this effect must be minimized using cells with ‘starved electrolyte’, i.e. with only a small free volume-fraction of KOH electrolyte in the MnO2-carbon electrode structure. Soluble Mn(iii) species were detected in in situ experiments by means of (a) a spectro-electrochemical procedure, (b) a ‘collector’ electrode system at which dissolved Mn(iii) was reoxidized to MnO2, and (c) a rotating ring-disc electrode.

Monte Carlo and Poisson-Boltzmann study of electrolyte exclusion from charged cylindrical micropores

The exclusion of model electrolytes from charged cylindrical capillaries is studied using the grand canonical Monte Carlo method and the Poisson-Boltzmann equation. Concentration profiles inside the capillary, Donnan exclusion coefficients, and particle number fluctuations were evaluated for 2:1 (divalent co-ions, monovalent counterions) electrolytes and 1:2 electrolytes (monovalent co-ions, divalent counterions) in the restrictive primitive model approximation for a range of electrolyte concentrations and surface charge densities. The Poisson-Boltzmann equation appears to be a good approximation for 2:1 electrolytes (monovalent counterions) but breaks down completely for 1:2 (divalent counterions) electrolytes in the capillary. The Donnan exclusion coefficient [Gamma] is not an increasing function of the surface charge density, as predicted by the Poisson-Boltzmann equation; rather, it passes through a maximum and decreases with further increase in the surface charge. The same behavior has been observed previously for 2:2 electrolytes. The concentration fluctuations in the capillary, studied for the first time in this work, are much smaller than in the bulk electrolyte solution due to the strong correlations in the electrical double layer. As expected, the concentration fluctuations decrease by increasing the charge on the surface. Finally, a more realistic electrolyte model which fits bulk properties of lithium and cesium chloride solutionsmore » very well has also been studied as a function of the surface charge in the capillary. The differences in the electrolyte exclusion between lithium and cesium salt can be explained in view of the short-range forces due to the restructing of water molecules around the ions. 45 refs., 10 figs., 6 tabs.« less

Combined electrochemical and quartz crystal microbalance study on the temperature dependence of ion and solvent sorption in electroactive polymer films on electrodes

The electrochemical quartz crystal microbalance (EQCM) technique has been applied to monitor the changes of ion and solvent content within poly(tetracyanoquinodimethane) (PTCNQ) films as a function of the redox state at different temperatures. The effect of concentration of the LiCI supporting electrolyte was also studied. It was observed that the mass of the neutral film increased as the temperature was increased corresponding to a greater swelling of the surface film. The electroactivity of the film (the total charge consumed during the electrochemical transformation of the redox groups of the film) and the mass change increased simultaneously with the temperature. This is consistent with the model that with increasing temperature, the polymer film adopts a less compact structure and the diffusion of the counter-ions as well as segmental motion of the polymer chain becomes less hindered. It was established that in the mass changes accompanying the redox processes occurring in PTCNQ films, the solvent sorption is predominant and the incorporation of ions plays a lesser role. The extent of the swelling depends on the concentration of the bulk electrolyte solution due to the decrease in water activity and the diminishing number of available water molecules for the hydration of Li + ions. In contact with electrolyte solutions of low or medium concentration, the films are highly swollen at elevated temperatures and consequently an interplay between a rigid layer and viscous behaviour can be observed.

General thermodynamics of the diffuse double layer

Classical thermodynamic methods are applied to the diffuse electrical double layer between two differently charged parallel flat surfaces in equilibrium with a bulk electrolyte solution. Initially, an important result concerning the inner regions of the double layer obtained previously for the particular case where the diffuse region conforms to the Poisson-Boltzmann equation is shown to be more general. This result leads to local thermodynamic expressions, obtained previously by the method of local balance, from which previously derived expressions for individual ionic activity coefficients are readily obtained. A general expression for the free energy of interaction (Δ G) between overlapping double layers is derived which includes previous expressions based on the Poisson-Boltzmann equation as special cases. When the approximation that the solution permittivity does not vary with solution composition in the diffuse region is valid, it follows that the charge density ρ depends only on the potential Ψ relative to the bulk solution and the bulk solution composition itself. Under these circumstances Δ G may be evaluated as a function of separation if ρ(Ψ) and the boundary conditions at the edges of the diffuse region are known. When the further approximations — (a) that the dielectric constant does not depend on field strength in the diffuse region, and (b) that reasonable estimates of single ionic activities can be obtained from cells with liquid junctions — both hold, a combination of potentiometric titration and electrokinetic studies enable all the information required to determine Δ G to be obtained empirically from studies of isolated surfaces provided that there is one ionic species in the bulk solution which is effectively excluded from the inner regions of the double layer. Nowhere in the analysis is the Poisson-Boltzmann equation required.

Infrared Reflection-Absorption Spectroscopy of the Electrode/Electrolyte Solution Interface: Optical Considerations

Calculations based on the classical Fresnel equations have been carried out to investigate the effect of various experimental parameters on infrared spectra obtained for molecules adsorbed at the electrode/solution interface. The effects considered include the nature of window material, the variation in the angle of incidence, the incident beam diameter, the instrument focus, and the presence of an absorbing bulk electrolyte solution. It is shown that, under certain circumstances, the resultant spectra are enhanced with respect to those obtained in the absence of an IR window and bulk adsorbing phase. The significance of the present results with respect to practical experiments is discussed.

Tetanus toxin channel in phosphatidylserine planar bilayers: conductance states and pH dependence

Tetanus toxin (TeTx) forms ionic channel in phosphatidylserine bilayers. TeTx channels exhibit different modes of channel bursting activity, from a closed state to well defined open states of different amplitudes. At positive applied voltages, TeTx channels flicker continuously between a closed state and the various distinct open states. Furthermore, fast transitions into subconductance states are discernible within the bursts of channel activity. Elementary conductance steps submultiple of the open states were not identified in single channel records owing to rapid transitions between different states. However, statistical analysis shows that conductances cluster with amplitudes multiple of an elementary value: e.g. 25-30 pS at neutral pH. Single channel current amplitudes decrease with the pH of the bulk electrolyte solution. Conductance decrements can be accounted for by the relative decrease of permeant cation concentration at the membrane-water interface, by a relative enrichment of protons that block the channel or by the stabilization of a conformational state of the channel protein.

Electro-oxidative polymerization of aniline on platinum anodes coated with polyurethane film

The electropolymerization of aniline on polyurethane film-coated platinum electrodes (PU/Pt) in electrolyte solutions with pH = 1 – 13.7 yields polyaniline that is conducting and bears a structure similar to conducting polyaniline prepared in aqueous acid solutions as indicated by FT-i.r. spectra and optical absorption investigations. It is found that aniline is electropolymerized mainly at the polyurethane-platinum interface, which is strongly acidic even in a basic electrolyte solution due to the strong limitation of the polyurethane layer of the PU/Pt electrode on the diffusion of H + released during the reaction and OH − in the bulk electrolyte solution through the polyurethane layer. A mechanism for the electrodeposition of aniline on PU/Pt electrodes is suggested.