Electrolyte diodes with weak acids and bases. I. Theory and an approximate analytical solution

Until now acid-base diodes and transistors applied strong mineral acids and bases exclusively. In this work properties of electrolyte diodes with weak electrolytes are studied and compared with those of diodes with strong ones to show the advantages of weak acids and bases in these applications. The theoretical model is a one dimensional piece of gel containing fixed ionizable groups and connecting reservoirs of an acid and a base. The electric current flowing through the gel is measured as a function of the applied voltage. The steady-state current-voltage characteristic (CVC) of such a gel looks like that of a diode under these conditions. Results of our theoretical, numerical, and experimental investigations are reported in two parts. In this first, theoretical part governing equations necessary to calculate the steady-state CVC of a reverse-biased electrolyte diode are presented together with an approximate analytical solution of this reaction-diffusion-ionic migration problem. The applied approximations are quasielectroneutrality and quasiequilibrium. It is shown that the gel can be divided into an alkaline and an acidic zone separated by a middle weakly acidic region. As a further approximation it is assumed that the ionization of the fixed acidic groups is complete in the alkaline zone and that it is completely suppressed in the acidic one. The general solution given here describes the CVC and the potential and ionic concentration profiles of diodes applying either strong or weak electrolytes. It is proven that previous formulas valid for a strong acid-strong base diode can be regarded as a special case of the more general formulas presented here.     

Numerical calculation of the electrophoretic mobility of colloidal particles in weak electrolyte solutions

A numerical calculation of the electrophoretic mobility of colloidal particles in weak electrolyte solution is presented. It is based on a previous work (C. Grosse, V.N. Shilov, J. Colloid Interface Sci. 211 (1999) 160-170), where the analytical theory of the thin double layer concentration polarization is generalized to the case of weak electrolytes, i.e., when the dissociation-recombination equilibrium and rate constants both have finite values. The analytical results are first completed by including terms corresponding to co-ions that were neglected in the original presentation. It is shown that these terms that have little bearing in the case of strong electrolytes, become quite important in the case when the electrolyte is weak. The problem is then solved using the network method, leading to numerical results for the electric potential and the concentrations of counterions, co-ions, and neutral ion pairs. Finally, the electrophoretic mobility is calculated both analytically and numerically. It is shown that the hypothesis of a weak electrolyte leads to changes of mobility with respect to the classical results that are even stronger than predicted analytically.     

Determining the radius and the apparent charge of a micelle from electrical conductivity measurements by using a transport theory: explicit equations for practical use

We propose here a procedure which combines experiments and simple analytical formulas that allows us to determine good estimations of the size and charge of ionic micelles above the critical micellar concentration (cmc). First, the conductivity of n-tetradecyltrimethylammonium bromide and chloride (TTABr and TTACl, respectively) aqueous solutions was measured at 25 degrees C, before and above their cmc. Then, an analytical expression for the concentration dependence of the conductance of an ionic mixture with three species (monomers, micelles, and counterions) was developed and applied to the analysis of the experiments. The theoretical calculations use the mean spherical approximation (MSA) to describe equilibrium properties. Here, we propose new expressions for the electrical conductivity, adapted to the case of electrolytes that are dissymmetric in size, and applicable up to a total surfactant concentration of 0.1 mol L(-1). Moreover, we show that they are good approximations of the corresponding numerical results obtained from Brownian dynamics simulations. Since the analytical formulas given in the present paper involve a small number of unknown parameters, they allow one to derive the size and charge of macroions in solution from conductivity measurements.     

Not only pH. Specific buffer effects in biological systems

The aim of this work is to overview the specific effect of pH buffers in biological systems. The pH of a buffer solution changes only slightly when a small amount of a strong acid or bases is added to it. This is widely accepted and applied both in chemical and in biological (i.e. enzyme catalysis) systems. Here we show some examples – spanning from pH measurements, enzyme activities, electrophoretic mobilities, antibody aggregation, protein thermal stability – that demonstrate additional roles of buffers. They not only set pH, but also address specific ion effects, in terms of Hofmeister series, when strong electrolytes are also added. From the experimental data referred to some charged biological moieties it emerges that different buffers, at the same nominal pH, can specifically adsorb at the charged surface. Buffer specific adsorption modifies several molecular and macroscopic properties amongst which electrophoretic mobilities, and hence effective surface charges, are particularly significant. More importantly, buffers' weak electrolytes, even at low concentration, are found to compete for the adsorption at the charged surfaces with strong electrolytes, thus modulating Hofmeister effects.     

Activity coefficients of single electrolytes in concentrated solutions derived from a quasi-lattice model

This paper describes a new model to calculate the mean activity coefficients of dissociated electrolytes in concentrated solutions. It is based on three assumptions: (i) a quasi-lattice arrangements of ions in solution; (ii) a contribution from ion-water interactions to the mean activity coefficients; (iii) a concentration dependence of the dielectric constant. The mean activity coefficients of thirteen strong electrolytes from moderately dilute solutions to saturated solutions are found to correlate well by this model. For dilute solutions, a limiting equation in which only ion-specific parameters are required is proposed. It is suggested that specific ionwater interactions might be the major source of the nonideality of strong electrolyte solutions at high concentrations.     

Physico-chemical studies in non-aqueous solvents: Part XII. Thermochemical studies of some 1:1 electrolytes in N,N-dimethylacetamide

Heats of solution of alkali metal, silver, ammonium and substituted ammonium perchlorates, lithium chloride and potassium thiocyanate have been measured in the concentration range 1.26–112.8×10^?3 mol l^?1 in N,N-dimethylacetamide (DMA) at 26.9°C. Slopes of the linear plots of heat of solution vs. square root of concentration are two to three times higher than the Debye-Hückel limiting slope indicating thereby complete dissociation of the electrolytes. Standard heats of solution of the electrolytes in DMA are higher than those in formamide and N-methylformamide and are comparable to those in N,N-dimethylformamide. Buckingham's model has been used to determine the solvation enthalpies of various ions in this solvent. The results indicate that ion solvation enthalpies decrease with increase in size of the ions and that the anion solvation enthalpies are higher than cation solvation enthalpies which is consistent with the findings in various solvents.     

Asymmetry of current-voltage characteristics of ion-exchange membranes: Model of charge density of fixed groups linear by membrane thickness

The model of a finely porous membrane with a linear distribution of charges of its fixed groups along its depth is used in the work to explain the phenomenon of asymmetry of current-voltage characteristics of perfluorinated MF-4SK membranes surface–modified by polyaniline or halloysite nanotubes. A new exact analytical method of solution of a system of transport equations is suggested that is based on the assumption of the monotonous behavior of the electric potential inside the membrane and allows finding current- voltage characteristics (CVCs) of the membrane using a system of two implicit algebraic equations. Algebraic equations for limiting currents are obtained for the first time for different orientations of the anisotropic membrane in an electrodyalisis cell and existence of two inflection points in CVCs is explained when its less charge side is oriented towards the anode.     

Conductance equation of dilute mixed strong electrolytes. II. Hydrodynamic and osmotic terms in relaxation field

The hydrodynamic and osmotic terms in the relaxation field are computed up to order linear in concentration for a dilute solution of mixed strong electrolytes using the primitive model. The computation is based on the relative function μ_ji computed in part earlier for the purely electrostatic interaction and in part in this work for the hydrodynamic interaction assuming the adequacy of the Fuoss velocity field. The results are compared with the earlier computations of Fuoss and Onsager, Murphy and Cohen, and Falkenhagen, Ebeling, and Kraeft. It is found that some terms of the new results are in agreement with the earlier computations and the other terms represent an improved computation.     

Asymmetry of current–voltage characteristics: a bilayer model of a modified ion-exchange membrane

The asymmetry of the current–voltage characteristics of ion-exchange membranes is explained in terms of the model of a bilayer fine porous membrane with constant charge distributions over the thickness of layers. This model has previously been proposed for determining diffusion permeability of membranes. In the case of one uncharged (neutral) layer, a set of two implicit algebraic equations is derived for determining the total current–voltage characteristics (CVC) of a membrane. For the first time, implicit algebraic equations are obtained for calculating the limiting currents at different orientations of an anisotropic membrane in an electrodialysis cell and explicit expressions are derived for determining specific conductivity of the membrane from the slope of the ohmic region of a CVC under the approximation of “excluded coions.” The model may be successfully used for describing the CVCs of perfluorinated MF-4SC sulfonic cation-exchange membranes, the surface layers of which are modified with polyaniline or halloysite.     

聚偏氟乙烯凝胶电解质组成间相互作用及其凝胶化研究

通过冷却聚偏氟乙烯 (PVDF) 丙烯碳酸酯 (PC)或PVDF PC LiClO4的溶液 ,制备了数个聚合物凝胶 .实验表明 ,聚合物凝胶的凝胶化时间 (tgel)与凝胶温度、聚合物浓度有关 ,且强烈地依赖于体系中盐的浓度 ,因为盐会缩短体系的tgel.凝胶体系中LiClO4的存在提高了其凝胶熔融温度 (Tgm) ,LiClO4的含量越大 ,相应凝胶的Tgm 越高 .用DSC和落球法所测凝胶的Tgm 有较大的差别 .这说明凝胶中可能存在热稳定性好和热稳定性相对较差的两种不同结构部分 .FT IR的研究结果表明 ,凝胶电解质的各组成 (LiClO4,PC和PVDF)间存在较强的相互作用 .对含盐和不含盐的两类凝胶体系的对比研究表明 ,两者不同的凝胶化现象和Tgm 归因于盐与聚合物或溶剂间的络合作用     

Refining of nonionic surfactant micellization theory based on the law of mass action

Two approaches to determining critical micelle concentration (CMC) are assessed, i.e., from the inflection point in the curve for the concentration dependence of the degree of micellization and as K^1/(1–n), where K is the constant of the law of mass action and n is the aggregation number. The latter approach makes the theory simpler, while the former explicitly expresses the critical degree of micellization via the aggregation number. The concentrations of monomers and micelles are analyzed as functions of the overall concentration of a surfactant in a micellar solution. These functions look much simpler in the graphical form as compared with their complex exact analytical representation. This has resulted in derivation of simple analytical approximations for these functions, with these approximations being useful for calculations. The concentration dependence of the surfactant diffusion coefficient has been considered based on these approximations. It turned out that this dependence not only provides the known method for determining the diffusion coefficient of micelles, but also gives the possibility in principle to determine the aggregation number from the slope of the dependence of the diffusion coefficient on the inverse concentration (counted from the CMC in the CMC units). This new method for determining the aggregation number has been tested using the literature data on the diffusion coefficient of penta(ethylene glycol)-1-hexyl ether in an aqueous solution.     

Mathematical modeling of interfacial gel polymerization for weak and strong gel effects

Frontal polymerization (FP) is a process in which a spatially localized reaction zone propagates into a monomer converting it into a polymer. Two types of FP processes have been observed experimentally. One is exothermic FP, which occurs due to diffusion of heat released in the polymerization reactions and which we have previously studied. The other is an isothermal FP process, also referred to as interfacial gel polymerization, which is due to mass diffusion of the species coupled with the gel effect. In a previous work we proposed and studied analytically a model of interfacial gel polymerization. That work discussed the case of an excessive amount of initiator in the initial mixture. In addition, it was assumed that the parameters of the problem were such that the steady‐state assumption (SSA) concerning the total concentration of radicals holds not only in the bulk region, which is typically the case, but also in the gel region, which may limit the applicability of the results. In this work we seek to resolve the limitations associated with these two main assumptions. We relax the SSA in the gel region, analyze the various situations of initiator consumption for a weak gel effect, and study the case of a strong gel effect. We obtain analytical results, including the time‐dependent propagation velocity of the reaction zone and the distance traveled by the front before it breaks down due to reactions ahead of the front, which are in good agreement with our numerical simulations.     

Low-conductivity background electrolytes in capillary zone electrophoresis--myth or reality?

The asymmetric triangle (fronting or tailing) concentration profiles and their broadening are the typical results of the electromigrational zone dispersion characterizing a system of the analyte in the background electrolyte (BGE). The present contribution suggests the parameter named the relative velocity slope, SBGE,X, which was introduced here as a quantity characterizing the peak broadening and the asymmetry. SBGE,X VS. analyte ionic mobility diagrams are suitable for the comparison of BGEs of given pH and the conductivity composed of electrolytes of different pKaS and ionic mobilities. The concept of SBGE,X diagrams is verified by capillary zone electrophoresis of the model analytes, which involve (i) the series of sulfobenzoylated poly(ethylene glycols) as examples of the strong electrolytes with different ionic mobilities and (ii) the series of monobasic phenols as weak electrolytes with different pKaS and similar ionic mobilities. It follows from both theoretical predictions of peak symmetry and their experimental verification that the optimum composition of BGEs is determined mostly by the suitable ionic mobility of the coion in dependence on the ionic mobility of the analyte. The low-conductivity BGEs based on low-molecular carrier ampholytes are at best only comparable with the properly chosen monobasic electrolytes.     

Empirical correlations between Krafft temperature and tail length for amidosulfobetaine surfactants in the presence of inorganic salt

Long-chain amidosulfobetaine surfactants, 3-(N-fattyamidopropyl-N,N-dimethyl ammonium) propanesulfonates (n-DAS, n > 18), are insoluble in pure water due to their high Krafft temperature (T(K)), while they are soluble when inorganic salt is added to the surfactant solution as the T(K) of these zwitterionic surfactants is decreased. The influence of the salt content and ionic species of the added electrolytes on the T(K) of the series of amidosulfobetaine surfactants was examined by means of UV-vis spectrophometry and visual inspection. It was found that the T(K) of these surfactants depends strongly on not only the hydrophobic alkyl length (n), but also the salinity of the aqueous environment. When the salt concentration is increased from 0 to 100 mM, the T(K) shows a sharp decrease; when the salinity is fixed between 100 and 2000 mM, the T(K) varies linearly with n with a slope of ~7.7 irrespective of the salt species and the salt content. When the salt concentration is further increased above 2000 mM, a linear function is still observed, but the slope increases slightly.     

Deformational,swelling, and potentiometric behavior of ionized poly(methacrylic acid) gels. II. Experimental results

The deformational, swelling, and potentiometric behavior of poly(methacrylic acid) gels was measured as a function of the degree of crosslinking, ionic strength, and degree of ionization. The comparison of the stress–strain behavior with theoretical relations derived in the preceding part has shown that the relations are valid only if an increase is assumed in the number of monomeric units in the statistical chain segment with increasing degree of neutralization of the gel. This dependence is affected by the salt content in the swelling solution and is also dependent on the activity coefficient of counterions. The pK₀ values for an undissociated gel approach pK₀ = 5.0 and increase somewhat with degree of neutralization. The swelling equilibria are in qualitative agreement with theoretical assumptions; their quantitative agreement depends on the activity coefficient of counterions.     

Non-linear calibration functions in ion chromatography with suppressed conductivity detection using hydroxide eluents

The linearity of calibration curves in ion chromatography with suppressed conductivity detection using hydroxide eluents was investigated. Theoretical calibration curves were derived for strong electrolytes and weak monobasic acids and the results compared with experimental data. At low concentrations up to 1 micromol l(-1) the autoprotolysis of water induces left-curved calibration functions even for strong electrolytes like nitrate. The experimental data are best described by a quadratic function, the differences between linear and quadratic regression being up to 10%. At higher concentrations the calibration curves for strong electrolytes are linear. Due to incomplete dissociation, the calibration curves for weak mono- and dibasic acids show a right curvature. Thus, depending on the analyte and the concentration range of interest, analysts should carefully choose between a linear and a quadratic regression function.     

The sub- and super-equivalence method of isotope dilution XIII. Theoretical error of the CVC and VCC variants

The theoretical error of both the CVC and VCC variants at non-quantitative complex forming reaction M+LML has been studied. The erros of analysis for the following analytical curve determinations are considered: (1) measurement with constant error of the analytical function, (2) measurement with registration of a constant number of disintegrations, (3) measurement at constant time of radioactivity measurement of isolated products. Conditions of isoconcentration point existence are determined. Intervals of parameters at which the relative error of the analysis result is comparable with that of the analytical curve are found out. The optimal conditions of analysis are discussed. Errors of both the CVC and VCC variants are compared with that of the CCV variant. Graphs of the theoretical error of analysis are given. The sub-super equivalent isotope dilution method (SSE IDA) has seven basic variants which can be classified by the CCV, CVC, VCC, VVC, VCV, CVV and VVV codes. The theroretical error of the CCV variant was studied earlier. In the present work, a study of errors of both the CVC and VCC variants is performed and extended to radioactivity measurement regimes that can be easily automated. The radioactive background of measurements is taken into account. Previous results and conclusions are corrected. Erros of both the CVC and VCC variants are compared with that of the CCV variant.     

Effect of different electrolytes on the swelling properties of calyx[4]pyrrole-containing polyacrylamide membranes

Calix[4]pyrrole (1) was synthesized and characterized and this macrocycle was incorporated in polyacrylamide gels. The presence of meso-octamethyl-porphyrinogen inside of gel was checked using infrared spectroscopy, differential scanning calorimetry, and swelling studies. The swelling degree of these hydrogels in equilibrium with different electrolytes (NaCl, LiCl, KCl, CaCl₂ and AlCl₃) was measured in a concentration range 0.1-0.5 mol dm⁻³. Although no significant alterations in the swelling degree can be found for the different 1:1 electrolytes, when the cation charge of unsymmetrical electrolytes increases, the gel swells in a significant way. This swelling process is enhanced by the presence of calyx[4]pyrrole. The effect of alkaline hydrolysis of polaycrylamide-based hydrogels was also studied. The hydrolysed hydrogels collapse in the presence of the electrolytes; this behavior is dependent on the hydrolysis degree, electrolyte charge and calyx[4]pyrrole presence and concentration; the latter leads to polyacrylamide with tailor-made properties.     

Interaction of amino acids, N-acetyl amino acid esters, thymine and adenine with sephadex LH-20 gel.

N-Acetyl aromatic amino acid esters, tryptophan, adenine and thymine show strong retention in Sephadex LH-20 gel in an aqueous phase. The decreased retention in 6 M urea, the absence of retention in absolute methanol and the increased retention at higher temperatures in the aqueous phase indicate that hydrophobic interaction is responsible for the observed retention of the amino acids and their esters in the gel. Adenine was found to be retained by polar interaction in the gel. The increased retention of the solutes in the presence of different electrolytes suggests that the lyotropic effect is more important than the ionic strength effect. The relevance of the results obtained with amino acids and esters to the conformational aspects of proteins in aqueous solution is discussed.     

Diffusion of sodium dodecyl sulfate micelles in agarose gels

The gradient diffusion of ionic sodium dodecyl sulfate micelles in agarose gel was investigated at moderate concentrations above the CMC. Of particular interest were the effects of micelle, gel, and sodium chloride concentration on the micelle diffusivity. Holographic interferometry was used to measure the gradient diffusion coefficient at three sodium chloride concentrations (0, 0.03, 0.10 M), three gel concentrations (0, 1, 2 wt%), and several surfactant concentrations. Time-resolved fluorescence quenching was used to measure aggregation numbers both in solution and gel. The micelle diffusivity increased linearly with surfactant concentration at the two larger sodium chloride concentrations and all gel concentrations. In general, the strength of this effect increased with decreasing sodium chloride concentration and increased with gel concentration. This behavior is evidence of decreasing micelle-micelle electrostatic interactions with increasing sodium chloride concentrations, and increasing excluded volume effects and hydrodynamic screening with increasing gel concentration, respectively. The only exception was at 0.1M sodium chloride and 2 wt% agarose, which showed a slight reduction in the slope compared to 1 wt% agarose. It was found that the concentration effect is quite strong for charged solutes: at a NaCl concentration of 0.03 M in a 2% agarose gel, in a solution with 3% SDS micelles by volume, the micelle diffusion coefficient is doubled relative to its value in the same gel at infinite dilution. The extrapolated, infinite-dilution diffusion coefficients and the rate at which the micelle diffusivity increased with surfactant concentration were compared with predictions of previously published theories in which the micelles are treated as charged, colloidal spheres and the gel as a Brinkman medium. The experimental data and theoretical predictions were in good agreement.