Transport Of Weak Electrolytes Research Articles

Electrodiffusion of ions in ion exchange membranes: Finite element simulations and experiments

Electrodiffusion of ions in both cation (CEM) and anion exchange membranes (AEM) has been studied with theoretical calculations and experimental studies. Calculations are based on the Finite Element Method (FEM) using COMSOL Multiphysics® software. Nernst-Planck equations are solved in multi-ionic systems where no closed form solutions are available. Simulations are compared with laboratory-scale experiments in terms of current efficiency and membrane selectivity. Simulations revealed unexpected features in transport, due to coupling of ionic fluxes when the local electroneutrality condition is assumed. Transport of weak electrolytes showed the importance of involving ionic equilibria along the concentration profiles in both solutions and membranes, compelling to consider ionic constituents. The advantage of the COMSOL simulations is the ease to find concentration and potential profiles across the entire system, and to split fluxes to diffusion and migration contributions, showing their coupling even in the absence of electric current.

Mass Transfer Phenomena during Electrodialysis of Multivalent Ions: Chemical Equilibria and Overlimiting Currents

Electrodialysis is utilized for the deionization of saline streams, usually formed by strong electrolytes. Recently, interest in new applications involving the transport of weak electrolytes through ion-exchange membranes has increased. Clear examples of such applications are the recovery of valuable metal ions from industrial effluents, such as electronic wastes or mining industries. Weak electrolytes give rise to a variety of ions with different valence, charge sign and transport properties. Moreover, development of concentration polarization under the application of an electric field promotes changes in the chemical equilibrium, thus making more complex understanding of mass transfer phenomena in such systems. This investigation presents a set of experiments conducted with salts of multivalent metals with the aim to provide better understanding on the involved mass transfer phenomena. Chronopotentiometric experiments and current-voltage characteristics confirm that shifts in chemical equilibria can take place simultaneous to the activation of overlimiting mass transfer mechanisms, that is, electroconvection and water dissociation. Electroconvection has been proven to affect the type of precipitates formed at the membrane surface thus suppressing the simultaneous dissociation of water. For some electrolytes, shifts in the chemical equilibria forced by an imposed electric field generate new charge carriers at specific current regimes, thus reducing the system resistance.

Impact of ionization equilibrium on electrokinetic flow of weak electrolytes in nanochannels

Weak electrolyte transport in nanochannels or nanopores has been actively explored in recent experiments. In this paper, we establish a new electrokinetic model where the ionization balance effect of weak electrolytes is outlined, and performed numerical calculations for H3PO4 concentration-biased nanochannel systems. By considering the roles of local chemical equilibrium in phosphorous acid ionization, the simulation results show quantitative agreement with experimental observations. Based on the model, we predict that enhanced energy harvesting capacity could be accomplished by utilizing weak electrolytes compared to the conventional strong electrolyte approaches in a concentration gradient-based power-generating system.

Experimental and theoretical study of nanofiltration of weak electrolytes: SO42–/HSO4–/H+ system

Over recent years, nanofiltration (NF) has been considered as an effective way to improve processing steps in metallurgical and hydrometallurgy applications dealing with mixtures of metal ions in sulphuric-acid-dominated solutions. The principal advantage of NF membranes over reverse osmosis (RO) membranes is their ability to allow for a practically free passage of acid, while metallic species, especially multi-charged species, are efficiently rejected. In general, these sulphuric solutions cover a range from strongly acidic solutions with pH below 1 up to moderately acidic solutions of pH 3. Over this range, changes in the feed acidity influence both the aqueous electrolyte solution speciation (SO42−/HSO4−/H+) and the membrane acid–base properties (protonation of carboxylic and amine groups). However, few studies have been published on the trans-membrane transport of inorganic species coupled to changes in their speciation as well as to the properties of the membrane phase.In this study, experimental data on the sulphuric acid rejection for pH values from 1 to 3 have been obtained with an aromatic poly(piperazine)amide membrane (NF270) at various trans-membrane pressures. The results were modelled by a novel version of the Solution-Electro-Diffusion model taking into account equilibrium reactions, and a general (quasi)analytical solution was obtained for the transport of weak electrolytes of arbitrary valence type. The equilibrium weak acid reaction made the total sulphate (SO42−/HSO4−) rejection decrease strongly as the fraction of single-charged hydrogen sulphate (HSO4−) in the feed increased. From the modelling procedure, permeances to H+, HSO4− and SO42− over the studied pH range were determined.

Ion transport through electrolyte/polyelectrolyte multi-layers

Ion transport of multi-ionic solutions through layered electrolyte and polyelectrolyte structures are relevant in a large variety of technical systems such as micro and nanofluidic devices, sensors, batteries and large desalination process systems. We report a new direct numerical simulation model coined EnPEn: it allows to solve a set of first principle equations to predict for multiple ions their concentration and electrical potential profiles in electro-chemically complex architectures of n layered electrolytes E and n polyelectrolytes PE. EnPEn can robustly capture ion transport in sub-millimeter architectures with submicron polyelectrolyte layers. We proof the strength of EnPEn for three yet unsolved architectures: (a) selective Na over Ca transport in surface modified ion selective membranes, (b) ion transport and water splitting in bipolar membranes and (c) transport of weak electrolytes.

Iontophoretic transport of weak electrolytes through the excised human stratum corneum

The effect of iontophoresis on the rate of permeation of a number of therapeutically active weak acids and bases through excised human stratum corneum has been examined, over a range of pH values. It has been shown that the amount of ionized drug species present in the drug solution is an important factor in the delivery of acids and bases by this route and that the molecular weight of the drug does not influence the rate of delivery by iontophoresis.

SCFAs: The Enigma of Weak Electrolyte Transport in the Colon.

Short-chain fatty acids are the predominant luminal anions in the colon (>75 mM) and thus create a rather unique environment for transporting epithelium. The colon absorbs short-chain fatty acids, either by diffusion of the protonated species across the apical membrane or by an anion exchange process with bicarbonate. Additionally, short-chain fatty acids modulate Na absorption, Cl secretion, intracellular pH, and cell volume.

Permeation of ammonia across bilayer lipid membranes studied by ammonium ion selective microelectrodes

Ammonium ion and proton concentration profiles near the surface of a planar bilayer lipid membrane (BLM) generated by an ammonium ion gradient across the BLM are studied by means of microelectrodes. If the concentration of the weak base is small compared with the buffer capacity of the medium, the experimental results are well described by the standard physiological model in which the transmembrane transport is assumed to be limited by diffusion across unstirred layers (USLs) adjacent to the membrane at basic pH values (pH > pKa) and by the permeation across the membrane itself at acidic pH values. In a poorly buffered medium, however, these predictions are not fulfilled. A pH gradient that develops within the USL must be taken into account under these conditions. From the concentration distribution of ammonium ions recorded at both sides of the BLM, the membrane permeability for ammonia is determined for BLMs of different lipid composition (48 x 10(-3) cm/s in the case of diphytanoyl phosphatidylcholine). A theoretical model of weak electrolyte transport that is based on the knowledge of reaction and diffusion rates is found to describe well the experimental profiles under any conditions. The microelectrode technique can be applied for the study of the membrane permeability of other weak acids or bases, even if no microsensor for the substance under study is available, because with the help of the theoretical model the membrane permeability values can be estimated from pH profiles alone. The accuracy of such measurements is limited, however, because small changes in the equilibrium constants, diffusion coefficients, or concentrations used for computations create a systematic error.

A Re-evaluation of the Properties of the Three-compartment Model of Intestinal Weak-electrolyte Absorption

The three-compartment model for intestinal weak-electrolyte transport provides an equation which predicts the flux ratio (the ratio of the forward and reverse fluxes of weak electrolyte across the intestine) from the dissociation constant, given that coefficients in the equation are specified. Using the equation to evaluate the coefficients from flux ratio and dissociation constant data fails to provide a unique set of values for the coefficients, although it does provide four dissimilar but related sets of values. Differentiation to determine the position and size of the maximum flux ratio provides a relationship between the model coefficients which shows why it is that four sets of coefficients can be used in the equation when it is applied to experimental data. Coefficient sets are derived for all available flux ratio data in the literature. These always contain a variant which requires the intermediate compartment to be more acid than the end compartment. Further specification of the flux ratio equation either by combining data from different physiological circumstances or by adding voltage terms, reduces the four possible coefficient sets to a single set, again requiring the intermediate compartment to be more acid than the end compartment. Estimates of the intermediate-compartment pH agree well with electrode estimates of mucosal surface pH. The present re-evaluation of the three-compartment model resolves an apparent contradiction between experimental and previous theoretical results which indicated a more alkaline intermediate compartment. Instead of contradicting experimental findings, results from the modelling process confirm that the "acid microclimate" version of the model is the better representation of the system that causes asymmetry in weak electrolyte flows across the small intestine.

Effect of sodium on transport of weak electrolytes across rat jejunum in vitro.

The effects of ion substitution and inhibitors on the transport of two representative weak electrolytes, benzoic acid and amphetamine, across short-circuited rat jejunum in vitro have been studied. The spontaneous asymmetries in the transport of the weak electrolytes were not dependent on the presence of either bicarbonate or chloride in the incubation medium. Net transport of the weak electrolytes was abolished when choline was substituted for sodium in the fluid bathing the mucosal surface of the tissue, but weak electrolyte transport was independent of the presence of sodium on the serosal side. Weak electrolyte transport was inhibited by ouabain and by orthovanadate. The effect of ouabain was abolished when potassium was substituted for sodium in the serosal fluid, and the effect of orthovanadate was potentiated in this condition. It was shown that the ability of the intestinal epithelium to discriminate between the ionized and non-ionized forms of weak electrolytes was not altered when choline was substituted for sodium in the incubation medium, indicating that the effect of sodium was associated with the driving force for weak electrolyte transport. Weak electrolyte transport was not abolished by raising the serosal potassium concentration, suggesting that the transport process was directly related with the function of the sodium pump, rather than correlated with gradients of electrical or chemical potential established as a consequence of pump activity.

Transport of acetate and chloroacetate weak electrolytes through a thin porous membrane in counter-current electrolysis

The transport of weak electrolytes through a porous membrane during counter-current electrolysis has been studied both theoretically and experimentally. The theoretical model is based on the Nernst–Planck equations and the obtained transport equations for the ion constituents are used to solve the fluxes of the ion constituents. Expressions for the diffusion coefficients and transport numbers are also presented. Experiments and calculations for two binary systems (acetic acid in water and monochloroacetic acid in water) and for three ternary systems (acetic acid and sodium acetate in water, monochloroacetic acid and sodium monochloroacetate in water and acetic acid and monochloroacetic acid in water) were performed at a total concentration of 0.01 mol dm–3 or less. The theoretically obtained fluxes of the ion constituents are compared with the experimental fluxes and it is shown that the theoretical model is able to predict the transport phenomena.

Intestinal transport of weak electrolytes: Determinants of influx at the luminal surface.

The determinants of weak electrolyte influx into everted segments of rat small intestine have been studied. Preliminary experiments showed that the observed influxes could be described as unidirectional, diffusional fluxes of the nonionized compound uncomplicated by a parallel ionic component. It is shown that the determinants of weak electrolyte influx in this situation may be described in terms of the resistance of the unstirred layer to movement from the bulk phase to the cell surface, the degree of ionization of the weak electrolyte at the cell surface, and the cellular permeability to the nonionized weak electrolyte. Quantitative considerations indicated that the unstirred layer was totally rate-limiting in the cases of some poorly ionized, or highly permeant compounds, but the unstirred layer was not totally rate limiting for most of the compounds studied. Calculation of cellular permeabilities for the nonionized forms of weak electrolytes required assumptions to be made concerning the pH value in the surface fluid layer. A uniform set of permeability data including both weak acids and weak bases was obtained only when it was assumed that the pH in the surface fluid layer was equal to that in the bulk phase, and it was concluded that these studies do not support the concept of a microclimate of distinctive pH at the epithelial surface as a determinant of weak electrolyte transport.

Epithelial transport of weak electrolytes. Properties of a model of the three compartment system

A study has been made of the properties of a theoretical model of a mechanism that has been proposed for the transport of weak electrolytes in several epithelia. The model consists of three aqueous compartments arranged in series, in which adjacent compartments are separated by barriers that may discriminate between the ionized and non-ionized forms of weak electrolytes, and where the pH of the intermediate compartment may be different than that of the bulk phases. Significant asymmetries in weak electrolyte transport may be associated with relatively modest driving forces. The abilities of the barriers to discriminate between the ionized and non-ionized forms of a weak electrolyte are critical parameters of transport in this system, and both the relative, and the individual absolute values of these parameters are important determinants of weak electrolyte transport. The degree of asymmetry in the fluxes of a weak electrolyte varies markedly with the discriminatory properties of the barriers within a range of values that may be appropriate to the barriers found in epithelia. The range of pK a values for which transport may be observed in the system varies with the discriminatory properties of the barriers, and some versions of the system may be effective in the transport of acids of pK a< 1, or of bases of pK a> 13, in addition to the transport of weak electrolytes. The study allows description of two main types of potential biological analogs of the model system, and criteria of tenability of each version are identified.

Relations of weak-electrolyte transport and acid-base metabolism in rat small intestine in vitro

The jejunal and ileal regions of rat small intestine in vitro exhibit different patterns of weak-electrolyte transport and acid-base metabolism. The jejunal pattern of weak-electrolyte transport is net transport of weak acids M yields S and of weak bases S yields M (M, mucosal; S, serosal), and the pattern of acid-base metabolism is luminal acidification and serosal alkalinization. In the absence of sodium or in the presence of metabolic inhibitors, weak-electrolyte transport and serosal alkalinization are inhibited, but luminal acidification is not inhibited. The ileal pattern of weak-electrolyte transport is the opposite of that of the jejunum and may be associated with the presence of a luminal alkalinization process observed in this region. In the absence of chloride the ileal patterns of weak-electrolyte transport and acid-base metabolism become similar to those of the jejunum. It is suggested that these observations support a previously proposed model for weak-electrolyte transport in the intestine consisting of a series three-compartment system in which the pH of the intermediate compartment is greater than that of the bulk phases.

Intestinal transport of weak electrolytes. Evidence in favor of a three-compartment system.

A study has been made of the transmural fluxes of benzoic, phenylacetic, and pentanoic acids, benzylamine, hexylamine, and D-amphetamine across rat jejunum incubated in vitro. The M to S fluxes of the weak acids were greater than their corresponding S to M fluxes, and the S to M fluxes of the weak bases were larger than their M to S fluxes. These patterns of asymmetric movements were observed when the transmural electrical potential difference was clamped at 0 mV, and when the pH values of the mucosal and serosal fluids were identical. The effects of a weak acid on the fluxes of other weak electrolytes were qualitatively similar when the effector weak acid was added to the mucosal fluid, and when it was added to the serosal fluid. But the effects of a weak base on the fluxes of other weak electrolytes were dependent upon its location, and the interactions observed when the effector weak base was added to the mucosal fluid were qualitatively different than those seen when it was added to the serosal fluid. The interactions between weak electrolytes could readily be explained in terms of the function of a system of three compartments in series, in which the pH of the intermediate compartment is greater than that of the bulk phases. But these observations could not be explained in terms of an analogous system involving an intermediate compartment of low pH, or in terms of a carrier mediated system. The transport function of the three-compartment system can be described in the form of an equation, and it is found that a pH difference of less than 0.5 unit may explain our observations on weak electrolyte transport.

Ultracytochemistry of Carbonic Anhydrase (CAH) in Erythrocytes and Intestinal Epithelium

Since the transport of fluid and weak electrolytes by the small intestine is markedly inhibited by acetazolamide we have studied the cytochemical localization of carbonic anhydrase (CAH) in rat jejunum using the erythrocyte as a model system. Pieces of jejunal tissue were glutaraldehyde-fixed, frozen to -20C and 10 micron sections were obtained with a cryostat microtome. Human erythrocytes were similarly fixed, saline-rinsed and embedded in gelatin which was then treated in the same manner as tissue.The histochemical reaction product in light microscopy sections is CoS derived from the precipitation of catalysed CoCO3 formation. The reaction time was 8 minutes and the visible CAH activity in both erythrocytes and jejunum (Fig. 1, 2 and 6) was abolished by 1 X 10-4M acetazolamide.

Ultrastructure and transport across epithelial membranes.

For epithelia, all too frequently this or that ultrastructure is spoken of as "absorptive" or "(secretory." Correlation between structure and function must relate to phenomena, such as the apparent active transport of weak electrolytes or water, that are unique for the epithelium. The macroscopic tubule of the gastric mucosa cannot sequester the active transport of hydrochloric acid from the solution bathing the mucosa. In the absence of both intracellular canaliculi and a microvillous border from the amphibian parietal cell, by exclusion, the endoplasmic reticulum or the plasma membrane proper must be considered as possible structures responsible for the sequestration.