Anion Partitioning Research Articles

Effect of Surface Hydrophilicity on Charging Mechanism of Colloids in Low-Permittivity Solvents

Electrophoretic mobilities of TiO2 colloids in an apolar solvent, toluene, were measured by differential-phase optical coherence tomography (DP-OCT). An electrode spacing of 0.18 mm, made possible by optical coherence tomography with transparent electrodes, enables measurement of the electrophoretic mobility with small samples (20 μL) of highly turbid colloids at low applied electric potential to avoid electrohydrodynamic instability and electrochemical reactions. In the presence of Aerosol-OT reverse micelles, which stabilized the countercharges, the zeta potential was positive for hydrophilic TiO2 (13 mV at 90 mM AOT) and negative for hydrophobic TiO2. The magnitudes of the zeta potentials were very similar for these two types of TiO2 and decreased at the same rate with AOT concentration. For both hydrophilic and hydrophobic TiO2, a general mechanism is presented to describe the zeta potential in terms of preferential partitioning of cations and sulfosuccinate anions between the particle surface and reverse micelle cores in bulk. This preferential partitioning is governed by the hydrophilicities and extents of the particle surfaces and reverse micelle cores, as a function of surfactant and water concentration. The emerging understanding of the complex charging and stabilization mechanisms for colloids in apolar solvents will be highly beneficial for the design of novel materials.

Anion Partitioning and Ion-Pairing Behavior of Anions in the Extraction of Cesium Salts by 4,5‘ ‘-Bis(tert-octylbenzo)dibenzo-24-crown-8 in 1,2-Dichloroethane

A systematic study of anion partitioning and ion pairing was performed for an extraction of individual cesium salts into 1,2-dichloroethane (1,2-DCE) using 4,5' '-bis(tert-octylbenzo)dibenzo-24-crown-8 as the cesium receptor. Equilibrium constants corresponding to the extraction of ion pairs and dissociated ions, formation of the 1:1 cesium/crown complex (confirmed by electrospray mass spectrometry), and dissociation of the ion pairs in water-saturated 1,2-DCE at 25 degrees C were obtained from equilibrium modeling using the SXLSQI program. The standard Gibbs energy of partitioning between water and water-saturated 1,2-DCE was determined for picrate, permanganate, trifluoromethanesulfonate, methanesulfonate, trifluoroacetate, and acetate anions. The dissociation of the organic-phase complex ion pair [Cs(4,4' '-bis(tert-octylbenzo)dibenzo-24-crown-8]+NO3- observed in the extraction experiments was shown to be consistent with the dissociation constant determined independently by conductance measurements. As attributed to the large effective radius of the complex cation, the evident anion discrimination due to ion pairing in the 1,2-DCE phase was relatively small, by comparison only a tenth of the discrimination exhibited by the anion partitioning. Only chloride and picrate exhibit evidence for significantly greater-than-expected ion-pairing tendency. These results provide insight into the inclusion properties of the clefts formed by opposing arene rings of the crown ether upon encapsulation of the Cs+ ion, whose weak anion recognition likely reflects the preferential inclusion of 1,2-DCE molecules in the clefts. Observed anion extraction selectivity in this system, which may be ascribed predominantly to solvent-induced Hofmeister bias selectivity toward large charge-diffuse anions, was nearly the same whether cesium salts were extracted as dissociated ions or ion pairs.

Solvation of Ions in Hydrophilic Layer of Polyoxyethylated Nonionic Micelle. Cooperative Approach by Electrophoresis and Ion-Transfer Voltammetry

Electrophoretic measurements of micellar mobility have revealed that polyoxyethylated nonionic surfactant micelles have negative zeta potential in various electrolytes, indicating that the partition of anions into the micelle dominates the entire electrolyte partition and the induced surface potential of the micelle. Although an excess of a negative charge is thus revealed in the micelle, it is uncertain whether anions are preferably solvated in the micelles or cations are expelled from the micelles. To determine the solvation energies of single ions in the hydrophilic layer of the micelle, we have performed ion transfer voltammetric measurements at microinterfaces between nitrobenzene and aqueous tetraethyleneglycol solution, which acts as a model for the palisade layer of the micelles. The cooperative utilization of these different methods has allowed us to determine the Gibbs free energy of transfer of a single ion without an extrathermodynamic assumption. On the basis of the resulting values, the partition of ions and the zeta potential induced by the imbalance of anionic and cationic partition have been quantitatively explained.

Anion Partitioning in and Diffusion through a Nafion Membrane

Although Nafion-117 is a cation-exchange membrane, anions can readily diffuse through it when it is in contact with a high ionic strength or low pH solution. In this study, diffusion coefficients of anions (bisulfate and Cr(VI)) through a Nafion membrane were measured under these two conditions using a two-compartment cell. In addition, the partition coefficients of these anions in the membrane were measured. For sulfate concentrations of about 0.1 M, the average bisulfate partition coefficient was about 0.23 (mg/cm 3 water)/(mg/cm 3 membrane). The adsorption isotherm for Cr(VI) in Nafion membranes equilibrated with 0.1 M NiSO 4 + CrO 3 solution was linear (C m =0.079C e +12.93, C e as mg/cm 3 CrO 3 , r 2 >0.99), indicating that the Cr(VI) partition coefficient decreased with increasing chromic acid concentration (1-4 M range). The apparent diffusivities of HSO 4 - and Cr(VI) anions in the Nafion membrane were 1.08x10 -6 and 8.06x10 -7 cm 2 /s, respectively. The bisulfate concentration variation resulting from diffusion in the system was well-simulated by a numerical program based on Fick's second law. An alumina ceramic diaphragm was also tested for comparison. The bisulfate partition and diffusion coefficients in the ceramic were 0.4 and 2.17x10 -6 cm 2 /s, respectively.

Synergistic pseudo-hydroxide extraction: synergism and anion selectivity in sodium extraction using a crown ether and a series of weak lipophilic acids.

The nature of the weak lipophilic acid used in synergistic combination with a model crown ether cation host was shown to have a strong effect on the strength and selectivity of sodium hydroxide separation from alkaline aqueous salt solutions. Sodium ion-pair extraction employing only cis-syn-cis-dicyclohexano-18-crown-6 (1) in nitrobenzene (NB) was correlated with the standard Gibbs energy (deltaG(p)o) of anion partitioning into NB and was notably weak and nonselective for the hydroxide ion, in accord with Hofmeister bias. The Hofmeister order can be selectively overcome for NaOH by utilization of acid-base chemistry coupled with complexation of sodium ion in the NB phase. Upon addition of a lipophilic organic acid into the solution of 1 in NB, sodium extraction was selectively enhanced due to the initiation of an exchange reaction between the aqueous sodium ion and the ionizable proton of the organic acid. A series of weak lipophilic hydroxy acids (HA) including fluorinated alcohols and phenols was tested. The resulting synergistic pseudo-hydroxide extraction correlates with the pKa of the employed HA; the most acidic cation exchangers provide the greatest synergism. The synergistic factor obtained using a fluorinated benzyl alcohol 7 was as high as 256. Ion-pair extraction of neutral sodium salts was not changed or only mildly enhanced by addition of HA into the NB solution of 1. This enhancement was explained by hydrogen bonding of HA with the anion as related to the hardness of the anion and the acidity of HA. In comparison with the synergism observed for NaOH, this enhancement was weak and unable to overcome the Hofmeister effect. Examination of extraction selectivity revealed that the combination of 1 and 7 preferentially extracted NaOH over all other sodium salts, including the normally preferred nitrate and perchlorate salts. Quantitative recovery of NaOH from the NB phase was demonstrated via hydrolysis of the organic acid upon a single contact of the loaded solvent with water.

Simultaneous control of electrostatic micellar partition and electroosmotic flow-rate by anion-dominated partition into zwitterionic micelles

The zeta potentials of zwitterionic micelles and capillary walls have been evaluated with capillary electrophoresis. The zeta potential of the micelles is predominantly determined by the nature of anions, while cations of identical valence have marginal effects; the linear relation has been found between the induced zeta potential and the hydration energy of an anion. The zeta potential of the capillary wall is also varied with anionic natures, and there is a good correlation between micellar and capillary wall zeta potential. This strongly suggests that the zeta potential of capillary walls is determined by the partition of anions into the surfactant layer formed on the capillary wall. Thus, we can simultaneously control both the electroosmotic flow-rate and micellar surface potential (in turn electrostatic interaction between micelles and ionic solutes) by varying the type and concentration of electrolytes. This idea has been applied to the separation of aromatic cationic solutes.

Prediction of the carrier-mediated cation flux through polymer inclusion membranes via fundamental thermodynamic quantities: complexation study of bis(dodecyloxy)calix[4]arene-crown-6 with alkali metal cations

In a systematic study of alkali metal cation transport through a polymer inclusion membrane (PIM) with calixcrown carriers, a model that postulates diffusion-limited flux successfully describes PIM transport behavior. The model developed herein is based on an ion-pair extraction equilibrium at the PIM interface and provides a convenient tool for the quantitative understanding and interpretation of the transport data. Cation permeability coefficients can be easily determined and used for the quantitative correlation of fluxes employing a range of aqueous and PIM compositions. The described approach can be readily extended to competitive-transport experiments. Transport of a single cation as well as several cations in a competitive experiment was related to the stability constants of the carrier-metal complexes and the Gibbs energies of ion partitioning between source and membrane phases. The expected dependence of Cs+ ion transport on the Gibbs energy of anion partitioning was validated in a series of 8 univalent anions. Accordingly, the permeability coefficient once determined for a given metal salt, carrier, and PIM provides the basis for the prediction of the transport fluxes under different initial conditions if the thermochemical quantities which govern the complexation and distribution of the metal species into the membrane phase are known or can be estimated. In support of these efforts, a calorimetric study was carried out to obtain thermodynamic parameters for the complexation of bis(dodecyloxy)calix[4]arene-crown-6 with alkali metal ions in acetonitrile.

Transfer of β-diketone and 4-acylpyrazolone anions across the electrified water ∣ nitrobenzene interface

Dependent on the pH of the aqueous phase, the transfer of substituted β-diketone and 4-acyl-5-pyrazolone anions as well as the transfer of protons facilitated by the ligand anions across a microhole supported water ∣ nitrobenzene microinterface has been studied by cyclic voltammetry. Above a certain pH the half-wave potential of the observed wave is independent of the pH and represents the simple transfer of the ligand anion between the two phases. This critical pH-value is dependent on the basicity and the hydrophobicity of the compound. Below this pH-value, the half wave potential is shifted linearly by ca. 52 mV with the pH and reflects the protonation/deprotonation process at the interface. The protonation constants of the ligands in the organic phase, the distribution coefficients, the diffusion coefficients and the formal potentials of the ion transfer were determined. With increasing chain lengths of the acyl-substituent, the formal potential of the transfer of the ligand anions across the interface shifts to more positive values which means that the standard Gibbs energy of the ligand anion partition between water and nitrobenzene decreases. The formal potentials for the 4-acyl-5-pyrazolone anion were found between 50 and −120 mV and the standard Gibbs energies of ion partition between −5 and 12 kJ mol −1 . The protonation constants of the ligands in the organic phase amount to 16.5±1.0.

Partitioning of Cations and Anions of Electrolytes in Electrostatic Ion Chromatography Using Pure Water as Mobile Phase

Abstract The elution behaviors of the mixture solution of strong electrolytes in electrostatic ion chromatography, using pure water as a mobile phase, have been investigated with a conductivity detector and ICP-AES, where a zwitterionic surfactant adsorbed on an ODS column was used as a stationary phase. The characteristic re-combinations of cations and anions as the ion-pairs were observed in the chromatograms.

Anion partitioning into highly lipophilic organic phases

AbstractThe partitioning of anions into different highly lipophilic media such as plasticizers used in the construction of ISEs was studied. Using both potentiometry and conductometry, precise partitioning values of a series of common potassium salts were obtained. The partitioning of the anions was shown to be significant and was dependent on the organic media used. These results stress the importance of the materials used in the construction of the anion selective electrodes since they gave anion partitioning sequences linked to the plasticizer, which differ from the common Hofmeister selectivity pattern. At the same time the partitioning values obtained can be used for the experimental estimation of the mobilities of anions in organic media.

Sorption Of Salicylic And 5-Sulfosalicylic Acids On Silica Gel Impregnated With Aliquat 336

Abstract Silica gel coated with Aliquat 336 was used as a sorbent for salicylic and sulfosalicylic acid or anions of these acids. It was found that both acids were extracted almost quantitatively if their concentrations in aqueous solutions did not exceed the theoretical capacity of the sorbent. Aliquat 336 extracts a large excess of salicylic acid and only a very small excess of sulfosalicylic acid relative to the amount of quaternary alkylammonium chloride in the sorbent in the form of undissociated molecules of acid. Although the main sorption process of both acids or their anions is an anion-exchange reaction, a physical partition of acids or anions between the two polar phases occurs to a lesser degree.

Human Skin Permeability Enhancement by Lauric Acid under Equilibrium Aqueous Conditions

An in vitro method was developed to investigate the enhancement of hydrocortisone transport across human stratum corneum (SC) by a model enhancer, lauric acid, in aqueous solutions under equilibrium conditions with respect to the enhancer. In contrast to classical (i.e., nonequilibrium) loading techniques, in which the enhancer is applied only to the donor side of SC either in pure form or in an organic solvent while enhancer‐free aqueous buffers are placed in the receptor phase, this method allowed the investigation of pH effects, concentration effects, and reversibility of both enhancer uptake and enhancement of drug transport under thermodynamically well‐defined conditions. The SC–buffer partition coefficients for lauric acid were linear with concentration and sigmoidal with pH, suggesting that both the neutral species and laurate anion partition into SC. Comparisons of partition coefficients in delipidized and untreated SC as a function of pH indicated that the uptake of lauric acid in neutral form is governed primarily by the lipid domain, whereas the protein domain accounts for anion uptake. The effects of lauric acid on skin permeability were >80% reversible upon extraction of the enhancer from the membrane. However, the degree of enhancement of hydrocortisone permeability was nonlinearly dependent on the equilibrium concentration of lauric acid in either the aqueous buffer or the membrane, exhibiting thresholds in the appearance of enhancement with concentration. The enhancer concentration necessary to achieve isoenhancement of about 6‐fold varied from ∼1 × 10−5M at pH < pKato ∼1 × 10−2M at high pH (pH > 8) demonstrating the higher influence of the free acid species. Maximum enhancement, however, occurred at high pH, reflecting the significantly higher solution concentration of laurate anion attainable in saturated solutions at high pH.

Determination of protonation macro- and microconstants and octanol/water partition coefficient of the antiinflammatory drug niflumic acid

The drug niflumic acid is an amphoteric substance with overlapping pKa values. The acid-base chemistry of the molecule has been characterized in terms of protonation macroconstants (with reference to stoichiometric ionizations) and microconstants (with reference to ionizations of individual species). The proton-binding sites were assigned using 1H and 13C NMR spectroscopy. Due to the very poor water solubility of niflumic acid, the aqueous pKa values were determined from the apparent ionization constants in methanol-water solutions of various proportions by extrapolation to zero co-solvent using the Yasuda-Shedlovsky procedure. The kz tautomerization microconstant of the equilibrium unionized form<-->zwitterionic form was determined from mixtures of organic solvent (dioxane or methanol) with aqueous buffer (at the pH of isoelectric point) by UV spectroscopy, and used for calculation of the other protonation microconstants. The zwitterionic form of the molecule predominates over the uncharged form, the concentration being maximal at the isoelectric pH. The apparent partition coefficients (Papp) of niflumic acid were measured in octanol/water solution by the shake-flask method over a wide pH range. The lipophilicity profile (logPapp vs pH) shows a parabolic shape near its maximum at the isoelectric point. A relationship derived between Papp, PXH0(micropartition coefficient of the uncharged microspecies) and PX-(partition coefficient of the anion) is valid for amphoteric drugs, in cases where the partition of the unionized form and the ion-pair partition of anion can be confirmed. The logP values of microspecies indicate the high lipophilicity of niflumic acid, which is consistent with its good skin penetration and absorption.

Separation of Acrolein and Its Possible Metabolites Using Different Modes of High Performance Liquid Chromatography

Abstract Ion-exclusion, reverse phase, anion exchange and partition chromatography were used to separate acroleinand related compounds including 3hydroxypropana1, 3-hydroxypropionic acid, 1,3-propanediol, ally1 alcohol, acrylic acid, HCO3 −, glycerol, glycidol, oxalic acid, malonic acid, propionic acid, ethanol, and propanol. The ion-exclusion chromatogmphy employed an Interaction ORH-801 organic acid analysis column and wds capable of Separating these small and polar molecules. The reverse phase chromatography was used to resolve coeluting compounds on the ionexclusion column. The anion exchange and partition chromatography were used to separate organic acids and carbohydrates, respectively.

Determination of thiocyanate anion as an organic derivative by gas chromatography.

A simple and sensitive gas chromatographic method has been established for the determination of biologically active thiocyanate anion as pentafluorobenzyl thiocyanate. The method is based on the partition of an ion-pair from the thiocyanate anion and a complex crypstand to a benzene layer for derivatization with pentafluorobenzyl bromide. The resulting derivative was analyzed by gas chromatography with electron capture detection. The quantitation range for thiocyanate anion was over 0.086 and 3.45 nmol. The detection limit of thiocyanate in 0.2 ml sample is about 34 pmol. The effects of several parameters such as base or acid, reaction time and the amount of pentafluorobenzyl bromide were evaluated for the optimization of partition and derivatization of thiocyanate anion. Application of the method to the analysis of thiocyanate in waste water and human saliva proved to be feasible.

Characteristics of corrosion inhibition admixtures in OPC paste with chloride additions: Part I: Chemistry and electrochemistry

The chemical modfications to OPC paste and pore fluid compositions produced by 1–5% nitrite, chromate and phosphate–ased admixes have been studied. Admixture partitioning and modes of corrosion inhibition have been determined. Constitutional differences between the solid paste with and without chloride additions were analysed, as well as pH, admixture anion and chloride ion concentrations of the extracted aqueous phase. Electrochemical rest potential measurements and linear polarization methods were employed to assess inhibitor efficiency, using mild steel electrodes embedded in cement paste cylinders in accelerated experiments at 50°C and 100% humidity. Admixture anion partitioning between the solid paste and pore fluid phase is variable. The CrO4 ion concentration in the pore fluids remains low; under 10% of the total is soluble, as compared to about 30% for nitrite and phosphate ions. Free CI ion levels in the pore fluid with and without admixture are found to range between 55 and 75% of the total after 75 days cure but the [Cl]/[OH] and [Cl]/[admix] ratios tend to be relatively constant after 28 days. Corrosion rate determinations indicate that phosphate and nitrite–based admixtures become effective as inhibitors during setting, but chromate begins to be effective only after about 28 days. This delayed action of the chromate ions may be linked to the initially high [Cl]/CrO4] ratio, which was about seven. he apparent differences in modes of inhibition result from the extent to which admixture partitioning modifies the chemical and electrochemical environment surrounding the steel reinforcement.

Comparison of transport of alkali halides and nitrates across asymmetric cellulose acetate and polyamide, composite poly(etherurea), and cation exchange membranes

Using typical synthetic membranes (asymmetric CA and polyamide, composite poly (ether—urea), strong cation exchange membrane), hydrodynamic permeabilties, lp, normalized salt permeabilities, P s/d, and asymptotic rejections, r ∞, for alkali chlorides and nitrates as well as for sodium halides were obtained from hyperfiltration experiments with 0.01 m and partially with 0.2 m brine solutions. Using the measured transport parameters, partition coefficients, K s, and diffusion coefficients, D sm, within the membranes were estimated. In general, the partition coefficients of the asymmetric CA membranes for the alkali chlorides increase in the following order CsCl < RbCl < KCl ⋍ LiCl < NaCl whereas no such clear sequence exists with the alkali nitrates. On the other hand, the sequence is nearly reversed with the cation exchange membrane. The normalized salt permeabilities, P s/d (d = effective membrane thickness), of sodium salts increase in the order Cl − < NO 3 − < Br − < I − for all membranes except the CK—1 membrane which exhibits a reversed sequence. Moreover, varying the cation or anion, respectively, the normalized salt permeabilities of the CK—1 membrane vary according to Walden's rule Λ·η = const.·T (Λ = equivalent conductivity, η = viscosity, T = absolute temperature). It is tried to correlate the variations of the cation and anion partition coefficients, K s = K co = 1 — r ∞, as well as of the normalized permeabilities, P s/d, with the Gibbs free energy of hydration of the alkali ions, Δ G hy.

5] Proton electrochemical potential gradient in vesicles, organelles, and prokaryotic cells

The measurement of a proton-electrochemical-potential gradient is central to the analysis of energy-conversion processes in mitochondria, chloroplasts, storage organelles, bacterial cells, algae, yeast, and other systems in which proton pumps play a primary role. The most direct measurement of the transmembrane-electrical-potential difference is obtained from the use of microelectrodes. The potential between two reference electrodes located at the bulk solutions bathing the two sides of a membrane is the membrane potential. However, the applications of microelectrodes to organelles, small cells, and vesicles are severely limited because of size limitation. Thus, the methods used in these systems are mostly indirect. This chapter discusses simple, two-compartment systems, such as vesicles, organelles, and prokaryotic cells. A transport mechanism that often ensures a true equilibration involves a partition of an ion into a membrane lipid phase, diffusion in the membrane, and dissociation.

Modeling ionic strength effects on anion adsorption at hydrous oxide/solution interfaces

The effects of changes in ionic strength on the adsorption behavior of selenite (SeO 3 2−) and selenate (SeO 4 2−) on goethite and hydrous ferric oxide have been modeled using a generalized version of the triple-layer surface complexation model. Selenite adsorption, which is relatively unaffected by changes in ionic strength, is best modeled assuming that selenite forms an inner-sphere (surface coordination) complex; selenate adsorption, which is markedly reduced by increasing ionic strength, is best modeled assuming that selenate forms an outer-sphere (ion-pair) surface complex. The modeling results suggest that it is possible to distinguish between inner-sphere and outer-sphere anion surface complexes by studying the effects of ionic strength on anion partitioning.

Modeling ionic strength effects on cation adsorption at hydrous oxide/solution interfaces

Abstract The effects of changes in ionic strength on the adsorption behavior of selenite (SeO32−) and selenate (SeO42−) on goethite and hydrous ferric oxide have been modeled using a generalized version of the triple-layer surface complexation model. Selenite adsorption, which is relatively unaffected by changes in ionic strength, is best modeled assuming that selenite forms an inner-sphere (surface coordination) complex; selenate adsorption, which is markedly reduced by increasing ionic strength, is best modeled assuming that selenate forms an outer-sphere (ion-pair) surface complex. The modeling results suggest that it is possible to distinguish between inner-sphere and outer-sphere anion surface complexes by studying the effects of ionic strength on anion partitioning.