Aggregation and adsorption of sodium dioctylsulfosuccinate in aqueous ammonium chloride solution: role of mixed counterions

The critical micelle concentration (cmc) of sodium dioctylsulfosuccinate (AOT) was determined at 25 °C from surface tension and fluorescence methods in aqueous NH(4)Cl solution for assessing the influence of mixed counterions on the special counterion binding behavior (SCB) of AOT. The SCB of AOT refers to a sudden twofold increase in the value of the counterion binding constant (β) in aqueous medium when the concentration (c(*)) of the added 1:1 sodium salt is about 0.015 mol kg(-1), and it has been tested so far for sodium ion only. In the presence of sodium and ammonium mixed counterions also the SCB of AOT exist, but with lower c(*) (0.009 mol kg(-1) NH(4)Cl). Synergism in the cmc occurs due to mixed counterions. In the case of inorganic counterions, unlike the case with organic counterions, the cmc is dependent on the total counterion concentration in solution and negligibly on the specific type of counterion. Na(+) and NH(4)(+) bind almost equally to the micelle in the region of low β (below c(*)), but in the region of high β (above c(*)) NH(4)(+) binds predominantly. It has been shown that the theoretical expression for the surface excess of ionic surfactant+electrolyte system containing a single counterion can also be used to evaluate the surface excess in the presence of mixed counterions if the two counterions are considered to undergo Henry-type adsorption at the air-solution interface.     

Colloido-chemical characteristics of nanoporous glasses with different compositions in solutions of simple and organic electrolytes. 2. Equilibrium electrochemical characteristics of membranes

The regularities of variations in the electrokinetic potential and surface charge of nanoporous glass membranes with different compositions have been studied as depending on the type of an electrolyte (sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium chlorides) and the structure of pore space. It has been shown that, in solutions containing specifically sorbed organic counterions, the range of positive values of electrokinetic potential arises due to the superequivalent absorption of counterions in the Stern layer. It has been found that the influence of the specific adsorption of counterions on the electrokinetic potential of porous glasses increases with the amount of secondary silica in the pore space. The effects of the counterion specificity, pore channel sizes, and composition of a porous glass on the value of the surface charge have been analyzed. The absolute value of the surface charge has been shown to significantly increase in the presence of organic counterions in comparison with inorganic ions throughout the examined range of background electrolyte concentrations.     

Salt-induced phase inversion in aqueous cationic/anionic surfactant two-phase systems

Phase inversion of aqueous two-phase systems with excess cationic surfactant (abbreviated as ATPS-C) formed by aqueous mixtures of 1,3-propanediyl bis(dodecyl dimethylammonium bromide) (abbreviated as 12-3-12) and sodium dodecyl sulfonate (abbreviated as AS) at 318.15 K was investigated. The experimental results indicate that addition of NaF, NaCl, NaHCO 3, or NaNO 3 can result in phase inversion of ATPS-C formed by 12-3-12/AS systems; however, addition of NaBr cannot lead to phase inversion. TEM micrographic experiments illustrate that there is no direct relationship between the microstructures of the concentrated phase in ATPS-C and phase inversion. To interpret the phase-inversion phenomena of ATPS-C, the phase composition, phase density, and phase volume ratio between the dilute phase and the concentrated phase in ATPS-C were investigated. Phase composition analysis results illustrate that for the ATPS-C formed by 0.10 mol.kg (-1) 12-3-12/AS mixed system, the concentration of Br (-) counterions in the dilute phase of ATPS-C increases with addition of NaF, NaCl, NaHCO 3, or NaNO 3. At the same time, the molar ratio between the F (-) (Cl (-), HCO 3 (-), or NO 3 (-)) counterions and Br (-) counterions in the concentrated phase of ATPS-C increases also. It illustrates that part of the bromide counterions which are the natural counterions of the surfactant 12-3-12 in excess are exchanged by other anionic counterions when an additional salt is added to the system. The investigation indicates that the common ground of the added F (-), Cl (-), HCO 3 (-), or NO 3 (-) counterions is that they all make a smaller density contribution than that of Br (-) counterions, although they have a weaker or stronger counterion binding ability with the mixed positively charged aggregates in ATPS-C than that of Br (-) counterion. Density experiments illustrate that the density increase of the dilute phase is larger than that of the concentrated phase in the ATPS-C with addition of NaF, NaCl, NaHCO 3, or NaNO 3; thus, phase inversion occurs. The densities of the added inorganic sodium salt aqueous solution and the order of the Hofmeister series for the added inorganic anions with respect to the chaotropic headgroup of 12-3-12 play important roles in the phase inversion of ATPS-C.     

Counterion binding in the aqueous solutions of bile acid salts, as studied by computer simulation methods

We investigate the structural and dynamical properties of counterion binding in sodium cholate and sodium deoxycholate micelles at three different concentration, namely, 30, 90, and 300 mM, by means of molecular dynamics simulations at the atomistic level. The obtained results can resolve a long-standing, apparent contradiction between different experiments that reported discordant values for the degree of counterion binding. Namely, our results suggest that certain experimental techniques, such as freezing point depression, are only sensitive to the contact counterions, and hence, the degree of contact binding of the counterions is measured. On the other hand, in experiments employing, e.g., electrode potential or nuclear magnetic resonance measurements, the solvent-separated counterions also contribute to the signal detected, and hence, the counterions that are measured as bound ones do include the solvent-separated counterions as well.     

反离子对氟表面活性剂的影响1. 表面活性及胶团化作用

通过表面张力和荧光探针法研究了全氟辛酸钠、全氟辛酸铵以及全氟辛酸四烷基铵[C₇F₁₅COON(C_nH_2n+1)₄, n＝1, 2, 3, 4]的表面活性以及胶团化作用, 系统地讨论了各种反离子, 特别是反离子大小的影响. 结果表明, 与普通碳氢表面活性剂不同, 反离子对这类氟表面活性剂的表面活性以及胶团化作用有很大影响. 表面活性剂的临界胶束浓度(cmc)随反离子的增大而下降; 饱和吸附层中平均每个分子所占的面积则大致随反离子的增大而增大. 而表面张力的变化则较为复杂. cmc时的表面张力随反离子的增大先上升(从全氟辛酸铵到全氟辛酸四乙铵)后下降(从全氟辛酸四乙铵到全氟辛酸四丁铵). 通过反离子的空间位阻、疏水性、插入以及电荷屏蔽效应对上述结果做了解释.     

Counterion release from adsorbed highly charged polyelectrolyte: an electrooptical study

The adsorption of sodium poly(4-styrene sulfonate) on oppositely charged beta-FeOOH particles is studied by electrooptics. The focus of this paper is on the release of condensed counterions from adsorbed polyelectrolyte upon surface charge overcompensation. The fraction of condensed Na+ counterions on the adsorbed polyion surface is estimated according to the theory of Sens and Joanny and it is compared with the fraction of condensed counterions on nonadsorbed polyelectrolyte. The relaxation frequency of the electrooptical effect from the polymer-coated particle is found to depend on the polyelectrolyte molecular weight. This is attributed to polarization of the layer from condensed counterions on the polyion surface, being responsible for creation of the effect from particles covered with highly charged polyelectrolyte. The number of the adsorbed chains is calculated also assuming counterion condensation on the adsorbed polyelectrolyte and semiquantative agreement is found with the result obtained from the condensed counterion polarizability of the polymer-coated particle. Our findings are in line with theoretical predictions that the fraction of condensed counterions remains unchanged due to the adsorption of highly charged polyelectrolyte onto weakly charged substrate.     

Explicit water molecular dynamics study of the mobility of halide ions in presence of ionene oligocations

We present explicit water molecular dynamics simulations of solutions of aliphatic 3,3- and 6,6-ionene oligocations neutralized with (i) fluoride, chloride, bromide, or iodide counterions, respectively, or (ii) with a 1:1 mixture of chloride and bromide anions in presence of a low molecular weight salt at 298 K. The SPC/E model was used to describe water molecules. Results of the simulation are presented in form of the pair distribution functions between various atoms on the ionene oligoion and counterions in solution. In addition, we were interested in the dynamics of counterions around model ionenes. We showed that counterions residing in the vicinity of the oligoion exchange rapidly with those in the bulk solution, with the frequency depending on the nature of the counterion and on the charge density of the oligoion. We calculated the average residence times of the various counterion species to the oligoions and proposed the model which divides the counterions into "free" and "bound" and calculated the fraction of "free" counterions. In the second part of the study, we investigated interaction of the sodium chloride and sodium bromide, being simultaneously present in the solution, with differently charged ionenes in water. The selectivity effect was clearly observed: bromide ions tend to replace chloride ions in the immediate vicinity of the ionene oligoions. Simulation results are discussed in light of our recent measurements of thermodynamic and transport properties of aqueous ionene solutions.     

Proton transport and the water environment in nafion fuel cell membranes and AOT reverse micelles

The properties of confined water and diffusive proton-transfer kinetics in the nanoscopic water channels of Nafion fuel cell membranes at various hydration levels are compared to water in a series of well-characterized AOT reverse micelles with known water nanopool sizes using the photoacid pyranine as a molecular probe. The side chains of Nafion are terminated by sulfonate groups with sodium counterions that are arrayed along the water channels. AOT has sulfonate head groups with sodium counterions that form the interface with the reverse micelle's water nanopool. The extent of excited-state deprotonation is observed by steady-state fluorescence measurements. Proton-transfer kinetics and orientational relaxation are measured by time-dependent fluorescence using time-correlated single photon counting. The time dependence of deprotonation is related to diffusive proton transport away from the photoacid. The fluorescence reflecting the long time scale proton transport has an approximately t-0.8 power law decay in contrast to bulk water, which has a t-3/2 power law. For a given hydration level of Nafion, the excited-state proton transfer and the orientational relaxation are similar to those observed for a related size AOT water nanopool. The effective size of the Nafion water channels at various hydration levels are estimated by the known size of the AOT reverse micelles that display the corresponding proton-transfer kinetics and orientational relaxation.     

Electro-optics of colloid-polyelectrolyte complexes: counterion release from adsorbed macromolecules

Complexation between sodium carboxymethylcellulose (NaCMC) and ellipsoidal particles of oppositely charged beta-FeOOH is studied using electric light scattering and electrophoresis. We focus on the complex behavior for overcharging of the particle surface. In this case, the fraction of condensed counterions on NaCMC is found to remain unchanged during polymer adsorption onto a weakly charged particle surface. Using the theory of Sens and Joanny, we evaluate the fraction of condensed counterions and compare it with results for nonadsorbed NaCMC. The polarization of condensed counterions along the chain of the adsorbed macromolecule is proved to create the electro-optical effect in suspensions stabilized by NaCMC adsorption.     

Effective self-diffusion coefficients of ions in sodium dodecyl sulfate micellar solutions

The effective self-diffusion coefficients of ions in premicellar and micellar solutions of sodium dodecyl sulfate are measured by the NMR self-diffusion method at 40°C. The obtained regularities are explained using a proposed model that takes into account the possible surface diffusion of counterions bound with micelles. This effect is shown to markedly influence the charge transfer in micellar solutions. Based on the results obtained, the self-diffusion coefficients of bound Na⁺ counterions are estimated and the causes and ranges of their variations are indicated.     

Transport,and thermodynamic investigations on sodium polystyrenesulfonate in ethylene glycol–water media

Polyion–counterion interactions in sodium polystyrenesulfonate dissolved in (ethylene glycol + water) mixed solvent media have been investigated conductometrically with special reference to their variations as functions of polyelectrolyte concentration, relative permittivity and temperature. Manning counterion condensation theory for polyelectrolyte solutions failed to describe the present experimental results. The data have, therefore, been analyzed using a new model for semidilute polyelectrolyte conductivity which takes into account the scaling arguments proposed by Dobrynin et al. The fractions of uncondensed counterions were found to depend on the polyelectrolyte concentration varying from 0.27 to 0.37, within the concentration range investigated here, indicating a strong interaction between counterions and polyion. A considerable fraction of the counterions is shown to migrate in the same direction as the polyions. The results further demonstrate that the monomer units experience more frictional resistance in solutions as the ethylene glycol content of the mixture increases or as the temperature decreases.     

Counterion binding behaviour of micelles of sodium dodecyl sulphate and bile salts in the pure state,in mutually mixed states and mixed with a nonionic surfactant

The counterion binding behaviour of micelles of sodium dodecyl sulphate (SDS) and several bile salts in the pure state have been studied, as well as in mutually mixed states, and in a mixed state with polyoxyethylene sorbitan monolaurate (PSML) as a nonionic surfactant. Electrochemical measurements have shown no counterion binding by the pure bile salt micelles and their mixtures with PSML; they can bind counterions when mixed with SDS, whereas the surfactant anions of SDS micelles bind counterions in the pure state and/or in mixed states with PSML. In the SDS-PSML and SDS-bile salts combinations, the counterion association is decreased by the increased proportions of the second component. The extent of counterion binding by the different systems is presented.     

Microstructure and rheology of stimuli-responsive nanocolloidal systems-effect of ionic strength

The effect of ionic strength on the rheological behavior of model pH-responsive nanocolloidal systems consisting of methacrylic acid-ethyl acrylate (MAA-EA) cross-linked with diallyl phthalate (DAP) was examined. Neutralization of acid groups increases the osmotic pressure exerted by counterions trapped in the polymeric network against ions in bulk solution, which is responsible for the swelling and increase in viscosity. Swelling decreases with increasing salt concentration as a result of reduced osmotic pressure inside the microgels, which is attributed to the charge shielding effect of counterions (salt) on the negatively charged carboxylate groups. Electromotive measurements using ion-selective electrodes confirmed that not all the counterions, that is, K+, remain mobile, but a fraction of these ions can penetrate the porous microgel particles to shield the negatively charged carboxylate groups. A consequence of this is that some of the Na+ counterions inside the particles are expelled, thus regaining their translational entropy, and become mobile sodium ions in the bulk solution. We successfully developed a new scaling law that relates the swelling ratio, Q, of microgels as a function of neutralization degree, alpha, cross-linked density, Nx, molar fraction of acidic units, y, and concentration of mobile counterions, CK+ and CNa+, represented as (Nx/c0)(CK+ + CNa+Q + Q2/3 proportional, variant yNxalpha. The new scaling law no longer assumes that all the counterions are trapped inside the microgels. The proportionality reduces to the form Q proportional, variant (yalphaNx)3/2 in the absence of salt, that is, CK+ + CNa+ approximately 0. By combining the results from light scattering and rheological measurements, we are able to correlate the microstructural evolution of the colloidal systems with their bulk rheological behavior.     

Electric transport and ion binding in solutions of fullerenehexamalonic acid Th-C66(COOH)12 and its alkali and calcium salts

The transport numbers of the macroion constituent in aqueous solutions of fullerenehexamalonic acid, C66(COOH)12, and its lithium, sodium, cesium, and calcium salts were determined by the indirect moving boundary method. By combining the transport data with the literature data for the electrical conductivity of the same solutions, the fraction of bound counterions to the fullerenehexamalonate macroion was calculated. It was found that about 20-40% of counterions are associated with the macroion in the concentration range from about 0.1 to 0.002 mol COO-/L. Consequently, under the influence of a potential field, these counterions move toward the anode as an integral part of the migrating macroion in opposition to the electrical field. Therefore, the absolute values of the macroion constituent transport numbers are relatively high. The experimental values of the fractions of bound counterions were compared with the theoretical predictions based on the solution of the Poisson-Boltzmann equation for the spherical cell model, and a reasonable agreement was obtained.     

Reentrant behavior of grafted poly(sodium styrenesulfonate) chains investigated with a quartz crystal microbalance

Poly(sodium styrenesulfonate) (PSSNa) chains have been grafted onto a SiO(2)-coated resonator surface. The conformational changes of grafted chains have been investigated using a quartz crystal microbalance with dissipation (QCM-D) in the presence of monovalent or multivalent salts as a function of ionic strength. In the case of monovalent counterions, the changes in frequency (Δf) and dissipation (ΔD) indicate that the highly extended PSSNa chains first shrink into a loose and inhomogeneous layer as the ionic strength increases. As the ionic strength increases further, the chains will collapse and form a denser and more homogeneous layer. In the case of divalent or trivalent counterions, the grafted PSSNa chains also collapse into a dense layer as the ionic strength increases. However, when the ionic strength is above a critical value, the chains would re-expand so that the layer becomes partially extended due to the charge inversion. Additionally, the effect of ion-specificity on the conformational changes of the chains has also been examined.     

Counterions in poly(allylamine hydrochloride) and poly(styrene sulfonate) layer-by-layer films

The amount of counterions in layer-by-layer (LBL) films of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) has been determined with X-ray photoelectron spectroscopy (XPS) for films prepared from solutions with various NaCl concentrations. Sodium and chloride counterions are present in LBL films produced from salt solutions, which are located at the surface and in the bulk of the films. The percentage of bulk counterions increases with the ionic strength of the polyelectrolyte before reaching a constant value. The bulk sodium/sulfur percentage ratios tend to 0.8 for samples washed with pure water and for samples washed with NaCl aqueous solutions, while the bulk chlorine/nitrogen percentage ratios tend to 0.5 for the same samples. The ratio between the percentages of polyelectrolyte ionic groups lies close to unity for all samples, indicating that counterions do not contribute to charge compensation in the polyelectrolyte during the adsorption process. The presence of counterions in LBL films is explained by Manning condensation near the polyelectrolyte ionic groups, leading to inter-polyelectrolyte ionic bondings via ionic networks. It is believed that condensation leads to the formation of NaCl crystallites in these LBL films, which was confirmed by X-ray diffraction measurements.     

Binding of counterions by micelles of ionic surfactants

The dependence of the concentration of free counterions on the overall concentration of an ionic surfactant is analyzed in terms of the quasi-chemical theory of micellization. The degree of binding of counterions by micelles of sodium dodecyl sulfate is determined from the results of potentiometric measurements performed using a sodium-selective electrode. It is disclosed that experimental and theoretical data are in satisfactory agreement, provided that the model used allows for the association of surface-active ions in a solution, with this association reducing the average coefficient of sodium dodecyl sulfate activity in an intermicellar medium.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 2, 2005, pp. 213–217.Original Russian Text Copyright © 2005 by Podchasskaya, Usyarov.     

Counterion-accelerated diffusion of aqueous serum albumin

A modified Harned conductimetric procedure has been used to measure binary diffusion coefficients at 25°C for aqueous sodium and potassium salts of the protein bovine serum albumin (M _n BSA; M=Na⁺ or K⁺; n=2 to 23). The condition of electroneutrality requires that the BSA ions diffuse at the same speed as the relatively mobile sodium or potassium counterions. Consequently, the protein ions diffuse 3 to 8 times more rapidly than the isoionic protein diffuses. An approximate analysis of counterion-accelerated diffusion of high molecular weight solutes is presented which shows that the increase in diffusivity with the number of counterions is a thermodynamic effect brought about by an increase in the free energy gradient of the solutes.     

Transport Numbers of Singly-, Doubly- and Triply-Charged Cations in Porous Glasses

Colloid Journal - Transport numbers of sodium, nickel, and lanthanum counterions have been studied in microporous (average pore radius is r = 2–3.4 nm) and macroporous (r = 16–27 nm)...     

Counterion binding on mixed anionic/cationic micelles

Measurements of counterion binding in mixtures of surfactant aqueous solutions have been performed to study the structure of the anionic/cationic mixed micelle/solution interface. The mixtures studied were SDS/DDAC and STS/TDPC. The binding of chloride and sodium ions to mixed anionic/cationic micelles was measured using ion-specific electrodes. Counterion binding was found to be strongly dependent on the molar ratio of surfactants present. The mixed micelle/solution interface includes the headgroups of both surfactants and counterions of surfactant in excess. The addition of oppositely charged surfactant caused an increasing dissociation of counterions.