Monovalent Counterions Research Articles

Interaction of cationic surfactants with carboxymethylcellulose in aqueous media

We have examined the polymer–surfactant interaction in mixed solutions of the cationic surfactants, i.e., dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, tetradecyltriphenylphosphonium bromide, and tetradecylpyridinium bromide and a semiflexible anionic polyelectrolyte carboxymethylcellulose in water and aqueous salt solutions by various techniques: tensiometry, viscosimetry or ion-selective electrode method, and dynamic light scattering. We have investigated the effect of varying surfactant chain length, head group size, counterion, and ionic strength on the critical aggregation concentration (CAC) of mixed polymer surfactant systems and the collapse of the polymer molecule under different solution conditions. The CAC decreases with increasing alkyl chain length. Above a certain surfactant concentration, mixed aggregates start growing until their macroscopic phase separation. The growth is more rapid with greater surfactant tail length and with increasing head group size. This is attributed in both cases to the increasing hydrophobic interaction between polymer and surfactant. Among surfactants with monovalent halide counterions, iodide induces the strongest binding, reflected by the onset of growth of the mixed aggregates at low surfactant concentration. This is perhaps related to the decreasing hydration of the counterion from chloride to iodide. The surfactant concentration at which the viscosity of the solution starts to decrease sharply is smaller than the CAC, and probably reflects polymer chain shrinkage due to noncooperative binding.

Electric Double Layers with Electrolyte Mixtures: Integral Equations Theories and Simulations

A study of a planar electric double layer (EDL) in the presence of mixtures of electrolyte is presented. In particular, results from the Hyper-Netted-Chain/Mean-Spherical-Approximation (HNC/MSA) theory are compared with Monte Carlo (MC) simulations. In this way, the charge inversion induced by mixtures of multivalent and monovalent counterions is probed. Since overcharging phenomena in nature emerge under such conditions, the role of ion-ion correlations in the EDL appears as a crucial point in this kind of study. Unlike previous related works, a realistic hydrated ion size is used in the HNC/MSA calculations and simulations. In this way, a qualitative agreement between the results obtained from the theory and MC simulations is found. However, some discrepancies arise when the charge inversion is expected to be more noticeable, namely at high surface charges and/or elevated concentrations of multivalent electrolytes. Such differences are explained in terms of an overestimation of the charge inversion by the integral equation (IE) formalism.

Monovalent counterion distributions at highly charged water interfaces: Proton-transfer and Poisson-Boltzmann theory

Surface sensitive synchrotron-x-ray scattering studies reveal the distributions of monovalent ions next to highly charged interfaces. A lipid phosphate (dihexadecyl hydrogen phosphate) was spread as a monolayer at the air-water interface, containing CsI at various concentrations. Using anomalous reflectivity off and at the L3 Cs+ resonance, we provide spatial counterion distributions (Cs+) next to the negatively charged interface over a wide range of ionic concentrations. We argue that at low salt concentrations and for pure water the enhanced concentration of hydroniums H3O+ at the interface leads to proton transfer back to the phosphate group by a high contact potential, whereas high salt concentrations lower the contact potential resulting in proton release and increased surface charge density. The experimental ionic distributions are in excellent agreement with a renormalized-surface-charge Poisson-Boltzmann theory without fitting parameters or additional assumptions.

A new approach to DNA bending by polyamines and its implication in DNA condensation

Polyamines are known to induce dynamical bending of DNA molecules. This mechanism is very important since many DNA binding proteins (DNAse, transcription factor, etc.) exert their action by their ability to bend DNA. We propose an analytical model which describes the dynamical bending of DNA by polyamine ions in highly diluted DNA solutions. The bending probability depends on the entropy loss of polyamines due to their localization. This localization is facilitated by the electrostatic repulsion between multivalent counterions condensed on DNA, which reduces the entropy loss in counterion localization. Therefore DNA bending by polyamines depends on the competition between monovalent counterions and polyamines. We find that the bending probability is weak for a low binding ratio of polyamines (i.e. number of bound polyamines per base pair), whereas a high bending probability can be reached at large polyamine binding ratio. In addition, we describe a new mechanism of DNA bending. It occurs with the help of thermal agitation, which initiates the bending and favours the polyamine localization. This model provides further insights into DNA bending by polyamines and its implication in DNA condensation. A qualitative estimation of the DNA bending probability is obtained by measuring the cleavage efficiency of DNA by bleomycin versus spermidine concentration. Indeed, a local helix distortion by polyamines results in an amplification of the double-strand cleavage by bleomycin. The measurement of the bleomycin amplification is performed by analysing images of DNA molecules with atomic force microscope. Some features of the dynamical bending indicate that condensation and bending are interrelated.

Scattering Functions of Flexible Polyelectrolytes in the Presence of Mixed Valence Counterions: Condensation and Scaling

We have investigated by small-angle X-ray and neutron scattering the structure of salt-free aqueous solutions of sulfonated polystyrene with variable fractions of monovalent (sodium, Na) and divalent (calcium, Ca) counterions. We follow the variation of the position of the maximum in the scattered intensity, q*, as a function of the concentration, c, and the fraction of Ca counterions. A relevant parameter is the degree of condensation of Na and Ca counterions in the different mixtures. We survey the literature and check that, in the case of monovalent counterions, the variation of q* with c can be described by the scaling theory of Dobrynin, Colby, and Rubinstein, provided the counterion condensation is included according to the Manning-Oosawa approach. Scattering data from mixed valence systems are still in agreement with this scaling model at lower concentrations, except for pure CaPSS 2 aqueous solutions. For increasing concentration, a discrepancy is enhanced that extends toward higher Ca fractions. This indicates that counterion condensation is not the only process. Another possible one is a retraction of the macroions due to divalent counterions that induce bridging-type correlations between monomers, as already observed on the form factors of CaPSS 2 and NaPSS plus added CaCl 2 salt. Such a retraction modifies the scaling, in particular, via a change in the overlap concentration c*. Furthermore, it is noticeable that q* measurements have to be handle with care. We show that misinterpretation of the characteristic maximum in the small-angle scattering of polyelectrolyte solutions may occur, especially at high concentration, if the contribution of the condensed counterions to the total scattering function is predominant.

Abrupt Structural Changes in Polyanionic Gels Evoked by Na‐Ca Ion Exchange: Their Biological Implications

AbstractAnionic polyelectrolyte gels, synthetic or natural, undergo abrupt structural changes when the monovalent counterions in the gel, e.g., Na+, are replaced with divalent cations, e.g., Ca2+. These structural changes are caused by the preferential binding of Ca2+ to the polyelectrolyte chains and the ability of Ca2+ to form crosslinks between two neighboring chains. Strong lyotropic (Hofmeister) effects of anions, such as fluoride, phosphate, aspartate and chloride, on the repetitiveness of abrupt structural changes in the gel strands are demonstrated. These findings have important biological implications.

Effect of counterions on the swelling of spherical polyelectrolyte brushes

We investigate the swelling of colloidal spherical polyelectrolyte brushes in the presence of different counterions. The colloidal particles consist of a solid poly(styrene) core of ca. 100 nm diameter onto which linear polyelectrolyte chains are chemically grafted. Two types of polyelectrolyte chains have been used here: The cationic polyelectrolyte poly(2-(acryloyl)ethyltrimethylammonium chloride)) (PATAC) and the anionic poly(styrenesulfonate) (PSS). Both systems are dispersed in water and the degree of swelling of the surface layer is studied by dynamic light scattering. Adding more and more salt leads to a strong shrinking of the surface layer as expected for polyelectrolyte brushes. It is shown that data obtained at low ionic strength can be collapsed on suitable master curves for monovalent and divalent counterions, respectively. For some ions, however, high salt concentrations may lead to a re-swelling of the brush layer in case of the cationic systems. This points to specific interactions of the counterions with the PATAC chains. This strong specific interaction between the counterions and the attached polyelectrolyte may even lead to flocculation of the particles at intermediate salt concentration. Surprisingly, for iodide and magnesium counterions the solubility increases again if the salt concentration is raised to 1 mol/l. Hence, specific interaction leads to salting-out effects as well as to salting-in effects for these colloidal particles. All specific effects seen at high concentrations of added salt can be explained by the increase of the reduced excluded-volume parameter which is due to the adsorption of salt ions.

Influence of the Diffuse Layer Overcharging or Undercharging on the Stability of Charged Interfaces: A Restricted Grand Canonical Monte Carlo Study

We have applied a restricted grand canonical Monte Carlo procedure to describe, in the framework of the primitive model, the counterion exchange mechanism between diffuse layers of counterions surrounding segregated charged lamellae. The net charge transfer between the dense and dilute domains is shown to vary as a function of the valence of the neutralizing counterions: undercharging of the dense interlayer is detected in the presence of monovalent counterions and overcharging with divalent counterions. Furthermore, no net reduction of the swelling pressure is detected for monovalent counterions, while a large enhancement of the net interlamellar attraction is found for charged lamellae neutralized by divalent counterions.

Range-Optimized Theory of Polyelectrolyte Solutions: Basic Theory and Application to Rod Polymers

We present a theory for the equilibrium structure of polyelectrolyte solutions. The main element is a simple and general optimization scheme that allows theories such as the random phase approximation to handle the strong repulsive forces present in such systems. Charged hydrophilic rods in salt-free solution at semidilute densities are examined. The effect of including condensed counterions is analyzed. Comparison with other theories is made. Results of experiments of hydrophilic polyelectrolytes with monovalent counterions in salt-free solution have shown that the structure factor peak position and height can become invariant at high charge fraction. We show that this invariance may be the result of polymer−polymer correlations rather than the conventional explanation involving counterion condensation. On the other hand, as determined by the osmotic pressure, it is found that the solution can become mechanically unstable to macrophase separation at high charge fraction. In this case, adding explicit condensed counterions acts to stabilize the liquid, that is, intrachain correlations between condensed counterions, not contained in the idealized Manning−Oosawa theory, must be included.

Mechanical creep and recovery of poly(styrene‐b‐ethylene/butylene‐b‐styrene) (SEBS), sulfonated SEBS (sSEBS), and sSEBS/silicate nanostructured materials

AbstractCreep and recovery of poly(styrene‐b‐ethylene/butylene‐b‐styrene) (SEBS) and sulfonated SEBS (sSEBS) materials having various percent sulfonation, some of which were exchanged with monovalent counterions, and sSEBS/silicate hybrids, were investigated. Explanations offered for tensile and shear creep reduction for low degrees of sulfonation involved hydrogen bonding between SO3H groups and/or observed frustrated morphologies. At higher sulfonation levels, an increase in overall percent strain was rationalized in terms of either a more distinct EB phase morphology, or to residual water bound to sulfonic acid groups. This effect was also observed when the acid form materials were ion‐exchanged with monovalent counterions. The behavior of silicate‐containing systems is situational and less understood. An equation for a modified Burgers viscoelastic model was seen to fit the creep data very well. Copyright © 2005 John Wiley & Sons, Ltd.

Effective interaction between charged plane and charged colloidal particle in electrolyte

The effective interaction between two infinite, uniformly charged planes immersed in electrolyte solution having only monovalent counterions is calculated using Poisson-Boltzmann equation. Meanwhile, the influence of the boundary (a infinite, uniform charged plane) on a charged colloid sphere is investigated using the Derjaguin approximation. It is found that there is only repulsive force between two charged planes in the absence of microion correlation. Furthermore, the variation in the boundary condition has little effect on their effective interaction. The conclusions are similar with that of the charged-plane and charged-colloid-sphere system.

Theoretical Study of Catalytic Effects in Micellar Solutions

The catalytic effect of charged micelles as manifested through the increased collision frequency between the counterions of an electrolyte in the presence of such micelles is explored by the Monte Carlo simulation technique and various theoretical approaches. The micelles and ions are pictured as charged hard spheres embedded in a dielectric continuum with the properties of water at 298 K with the charge on micelles varying from zero to z(m) = 50 negative elementary charges. Analytical theories such as (i) the symmetric Poisson-Boltzmann theory, (ii) the modified Poisson-Boltzmann theory, and (iii) the hypernetted-chain integral equation are applied and tested against the Monte Carlo data for micellar ions (m) with up to 50 negative charges in aqueous solution with monovalent counterions (c; z(c) = +1) and co-ions (co; z(co) = -1). The results for the counterion-counterion pair correlation function at contact, g(cc)(sigma(cc)), are calculated in a micellar concentration range from c(m) = 5 x 10(-)(6) to 0.1 mol/dm(3) with an added +1:-1 electrolyte concentration of 0.005 mol/dm(3) (for most cases), and for various model parameters. Our computations indicate that even a small concentration of a highly charged polyelectrolyte added to a +1:-1 electrolyte solution strongly increases the probability of finding two counterions in contact. This result is in agreement with experimental data. For low charge on the micelles (z(m) below -8), all the theories are in qualitative agreement with the new computer simulations. For highly charged micelles, the theories either fail to converge (the hypernetted-chain theory) or, alternatively, yield poor agreement with computer data (the symmetric Poisson-Boltzmann and modified Poisson-Boltzmann theories). The nonlinear Poisson-Boltzmann cell model results yield reasonably good agreement with computer simulations for this system.

C-di-GMP Displays A Monovalent Metal Ion-Dependent Polymorphism

Cyclic diguanosine monophosphate (c-di-GMP) is known to be an important signaling molecule that regulates the formation of bacterial biofilms. We demonstrate here by UV, CD, and NMR that it displays a surprising polymorphism that varies with the monovalent counterion and is consistent with the formation of a G-quartet structure in K+ and a stacked structure in Li+ or Na+. The ability of this rigid molecule to adopt different alignments may be important to its biological role.

Effective charge of colloidal particles

A new dynamical definition of the effective colloidal charge, especially suitable for the Monte Carlo and Molecular-dynamics simulations, is proposed. It is shown that for aqueous colloidal suspensions containing monovalent counterions the "dynamical" effective charge agrees perfectly with the "statistical" effective charge calculated using the Alexander prescription. In the case of multivalent ions, the effective charge behaves in a qualitatively different way from the predictions of the Poisson-Boltzmann theory.

Self-diffusion coefficient of water in Nafion-117 membrane with different monovalent counterions: a radiotracer study

The self-diffusion coefficient of water in Nafion-117 membrane in different cationic forms was measured by the transient radiotracer method, which is based on an analytical solution of Fick's second law. The self-diffusion coefficient of water in the membrane was obtained from the analysis of time-dependent isotopic-exchange rates of tritium tagged water between sample of Nafion-117 membrane and equilibrating water. This transient method does not require the knowledge of partition coefficient of water, which is an essential parameter in the radiotracer permeation method. In present work, self-diffusion coefficients of water in the Nafion-117 membrane with H +, Li +, Ag +, Na +, K +, Rb + and Cs + monovalent counterions were obtained. The values of logarithm of self-diffusion coefficients were found to vary linearly as a function of polymer volume fraction except for membrane sample with H + counterions. The self-diffusion coefficient of water in Nafion-117 membrane with H + counterions was significantly different from the trend observed in the variation of self-diffusion coefficient of water as a function of polymer volume fraction in the membrane with other monovalent counterions. This observation seems to suggest that the physical structure of Nafion-117 membrane in H + form may be quite different from the Nafion-117 membrane with other monovalent counterions. The high self-diffusion coefficient of water (1.67 × 10 −6 cm 2/s) in Nafion-117 membrane with Cs + counterions indicates that the ionic clusters in Nafion-117 membrane are well connected even at low water content (8.2 wt.%) in the membrane.

Charged designed copolymers in the presence of multivalent counterions: a molecular dynamics study

We present the results of molecular dynamics simulations of charged proteinlike hydrophobic–hydrophilic (HP) copolymers in a dilute salt-free solution with multivalent counterions under poor solvent conditions. The primary sequence of these copolymers is constructed such that they can self-assemble into a segregated core–shell microstructure thus resembling some of the basic properties of globular proteins. The results are compared with those obtained earlier for the system containing monovalent counterions. The processes of coil-to-globule transition, aggregation of polyions, and counterion condensation are studied in detail as a function of temperature. The main attention is paid to the influence of the counterions of different charge on the aggregation of the copolymers. It is found that multivalent counterions considerably increase the aggregation of the chains, which form in their presence the finite-size aggregates built up from several polyions. However, the striking feature of the aggregation is that this process does not appear to lead to macroscopic phase separation. The mechanism that prevents the phase separation is discussed.

Dispersion of alumina-coated TiO 2 slurries in the presence of poly(acrylic) acid influence of monovalent counterions

The rheological behavior of concentrated alumina-coated TiO 2 slurries has been investigated in connection with the type of surface counterions (monovalent cations: X = Li +, Na +, TMA +) in the absence and in the presence of polyacrylic acid (PAA). The study has been conducted in a pH range of 4–10 and with ionic strengths lower than 0.01 M. The pH and ionic strength were adjusted with XOH and XCl, respectively. The surface properties have been investigated by titration of surface counterions and the apparent yield stress has been measured using a dynamic stress rheometer. It appears from the results that the pH at the maximum yield stress and the magnitude of the yield stress depend on the nature of the counterion. The yield stress measurements were also conducted in the presence of PAA (0.5 segment/nm 2) adsorbed on the particle surface. In that case, the mineral surface and adsorbed polymer were neutralized with XOH. The results show that the dispersion efficiency depends on the polymer counterion. In general, it is found that the maximum yield stress and the corresponding counterion surface density both follow the sequence TMA + < Na + < Li +. The adsorption of PAA apparently amplifies the effects observed with the corresponding cation. An electrostriction effect of the hydration layer at the interface is suggested in order to explain the increasing yield stress as the surface density of Li + increases. The so-called structure-making/structure-breaking model explains the yield stress reduction with the TMA + surface density.

Simulation of Phase Equilibria in Lamellar Surfactant Systems

The coexistence of two lamellar liquid crystalline phases has been investigated by means of Monte Carlo simulations. The surfaces of the negatively charged bilayers formed by the surfactant molecules are modeled as planar infinite walls with a uniform surface charge density. Water is treated as a dielectric continuum, and only electrostatic interactions are considered. The counterions are mono- and divalent point ions, and their ratio is allowed to vary. Monovalent counterions lead to a repulsive osmotic pressure at all separations, while an attractive region exists when the counterions are divalent. In the latter case, one would expect a phase separation to take place, although it is not observed experimentally due to the limited stability of the lamellar phase at high water content. In a system with mixed counterions, however, the osmotic pressure exhibits a van der Waals loop under such conditions that two phases can coexist. A phase diagram is constructed, and the agreement with experimental data is excellent.

Effects of counterion fluctuations in a polyelectrolyte brush.

We investigate the effect of counterion fluctuations in a single polyelectrolyte brush in the absence of added salt by systematically expanding the counterion free energy about Poisson-Boltzmann mean-field theory. We find that for strongly charged brushes, there is a collapse regime in which the brush height decreases with increasing charge on the polyelectrolyte chains. The transition to this collapsed regime is similar to the liquid-gas transition, which has a first-order line terminating at a critical point. We find that, for monovalent counterions, the transition is discontinuous in theta solvent, while for multivalent counterions, the transition is generally continuous. For collapsed brushes, the brush height is not independent of grafting density as it is for osmotic brushes, but scales linear with it.

Dynamics, Site Binding, and Distribution of Counterions in Polyelectrolyte Solutions Studied by Electron Paramagnetic Resonance Spectroscopy

A microscopic picture of counterion condensation in liquid and glassy frozen solutions of the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) is derived from data obtained by a combination of continuous-wave and pulse electron paramagnetic resonance techniques. The condensation of such divalent anions to the polyelectrolyte chain in the presence of a large excess of polyelectrolyte and monovalent counterions can be described by a dynamic equilibrium between specifically site-bound and nonspecifically territorially bound counterions with exchange between the two states proceeding on time scales significantly shorter than 1 ns. No free divalent counterions are detected. The dynamic electrostatic attachment is manifest in an axially symmetric rotational diffusion tensor, with the unique axis of fast rotation corresponding to the electrostatic bond between one sulfonate group of Fremy's salt and the quaternary ammonium group of the PDADMAC repeat unit. A distance of 0.43 nm between the electron spin and the 14N nucleus of the ammonium group is found by electron spin−echo envelope modulation spectroscopy, suggesting that the site-bound state corresponds to contact ion pairs. The same experiment provides an estimate of 20% site-bound and 80% territorially bound divalent counterions in glycerol/water glassy frozen solution. Pulse electron−electron double resonance measurements show that on nanometer length scales the counterions are virtually homogeneously distributed in three dimensions for high polyelectrolyte concentration but linearly distributed along stretched chains at low polyelectrolyte concentrations where no overlap of chains is expected.