Monovalent Counterions Research Articles

Monte Carlo Study of Coulombic Criticality in Polyelectrolytes

The role of charges in determining the water solubility of polyelectrolytes, a question of considerable relevance to biology, is currently unresolved. We use computer simulations to study the purely Coulombic phase separation of flexible polyelectrolytes with monovalent counterions in an athermal solvent. In agreement with recent theories we find that the critical temperature for this transition increases with chain length, but that the critical density remains unchanged. We therefore stress that the phase behavior of polyelectrolytes is qualitatively different from uncharged polymers, where the critical density decreases towards zero for long chains.

A novel positively charged composite membranes for nanofiltration prepared from poly(2,6-dimethyl-1,4-phenylene oxide) by in situ amines crosslinking

A novel positively charged membrane for nanofiltration was prepared from linear engineering plastics poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) by composting the aryl-substitution substrate with brominated polymers of proper benzyl and aryl bromine content (ABC) followed by in situ amine crosslinking. The resultant membrane was characterized with pure water flux and rejection of electrolytes MgCl 2 and NaCl. The results show that the membrane nanofiltration properties are significantly affected by the type of brominated polymers of the top layer, the amination time, the composition of amination medium. The pure flux is enhanced by the benzyl substitution of the top layer, amination time, ratio of trimethyl amine (TMA) in amination medium etc.; while the rejection of electrolyte is increased with the aryl substitution of the top layer, ratio of ethylenediamine (EDA) in amination medium, etc. In addition, due to positive charge on the top layer, the membrane behaves much higher rejection for multi-valent cations (e.g. Mg 2+) than for monovalent cations (e.g. Na +) if the electrolytes have the same monovalent counterion (e.g. Cl −). Therefore, by properly balancing bromination and amination conditions, a desired positively charged membrane could be obtained for removal of multivalent cations from water or for separation of monovalent cations and multivalent cations.

Thermal behavior of the polyoxometalates derived from H3PMo12O40 and H4PVMo11O40

The aim of this study is to investigate the influence of some monovalent counter-ions (NH4+, K+ and Cs+) on thermal behavior of polyoxometalates derived from H3PMo12O40 (HPM) and H4PVMo11O40 (HPVM) by replacing the protons. The IR and UV-VIS-DRS spectra of some acid and neutral NH4+, K+, Cs+ salts, which derived from HPM and HPVM, confirmed the preservation of Keggin units (KU) structure. The X-ray diffraction spectra clearly showed the presence of a cubic structure. The non-isothermal decomposition of studied polyoxometalates proceeds by a series of processes: the loss of crystallization water; the loss of O2 accompanying with a reduction of V5+→V4+ and Mo6+→Mo5+; the loss of constitution water started at 360°C for HPVM salts and 420°C for HPM salts; the decomposition of ammonium ion over 420°C with NH3, N2 and H2O elimination and simultaneous processes of reduction (V5+→ V4+ and Mo6+→ Mo5+ or Mo4+) associating with endothermic effects; reoxidation of Mo5+, Mo4+ and V4+with a strong exothermic effect; destruction of KU to the oxides: P2O5, MoO3 and V2O5 and the crystallization of MoO3.

Counterion layering at high surface charge in an electric double layer. Effect of local concentration approximation

A systematic study of the counterion layering phenomenon at high surface charge in a planar electric double layer is reported. Extensive canonical Monte Carlo simulations are performed on a 1:1 electrolyte to investigate the effect on layering due to a variation of electrode charge density, electrolyte concentration, and ionic size. For a 1 mol dm −3 salt at room temperature and ionic diameter 400 pm, the second layer begins to be formed at around a surface charge density of ∼0.4 C m −2. It is seen that the variation of ionic size (at constant surface charge and salt concentration) leads to the most pronounced changes in the counterion layering. At an ionic diameter of 600 pm a third layer of counterions is observed. The simulations are extended to asymmetric valency 2:1 and 1:2 salts where layering effects are observed for monovalent counterions only. The calculated double layer capacitance is found to decrease in the presence of layering. A mean-field Poisson–Boltzmann equation coupled with a recently derived ionic exclusion volume term is found to predict this layering in qualitative agreement with the simulations. The use of a local concentration approximation improves the agreement in most cases.

Competitive binding of Na + and Ca 2+ ions to teichoic acid analogues

The competitive binding of monovalent and divalent counterions to poly(alkylene phosphate) related to bacterial teichoic acids and poly(styrenesulfonate) was studied experimentally by potentiometry with ion-selective electrodes. The binding of calcium ions and the release of sodium ions accompanying calcium ions binding in aqueous solutions of the polyelectrolytes was analysed and the mean exchange ratio Na+/Ca2+ was estimated. It was found that in the process of addition of calcium ions to sodium poly(alkylene phosphate) and sodium poly(styrenesulfonate) solutions all the Ca2+ ions added are bound to polyions and the initially condensed Na+ ions are released proportionally to the concentration of the added Ca2+ ions up to the critical concentration of the Ca2+ ions added. For a molar concentration ratio of calcium counterions to charged groups on the polyion up to 0.20 the exchange ratio was approximately equal to 1 or 2 for the sodium poly(alkylene phosphate)/CaCl2 and sodium poly(styrenesulfonate)/CaCl2 systems, respectively.

Non-ideal effects in highly asymmetric ionic mixtures at moderate coupling

A binary mixture of hard-sphere polyions and point-size counterions is considered using the hypernetted-chain equation and the symmetric Poisson–Boltzmann equation. It is shown that such non-ideal effects as the polyion–polyion attraction, counterion association near the polyion surface, and the polyion charge inversion that were noticed mainly in strongly coupled systems with multivalent counterions can appear also at moderate coupling with monovalent counterions. It is proposed to use the change of the sign of the polyion surface potential as an indicator for the polyion charge inversion. Both theories demonstrate the latter effect at moderate coupling. In the case of monovalent counterions, the counterion association appears at higher total ion concentration than the polyion–polyion attraction does. In the case of multivalent counterions, these two effects appear in the opposite order.

Intermediate-range order in aqueous solutions of salts constituted of divalent ionscombined with monovalent counter-ions

Concentrated aqueous solutions of salts constituted by divalent ions combinedwith monovalent counter-ions were investigated by x-ray diffraction and Ramanspectroscopy at room temperature. The salts studied were strontium, barium, andmagnesium chlorides and bromides, and lithium and caesium sulphates. For manyof these solutions, intensity maxima were detected in their x-ray diffractionpatterns close to 0.6–1 Å−1. Interpretation of these maxima, pre-peaks, is discussed,taking into account the results of previous investigations of the authors onconcentrated aqueous solutions of trivalent cations. In these solutions thepre-peaks appear to be narrower and much more intense. In the ionic solutionsstudied here, a slight contrast between values of the scattering power of twospatial domains is suggested as the origin of the observed pre-peak. One of thesecomes from an accumulation of scattering power around the ion with the higherelectric charge, locally organized into a subtle close packing; the other comes fromthe holes of this structure. Molecular models of the structure of theseelectrolytes are used to demonstrate the plausibility of this interpretation.

Forces between charged surfaces in a solvent primitive model electrolyte

Grand canonical Monte Carlo (GCMC) simulations have been performed to investigate the components of the force between parallel charged surfaces in an electrolyte. The solvent primitive model (SPM) was used to investigate the effect of neutral hard sphere solvent particles on the force between the surfaces. The effects of particle size, wall charge density, charge valency of the electrolyte, and the exclusion of neutral hard sphere are discussed. When solvent particles are considered, the total force between the charged surfaces is always repulsive, even for divalent counterions. This is different from the earlier conclusion reached with a restricted primitive model electrolyte. The repulsive force decreases in going from monovalent counterions to divalent counterions.

Association of two semiflexible polyelectrolytes by interchain linkers: Theory and simulations

The aggregation of two highly charged semiflexible polyelectrolytes in the presence of generalized linkers is studied theoretically. This model provides insight into biological processes such as DNA condensation and F-actin self-assembly induced by either multivalent counterions or bundling proteins. The interplay between the bending rigidity of the chains and their electrostatic interactions leads to an effective interlinker interaction that is repulsive at large distances and attractive at short ones. We find a rounded phase transition from a dilute linker gas where the chains form large loops to a dense disordered linker fluid where the chains are almost parallel. The onset of chain pairing occurs as soon as the free energy of a pair of chains becomes lower than that of two isolated chains and is located well within the crossover regime between the two linker phases. Our main findings are confirmed by molecular dynamics simulations of two semiflexible charged chains in a mixture of monovalent and polyvalent counterions. This simple model allows us to recover qualitative features of experimental aggregation diagrams of DNA and F-actin and can also be used to study DNA denaturation.

Interaction of Mg 2+ with DNA investigated by dielectric spectroscopy

Divalent ions, as Mg 2+, strongly interact with polyelectrolytes, in particular with DNA and this binding involves two phosphate groups. In our laboratory we studied this interaction by dielectric spectroscopy. At radiofrequencies DNA shows a well-defined dielectric relaxation, due to monovalent counterion oscillations along the surface of the biopolymer. The dielectric response of the biopolymer is thus modified by the interaction with magnesium. In this communication we report on measurements at radiofrequencies, performed on Na-DNA aqueous solutions in the presence of increasing amounts of Mg 2+ ions. The dielectric relaxation is reduced as the magnesium content increases. This effect tends to saturate when the molar ratio Mg 2+/nucleotide approaches the value of 0.25. A significant parameter i.e. length of the biopolymer subunit, estimated by dielectric relaxation time, appears to be modified, denoting also the existence of a structural effect. This new approach proved to be a valid tool in studying interaction processes among nucleic acids and divalent ions or, in general, charged macromolecules.

Spherical colloids: effect of discrete macroion charge distribution and counterion valence

We report the coupled effects of macroion charge discretization and counterion valence in the primitive model for spherical colloids. Instead of considering a uniformly charged surface, as it is traditionally done, we consider a more realistic situation where \textit{discrete monovalent microscopic charges} are randomly distributed over the sphere. Monovalent or multivalent counterions ensure global electroneutrality. We use molecular dynamics simulations to study these effects at the ground state and for finite temperature. The ground state analysis concerns the counterion structure and \textit{charge inversion}. Results are discussed in terms of simple analytical models. For finite temperature, strong and weak Coulomb couplings are treated. In this situation of finite temperature, we considered and discussed the phenomena of ionic pairing (pinning) and unpairing (unpinning).

Influence of Counterion on the Interaction of Dodecyl Sulfates and Cellulose Ethers

Fluorescence probe techniques together with microcalorimetry and dye solubilization were used to study the interaction between nonionic polymers and anionic surfactants with different monovalent counterions in order to examine the effects of the counterion. The polymers used were the cellulose ethers hydroxypropyl methyl cellulose (HPMC) and ethyl hydroxyethyl cellulose (EHEC). The surfactants were dodecyl sulfates with potassium, sodium, and lithium as counterions (KDS, NaDS, LiDS). The counterion influenced the interaction start concentration as well as the nature of the mixed aggregates formed. The interaction start, according to surfactant concentration, was found to be in the order KDS<NaDS<LiDS for both polymers as well as in aqueous solution. From fluorescence measurements it was found that the KDS–polymer aggregates shield pyrene from water better than the other surfactants, indicating larger aggregates with a more fluid interior. The microcalorimetry measurements confirm that the adsorption of the surfactants onto the polymer is endothermic and entropy driven at the start and as more clusters are formed on the polymer chains the process converts to being exothermic and driven by both enthalpy and entropy.

Mechanism of Macroion−Macroion Clustering Induced by the Presence of Trivalent Counterions

The results of molecular dynamics simulations for a soft sphere model of highly asymmetric electrolyte solutions containing macroions and counterions are presented. The macroions carry a negative charge of 12 or 24 elementary charges and the solutions contain monovalent, divalent or trivalent (zc = +1, +2, or +3) counterions. The size ratio between macroion and counterion was chosen to be about 5:1 and 7:1. From analysis of both static and dynamic properties we propose a mechanism for the observed formation of clusters between equally charged macroions in these solutions.

Behaviour of cationic polyelectrolytes upon binding of electrolytes: effects of polycation structure, counterions and nature of the solvent

The effects of polycation structure, counterions and the nature of the solvent on the interaction between low-molecular-weight salts with some cationic polyelectrolytes in water and methanol were investigated. The polyelectrolytes used in this study were cationic polymers with quaternary nitrogen atoms in the backbone with or without a nonpolar side chain (polymer type PCA5H1, PCA5D1 and PCA5) or tertiary amine nitrogen atoms in the main chain (polymer type PEGA). LiCl, NaCl, KCl, NaBr, NaI and Na2SO4 were used as low-molecular-weight salts. The interaction between polycations and salts was followed by viscometric and conductometric measurements. The study of the interaction of monovalent counterions with cationic polyelectrolytes emphasized an increase in the interaction with the decrease in the radius of the hydrated counterion, both for strong polycations and for weak polycations, suggesting that counterion binding is nonspecific. In the case of SO2−4 anions, the Λm−c1/2 curve passes through a minimum at cp values between 1 × 10−3 and 3 × 10−3 unit mol/l; this phenomenon can be explained by the maximum counterion interaction owing to the capacity of the polyvalent counterion to bind two charged groups by intra- or interchain bridges. The investigation of the influence of the polycation structure on the counterion binding indicated an increase in charged group–counterion interactions with a decrease in the nonpolar chain length and an increase in the quaternary ammonium salt group content (charge density) in the chain. The polyelectrolyte with tertiary amine groups in the chain, PEGA, showed, on one hand, a cation adsorption order as K+>Na+>Li+ and, on the other hand, a stronger association between ions and PEGA chains in methanol than in water owing to the poorer solvating effect of methanol on the cations.

Phase transitions in 2:1 and 3:1 hard-core model electrolytes.

Critical temperatures, T(c), densities, rho(c), and coexistence curves for 2:1 and 3:1 hard-core model electrolytes have been found by fine-discretization Monte Carlo simulation. The size ratio of + and - ions strongly affects T(c) and rho(c); the trends contradict most current theories. Large multivalent ions screened by small monovalent counterions exhibit normal gas-liquid transitions of direct relevance to phase separation in charge-stabilized colloids. Conversely, extrapolation suggests the absence of such transitions for sufficiently small multivalent ions with large monovalent counterions.

Enzymatically and chemically de-esterified lime pectins: characterisation, polyelectrolyte behaviour and calcium binding properties

A series of pectins with different levels and patterns of methyl esterification was produced by treatment of a very highly methylated pectin with acid, alkali, plant pectin methyl esterase and fungus pectin methyl esterase. The intrinsic p K values, as well as the free fractions of monovalent and calcium counterions, were determined on pectin salt-free solutions. The variations of p K a versus the ionisation degree were found to depend on the de-esterification process but a unique value of 2.90±0.15 was estimated for the intrinsic p K value. Calcium binding properties of chemically and enzymatically de-esterified pectins were investigated and experimental results were compared to Manning's theoretical values. A progressive dimerisation process for pectins with a blockwise distribution of carboxyl groups in the presence of calcium ions is hypothesised.

Influence of the counter-ion on the effective charge of polyacrylic acid in dilute condition

Osmotic pressure measurements were satisfactorily used to compute the effective charge of poly(acrylic) acid (PAA) at different pHs and with different monovalent and divalent counterions. This experimental method is sensitive to the osmotically active species (polymer and counterions). After correction for the polymer contribution to the total osmotic pressure (solvency and excluded volume), the remaining pressure can be attributed to the polyelectrolyte counterions and processed with Donnan and equation of state in order to compute the effective charge number per polymeric chain ( Z eff). The behavior of Z eff against the chain concentration and pH was investigated after neutralization of PAA with LiOH, NaOH, and TMAOH. The results clearly indicate that the nature of the monovalent counterion has no effect on Z eff leading to the conclusion that the interaction between monovalent counterions and the acrylate functionality is purely electrostatic in agreement with conductimetric and potentiometric results reported in the literature. The behavior of Z eff against the degree of ionization of the polymer and its concentration is also in good agreement with the theoretical expectations of the theory of ionic condensation. Osmotic measurements were also used in order to understand the influence of divalent cations (Mg 2+, Ca 2+, Ba 2+) on Z eff of the sodium salt of PAA at pH 9 and at different divalent/acrylate molar ratio. All the divalent cations depress Z eff each of one at different degrees, confirming a specific divalent/polymer interaction. The energy of hydration of cations can explain most of the observed results with divalents.

From Powders to Dispersions in Water: Effect of Adsorbed Molecules on the Redispersion of Alumina Particles

Aqueous dispersions of colloidal alumina particles have been submitted to processes where the dispersion is aggregated, dried, and then redispersed in water. This cycle was performed with alumina particles that had different surface states: bare surfaces with monovalent counterions; surfaces that have been covered with small molecule ligands and have their surface charge reversed. In all cases, redispersion was produced by an increase in the ionic pressure due to the counterions, after immersion of the powder in water at the appropriate pH. In all cases, the pH that produced the appropriate ionic pressure was 5−6 pH units away from the isoelectric point of the surfaces. This was related to the fact that the surfaces were, in all cases, separated by a thin water film, which had a constant thickness (8 Å) set by the strength of binding of water to the alumina surfaces. However, for surfaces that had been covered with high amounts of macromolecular ligands, the minimum separation of surfaces was increased, and redispersion could be obtained at a pH which was closer to the isoelectric point. These phenomena can be accounted for, quantitatively, through a calculation of the balance of surface forces, including van der Waals attractions, hydration forces, and ionic pressures.

Salting-In and Salting-Out of Hydrophobic Solutes in Aqueous Salt Solutions

We present results on the thermodynamic and structural aspects of the hydration of hydrophobic solutes in three tetramethylammonium [N(CH3)4+] salt solutions at various concentrations obtained from molecular dynamics simulations. Monovalent counterions of different sizesF-, Cl-, and a relatively large model ion BI-are chosen in order to cover a range of kosmotropic to chaotropic behaviors. Chemical potentials of hard-sphere solutes obtained using test particle insertions display both salting-in and salting-out effects depending on the type of salt. Water and salt-ion densities in the vicinity of hard-sphere solutes are calculated. Small and strongly hydrated F- ions (kosmotropes) are excluded from the vicinity of hydrophobic solutes, leading to an increase in local water densities near hydrophobic solutes (i.e., preferential hydration). This increases the excess chemical potential of hydrophobic solutes in solution which leads to salting-out. Opposite behavior is observed for large, less favorably hydrate...

Structure of a 3-component polyelectrolyte solution model with dimerizing counterions and coions

The effect of dimerization between counterious and/or coions on the structure of the three-component primitive model of polyelectrolyte solution is studied using the integral equation technique for associating fluids. The three-component system contains large and highly charged macroions, intermediate size and charge coions and small monovalent counterions. In addition to the Coulomb forces, the counterions and coions also interact via short-range directional attractive forces which cause the formation of the dimers. It is demonstrated that dimerization effects are primarily of electrostatic origin and can be seen at lower concentrations. For the higher concentration regime, dimerization of the coions does not appreciably change correlation between the particles. In particular, the depletion interaction between the macroions, which is of packing origin, is not effected by the dimerization of the coions. The effect of the macroionic charge on the dimerization of the counterions is also studied. It is shown, that the increase of the macroionic charge causes the dimerization process to be more intensive.