Bridging like-charged macroions through long divalent rodlike ions

Like-charged macroions in aqueous electrolyte solution can attract each other because of the presence of inter- and/or intramolecular correlations. Poisson-Boltzmann theory is able to predict attractive interactions if the spatially extended structure (which reflects the presence of intramolecular correlations) of the mobile ions in the electrolyte is accounted for. We demonstrate this for the case of divalent, mobile ions where each ion consists of two individual charges separated by a fixed distance. Variational theory applied to this symmetric 2:2 electrolyte of rodlike ions leads to an integro-differential equation, valid for arbitrary rod length. Numerical solutions reveal the existence of a critical rod length above which electrostatic attraction starts to emerge. This electrostatic attraction is distinct from nonelectrostatic depletion forces. Analysis of the orientational distribution functions suggests a bridging mechanism of the rodlike ions to hold the two macroions together. For sufficiently large rod length, we also observe "overcharging", that is, an over-compensation of the macroion charges by the diffuse layer of mobile rodlike ions. Our results emphasize the importance of the often rodlike internal structure that condensing agents such as polyamines, peptides, or polymer segments exhibit. The results were compared with Monte Carlo simulations.     

Structuring of macroions confined between like-charged surfaces

We investigate the structuring of charged spherical nanoparticles and micelles (i.e., "macroions") between two surfaces as a function of bulk macroion concentration. Structuring is deduced from measured force profiles between a silica particle and a silica plate in the presence of an aqueous macroion (Ludox silica nanoparticle or sodium dodecyl sulfate micelle) solution, obtained with an atomic force microscope. We observe oscillatory force profiles that decay with separation. We find that the wavelength of the force profiles scales with the bulk number density as rho(-)(1/3), rather than with the effective macroion size. Only at very high silica nanoparticle concentration (above 10 vol %) in a low ionic strength solution does the wavelength become smaller than that predicted by the simple rho(-)(1/3) scaling; however, the original scaling is recovered upon the addition of a small amount of electrolyte. A comparison between the measured wavelength and the predicted spacing between the macroions in the bulk shows that the two variables differ in both magnitude and bulk density scaling. This finding suggests that confined macroions are more ordered than those in the bulk and the nature of this ordering is maintained over a relatively wide range of bulk concentration.     

Complex formation of N-donor ligands with group 11 monovalent ions

Thermodynamic data on complex formation between nitrogen donor ligands (amines, pyridines) and group 11 monovalent ions in water and non-aqueous media are reviewed here. Particular emphasis is paid to Ag(I) complex formation in water and dimethylsulfoxide (DMSO), due to the amount and quality of data available. The influence of different basicities and steric properties of ligands, together with the solvation of the species involved, on the stability and nature of the resulting complexes is discussed. It emerges generally that the coordination properties of amines towards 1+ ions are all modulated through the number and basicity of nitrogen atoms present in the ligand, chelate ring sizes, degree of N-functionalisation, and the nature of the solvent. When possible, the thermodynamic properties of the complexes are related to the structural features of the ligands.     

Perturbation of water structure due to monovalent ions in solution

The ion induced modification to the tetrahedral structure of water is a topic of much current interest. We address this question by interpreting neutron diffraction data from monovalent ionic solutions of NaCl and KCl using a computer assisted structural modeling technique. We investigate the effect that these ions have on the water-water O-O, O-H and H-H radial distribution functions as a function of ionic concentration. It is found that the O-H and H-H functions are only marginally affected by ionic composition, signaling that hydrogen bonding between water molecules remains largely intact, even at the highest concentrations. On the other hand the O-O functions are strongly modified by the ions. In particular the position of the second peak in g(OO)(r), is found to move inwards with increasing salt concentration, in a manner closely analogous to what happens in pure water under pressure. Furthermore by recalculating g(OO)(r) after excluding all the water molecules in the first hydration shell of each ion, we show that this structural perturbation exists outside the first hydration shell of the ions.     

Electrostatic interactions in phospholipid membranes I: Influence of monovalent ions

Electrostatic interactions in monolayers and vesicles of acidic phospholipids are studied by thermodynamical and optical techniques in conjunction with numerical calculations. A nonmonotonic ionic strength dependence with an extremum at 0.1 M (NaCl) is observed for the phase transition temperature of vesicles as well as for the surface pressure of monolayers at low molecular density. This finding is in accordance with the calculations predicting the dominance of charge screening by monovalent counterions only for concentrations above 0.1 M. For lower salt content, however, its increase causes an elevation of the degree of dissociation and thus also electrostatic repulsion. This leads to a higher surface pressure, a lower transition temperature and a smaller size of solid domains observed in the liquid/solid coexistence range of monolayers. This supports the previously published idea, that finite size and repulsion of the domains arise from a different surface charge density in fluid and solid lipid phases.Abbreviations DLPA L--dilauryl-phosphatidic-acid - DMPA L--dimyristoyl-phosphatidic-acid - EDTA Ethylenediamine-tetraacetic acid (sodium salt) - DP-NBD-PE L--dipalmitoyl-nitrobenzoxadizol-phosphatidylethanolamine     

Interaction forces between colloidal particles in a solution of like-charged, adsorbing nanoparticles

We have measured the force between a weakly charged micron-sized colloidal particle and flat substrate in the presence of highly charged nanoparticles of the same sign under solution conditions such that the nanoparticles physically adsorb to the colloidal particle and substrate. The objective was to investigate the net effect on the force profile between the microparticle and flat substrate arising from both nanoparticle adsorption and nanoparticles in solution. The experiments used colloidal probe atomic force microscopy (CP-AFM) to measure the force profile between a relatively large (5 μm) colloidal probe glass particle and a planar glass substrate in aqueous solutions at varying concentrations of spherical nanoparticles. At very low nanoparticle concentrations, the primary effect was an increase in the electrostatic repulsion between the surfaces due to adsorption of the more highly charged nanoparticles. As the nanoparticle concentration is increased, a depletion attraction formed, followed by longer-range structural forces at the highest nanoparticle concentrations studied. These results suggest that, depending on their concentration, such nanoparticles can either stabilize a dispersion of weakly-charged colloidal particles or induce flocculation. This behavior is qualitatively different from that in nonadsorbing systems, where the initial effect is the development of an attractive depletion force.     

Van der Waals Attraction between Spherical Particles

A new and single correction function which could be used with the London equation to account for the retardation effect on van der Waals attraction, was recently proposed. The function is simple enough to permit closed-form solutions to be obtained by the Hamaker–De Boer approach for macrobodies of a few different shapes. In this paper, closed-form solutions are presented for sphere–half space and sphere–sphere systems. The results are shown to agree well with those which employ Overbeek correction functions.     

Calcium mediated polyelectrolyte adsorption on like-charged surfaces

Monte Carlo simulations within the primitive model of calcium-mediated adsorption of linear and comb polyelectrolytes onto like-charged surfaces are described, focusing on the effect of calcium and polyion concentrations as well as on the ion pairing between polymers and calcium ions. We use a combination of Monte Carlo simulations and experimental data from titration and calcium binding to quantify the ion pairing. The polymer adsorption is shown to occur as a result of surface overcharging by Ca(2+) and ion pairing between charged monomers and Ca(2+). In agreement with experimental observations, the simulations predict that the polymer adsorption isotherm goes through a maximum as the calcium or the polymer concentration is increased. The non-Langmuir isotherms are rationalized in terms of charge-charge correlations.     

Separation of sodium ions from trivalent chromium by electrodialysis using monovalent cation selective membranes

Trivalent chromium Cr(III) in wastewaters produced by leather tanning processes must be treated before discharge in the environment. Electrodialysis was studied for this application. Cr(III) ion is separated from sodium ion by using modified cation-exchange membranes. The membrane modification consists of a polyethylenimine layer electrodeposited on the membrane surface. This layer is positively charged in acidic media and repels multivalent ions while monovalent ions cross the membrane. The modified membrane in this study was a Nafion^® 324 membrane. The transfer of chromium, sodium, calcium, magnesium, chloride and sulphate ions from a mixture was investigated. The pH must be regulated in order to avoid chromium hydroxide precipitation in the dilute chamber. The behaviour of sulphate chloride system is unusual for the AMX membrane. Adsorption of PEI on the membrane surface is assumed to explain this behaviour. The overall current efficiency was close to 96–98% for cations and anions.     

Effect of supporting electrolyte on the activation energy for diffusion of some monovalent ions

The effect of some alkali metal bromides, iodides and sulphates on the diffusion of bromide, iodide and thallium ions, respectively, is studied at various temperatures. The activation energy required for the process of diffusion of these three ions in different supporting electrolytes have been calculated. It is found that activation energy for a given ion decreases in the reverse order of the charge density of alkali metal ions of the supporting electrolyte. This observed trend in activation energy is explained qualitatively by considering the distortion in the water structure caused by these ions and agar molecules.     

Entropic effects in the electric double layer of model colloids with size-asymmetric monovalent ions

The structure of the electric double layer of charged nanoparticles and colloids in monovalent salts is crucial to determine their thermodynamics, solubility, and polyion adsorption. In this work, we explore the double layer structure and the possibility of charge reversal in relation to the size of both counterions and coions. We examine systems with various size-ratios between counterions and coions (ion size asymmetries) as well as different total ion volume fractions. Using Monte Carlo simulations and integral equations of a primitive-model electric double layer, we determine the highest charge neutralization and electrostatic screening near the electrified surface. Specifically, for two binary monovalent electrolytes with the same counterion properties but differing only in the coion's size surrounding a charged nanoparticle, the one with largest coion size is found to have the largest charge neutralization and screening. That is, in size-asymmetric double layers with a given counterion's size the excluded volume of the coions dictates the adsorption of the ionic charge close to the colloidal surface for monovalent salts. Furthermore, we demonstrate that charge reversal can occur at low surface charge densities, given a large enough total ion concentration, for systems of monovalent salts in a wide range of ion size asymmetries. In addition, we find a non-monotonic behavior for the corresponding maximum charge reversal, as a function of the colloidal bare charge. We also find that the reversal effect disappears for binary salts with large-size counterions and small-size coions at high surface charge densities. Lastly, we observe a good agreement between results from both Monte Carlo simulations and the integral equation theory across different colloidal charge densities and 1:1-electrolytes with different ion sizes.     

Differences on the conversion of celestite in solutions bearing monovalent ions under hydrothermal conditions

The replacement of SO₄²⁻ ions by monovalent ions in mineral SrSO₄ crystals was investigated under hydrothermal conditions by using aqueous solutions bearing F⁻ and OH⁻ ions. Experiments were conducted at various temperatures (150-250 °C) for different reaction intervals (1-96 h), with M⁻/SO₄²⁻ molar ratios of 1, 5 and 10, where M⁻=F⁻ or OH⁻. The celestite crystals were completely converted into SrF₂ crystals, at 200 °C using a F⁻/SO₄²⁻ molar ratio=5 for 24 h. The morphology of the converted SrF₂ crystals indicated that the heteroionic conversion proceeded by a pseudomorphic replacement process, because the transformed crystals maintained their original shape and dimensions. In contrast, the SrSO₄ crystals were instantaneously converted into the Sr(OH)₂ phase by a bulk dissolution-recrystallization mechanism, resulting in the formation of large transparent acicular Sr(OH)₂ crystals. The differences on the conversion process are mainly associated with the chemical interaction between the mineral crystal and the hydrothermal fluid. In addition, the chemical stability of the converted phase with low solubility is also essential for the heteroionic conversion to proceed by the pseudomorphic replacement process.     

Interaction of monovalent ions with hydrophobic and hydrophilic colloids: charge inversion and ionic specificity

Here we study experimentally and by simulations the interaction of monovalent organic and inorganic anions with hydrophobic and hydrophilic colloids. In the case of hydrophobic colloids, our experiments show that charge inversion is induced by chaotropic inorganic monovalent ions but it is not induced by kosmotropic inorganic anions. For organic anions, giant charge inversion is observed at very low electrolyte concentrations. In addition, charge inversion disappears for both organic and inorganic ions when turning to hydrophilic colloids. These results provide an experimental evidence for the hydrophobic effect as the driving force for both ion specific effects and charge inversion. In the case of organic anions, our molecular dynamics (MD) simulations with full atomic detail show explicitly how the large adsorption free energies found for hydrophobic colloids are transformed into large repulsive barriers for hydrophilic colloids. Simulations confirm that solvation free energy (and hence the hydrophobic effect) is responsible for the build up of a Stern layer of adsorbed ions and charge inversion in hydrophobic colloids and it is also the mechanism preventing charge inversion in hydrophilic colloids. Overall, our experimental and simulation results suggest that the interaction of monovalent ions with interfaces is dominated by solvation thermodynamics, that is, the chaotropic/kosmotropic character of ions and the hydrophobic/hydrophilic character of surfaces.     

Identification of two types of exchangeable sites for monovalent copper ions exchanged in MFI-type zeolite

Three different approaches have been used to characterize the state of exchanged copper ions in copper-ion-exchanged MFI (CuMFI) samples. (1) Two types of an ion-exchangeable site with different adsorption properties for N(2) or CO molecules were identified depending on the pre-treatment temperature (723 or 873 K) of a sample prepared by using an aqueous solution of CuCl(2). (2) The state of the active sites formed by the evacuation of a sample at 873 K that had been prepared using a mixture solution of aqueous NH(4)CH(3)COO and Cu(CH(3)COO)(2) was analysed utilizing both (13)C(18)O and (12)C(16)O to identify the two types of active adsorption sites for CO molecules. (3) CuMFI samples prepared by the ion-exchange method employing anhydrous CuCH(3)COO showed a surprising adsorption feature characterized by a single IR band occurring at 2159 cm(-1) due to the adsorbed CO molecules, but there was no corresponding IR band due to adsorbed N(2) molecules. A successful preparation of CuMFI, in which the monovalent copper ions exclusively occupied another one of the two types of ion-exchangeable sites, was also carried out utilizing the solid-ion exchange method using Cu(CH(3)COO)(2).H(2)O. This site exhibits an IR band occurring at 2151 cm(-1) for CO molecules and also acts as an active site for N(2) molecules. These experimental data correlate, and clearly indicate that there are at least two types of exchangeable sites for copper ions in MFI-type zeolites.     

Entropic attraction condenses like-charged interfaces composed of self-assembled molecules

Like-charged solid interfaces repel and separate from one another as much as possible. Charged interfaces composed of self-assembled charged-molecules such as lipids or proteins are ubiquitous. The present study shows that although charged lipid-membranes are sufficiently rigid, in order to swell as much as possible, they deviate markedly from the behavior of typical like-charged solids when diluted below a critical concentration (ca. 15 wt %). Unexpectedly, they swell into lamellar structures with spacing that is up to four times shorter than the layers should assume (if filling the entire available space). This process is reversible with respect to changing the lipid concentration. Additionally, the research shows that, although the repulsion between charged interfaces increases with temperature, like-charged membranes, remarkably, condense with increasing temperature. This effect is also shown to be reversible. Our findings hold for a wide range of conditions including varying membrane charge density, bending rigidity, salt concentration, and conditions of typical living systems. We attribute the limited swelling and condensation of the net repulsive interfaces to their self-assembled character. Unlike solids, membranes can rearrange to gain an effective entropic attraction, which increases with temperature and compensates for the work required for condensing the bilayers. Our findings provide new insight into the thermodynamics and self-organization of like-charged interfaces composed of self-assembled molecules such as charged biomaterials and supramolecular assemblies that are widely found in synthetic and natural constructs.     

Controlling the charge and organization of anionic lipid bilayers: effect of monovalent and divalent ions

It is shown that the organization of lipid bilayers containing phosphatidic acid (PA) and phosphatidlycholine (PC) can be controlled by altering the monovalent and divalent ion concentrations. At high pH and/or calcium concentration, 1:1 Ca(2+)-PA(2-) complexes form; these complexes demix, and PA-rich and PC-rich regions are observable with epifluorescence microscopy. The results are compared with predictions from electrostatic theory. It is noted that the complex formation correlates in a roughly linear fashion with the monovalent/divalent ion ratio, a parameter that cells adjust.     

Influence of monovalent ions on density fluctuations in hydrothermal aqueous solutions by small angle X-ray scattering

Synchrotron small angle X-ray scattering measurements on water and alkaline bromine aqueous solutions (XBr, with X = Li, Rb, or Cs) were carried out from ambient to supercritical conditions. The temperature was increased from 300 to 750 K along several isobars between 24 and 35 MPa. The correlation length and the structure factor were extracted from the data following the Ornstein-Zernike formalism. We obtained experimental evidence of the shift of the critical point and isochore and their dependence on the ions concentration (0.33 mol/kg and 1.0 mol/kg). We also observed that the size of the density fluctuations and the structure factor increase with the presence of the ions and that this effect is positively correlated with the atomic number of the cation. These behaviors were compared with ZnBr(2) and NaCl systems from the literature.     

Parameters of monovalent ions in the AMBER-99 forcefield: assessment of inaccuracies and proposed improvements

The monovalent ion parameters used by the AMBER-99 forcefield are shown to exhibit physically inaccurate behavior in molecular dynamics simulations of strong 1:1 electrolytes. These errors arise from an ad hoc adaptation of Aqvist's cation parameters. The result is the rapid formation of large, unphysical clusters at concentrations that are well below solubility limits. The observed unphysical behavior poses a serious challenge for simulating ions around highly charged polymers such as nucleic acids. In this communication, we explain the source of this unphysical behavior. To facilitate the continued use of the popular AMBER parameters, we prescribe a simple fix whereby Aqvist's cations and anions are used in conjunction with the AMBER forcefield for nucleic acids. A preliminary test of this strategy suggests that the proposed fix is reasonable and is likely to be generalizable for simulating diffuse and specific ion binding to nucleic acids.