Special features of ion association in concentrated LiCl aqueous solutions at various temperature

The influence of high (up to 623 K) and low (up to 138 K) temperatures on ion association in 18.5–3.7 molar aqueous LiCl solutions was studied. It was found that the ion association in concentrated and highly concentrated solutions takes different directions as temperature increases (298–623). The interrelation between thermal structure changes and types of interparticle interactions was considered. The concentration region of the appearance of melt-like state of the systems under study was found (9.25–13.88 mol kg^?1). Under the vitrification conditions (143–138 K) in 11.1 molal solution hydrate-separated ion pairs are the main structure units. It is probable that an ordering at medium distances typical for water-electrolyte glasses is formed on the basis of this units in the temperature range of 174–138 K.     

The structure around the perchlorate ion in concentrated aqueous solutions

A neutron diffraction study has been carried out on a 3.25 modal solution of NaClO₄ in heavy water, in which the first-order difference method of isotopic substitution was applied to chlorine atoms of the ClO₄⁻ ion. The results show that (a) the intramolecular separation of Cl-O is 1.43 ± 0.02 Å in good agreement with X-ray diffraction results from single-crystal studies. (b) there are between four and five nearest-neighbour deuterium atoms in the range 2.4 ⩽ r ⩽ 3.2 Å. and (c) the ClO₄⁻-water coordination is less well defined than that for Cl⁻-water, but similar to that for the NO₃⁻-water structure.     

Single ion activity coefficients in concentrated aqueous hydrogen chloride solutions

The adsorption behaviour of the steroids: ouabagenin, ouabain, proscillaridin, digoxin, and k-strophanthine was studied by a.c. polarography and capacitance—time curves at the dropping mercury electrode. The steroids behave like strongly surface-active substances: the relative capacitance decrease ΔC/C₀ depends linearly both on the concentration of the solution and on the root of the drop age. The surface area determined for ouabagenin to be 125 Ų on the basis of adsorption kinetics corresponds roughly to the maximum crosssection of the molecule. With glycosides, the surface area increases with the number of monosaccharide molecules being attached in position 3 of the steroid. It was inferred, therefore, that the adsorbed molecule rests with its steroid and sugar moiety flat on the electrode. A capacitance minimum with sharply defined edges (pit) occurs in the a.c. polarograms of ouabain. In this potential area, the dependences Δ/₀=f(c) and ΔC/C₀=f(t) show a step-like course with two plateaus, which is discussed to be due to association of the adsorbed molecules.     

Mechanism of gold metal ion reduction, nanoparticle growth and size control in aqueous amphiphilic block copolymer solutions at ambient conditions

Spontaneous formation and efficient stabilization of gold nanoparticles with an average diameter of 7 approximately 20 nm from hydrogen tetrachloroaureate(III) hydrate (HAuCl4.3H2O) were achieved in air-saturated aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer solutions at ambient temperature in the absence of any other reducing agent. The particle formation mechanism is considered here on the basis of the block copolymer concentration dependence of absorption spectra, the time dependence (kinetics) of AuCl4- reduction, and the block copolymer concentration dependence of particle size. The effects of block copolymer characteristics such as molecular weight (MW), PEO block length, PPO block length, and critical micelle concentration (cmc) are explored by examining several PEO-PPO-PEO block copolymers. Our observations suggest that the formation of gold nanoparticles from AuCl4- comprises three main steps: (1) reduction of metal ions by block copolymer in solution, (2) absorption of block copolymer on gold clusters and reduction of metal ions on the surface of these gold clusters, and (3) growth of metal particles stabilized by block copolymers. While both PEO and PPO blocks contribute to the AuCl4- reduction (step 1), the PEO contribution appears to be dominant. In step 2, the adsorption of block copolymers on the surface of gold clusters takes place because of the amphiphilic character of the block copolymer (hydrophobicity of PPO). The much higher efficiency of particle formation attained in the PEO-PPO-PEO block copolymer systems as compared to PEO homopolymer systems can be attributed to the adsorption and growth processes (steps 2 and 3) facilitated by the block copolymers. The size of the gold nanoparticles produced is dictated by the above mechanism; the size increases with increasing reaction activity induced by the block copolymer overall molecular weight and is limited by adsorption due to the amphiphilic character of the block copolymers.     

Protonation and sodium ion-pairing of the sulfite ion in concentrated aqueous electrolyte solutions

A protocol has been developed for the reliable titration of aqueous sulfite solutions which minimizes photodecomposition effects. This procedure has been used to measure the protonation constants of the sulfite ion in aqueous solution by glass electrode potentiometry at 25.0‡C and ionic strengths (I) from 0.1 to 5.0M in NaCI media and atI = 1.0M in KC1 and Me₄NCl media. These measurements provided evidence of weak but significant ionpairing between SO2/3 -and Na⁺ with a formation constant of logK _Na = 0.431 in 1.0M Me₄NCl. This was in very good agreement with the value logK _Na = 0.410 measured directly by Na⁺ ion-selective electrode potentiometry. Evidence is also presented for an extremely weak association of K⁺ and SO ₂ ³ -with logK _k = 0.22 in 1.0M Me₄NCl.     

Storage of aqueous solutions of selenium for speciation at trace level

 Different studies carried out in recent years on the preservation of trace selenium species in aqueous solutions are reviewed and experimental results showing a 29% oxidation of Se(IV) to Se(VI) in less than one month in acidic and oxygenated medium in the presence of chloride ions are presented. The hypothesis of an oxidation of Se(IV) to Se(VI) at acidic pH by traces of Cl₂, obtained by a reaction between chloride ions and dissolved oxygen, followed by chlorine oxidation, is examined. Received: 3 May 1996/Revised: 24 June 1996/Accepted: 1 July 1996     

Heat capacities of concentrated multicomponent aqueous electrolyte solutions at various temperatures

The specific heat capacities of the aqueous multicomponent system NaCl +KCl+MgCl₂+CaCl₂ with ionic strength between 8.3 and 9.6 (resembling Dead Sea waters) were measured between 15°C and 45°C. The obtained data were fitted to an empirical equation as a function of concentration and temperature. The thermodynamic functions of the studied multicomponent system were found to be strongly influenced by changes in MgCl₂ concentrations. The application of Young's rule to such concentrated systems was checked at 25°C. The calculated (by Young's rule) specific heat capacitiesC _p and apparent molar heat capacities C_p, of these multicomponent electrolyte solutions were in reasonable agreement with the measured values (–0.008 J-g^–1-K^–1 and –2.6 J-mol^–1-K^–1, respectively).     

Component dynamics in polyvinylpyrrolidone concentrated aqueous solutions

(2)H-nuclear magnetic resonance (NMR) and neutron scattering (NS) on isotopically labelled samples have been combined to investigate the structure and dynamics of polyvinylpyrrolidone (PVP) aqueous solutions (4 water molecules/monomeric unit). Neutron diffraction evidences the nanosegregation of polymer main-chains and water molecules leading to the presence of water clusters. NMR reveals the same characteristic times and spectral shape as those of the slower process observed by broadband dielectric spectroscopy in this system [S. Cerveny et al., J. Chem. Phys. 128, 044901 (2008)]. The temperature dependence of such relaxation time crosses over from a cooperative-like behavior at high temperatures to an Arrhenius behavior at lower temperatures. Below the crossover, NMR features the spectral shape as due to a symmetric distribution of relaxation times and the underlying motions as isotropic. NS results on the structural relaxation of both components-isolated via H/D labeling-show (i) anomalously stretched and non-Gaussian functional forms of the intermediate scattering functions and (ii) a strong dynamic asymmetry between the components that increases with decreasing temperature. Strong heterogeneities associated to the nanosegregated structure and the dynamic asymmetry are invoked to explain the observed anomalies. On the other hand, at short times the atomic displacements are strongly coupled for PVP and water, presumably due to H-bond formation and densification of the sample upon hydration.     

Ozone solubility in concentrated aqueous solutions of salts

The effect exerted by the concentration of salts (NaCl, Na₂SO₄, and NaNO₃) on the ozone solubility in aqueous solution at 20, 30, and 40°C was studied. The solubility coefficients of ozone were calculated. The Henry constants and the Sechenov coefficients were determined.     

Ozone decomposition in concentrated aqueous solutions of salts

Effect of high concentrations of electrolytes (Na₂SO₄, NaCl, and NaNO₃) on the rate of ozone decomposition in water was studied. The conversion kinetics of O₃ dissolved in these solutions was analyzed. The rate constants of ozone decomposition were determined.     

Ammonium ion coordination in concentrated aqueous ammonium chloride

Results of a neutron-diffraction study on 5.0 molal ammonium chloride in heavy water were used to determine the ND⁺₄ water conformation. There are between 10 and 11 water molecules coordinated to the ammonium ion in the range 2.60–4.35 Å. The coordination is not well defined and although in broad agreement with a molecular dynamics study, there are significant differences.     

Hydrogen evolution at mercury from acid concentrated aqueous solutions of indifferent salts

The possible reasons of the hydrogen overvoltage decrease at high supporting electrolyte concentrations have been considered. It follows from a comparative study of the kinetic isotope effect and overvoltage shifts that the overvoltage decrease cannot be ascribed to the Ψ₁ effect caused by the specific anion adsorption. The overvoltage decrease is associated with the discharge activation energy. It is suggested that in a high-concentration solution the hydroxonium ion is actually located in a different medium, which corresponds to a lower reorganization energy.     

Computer simulation of cluster assembling

Density functional theory is used to investigate the assembling of metallic clusters to yield stable or metastable cluster solids. Motivated by the observed high stability of the Al₁₃H cluster, which has a substantial highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO) gap, we have modeled the assembling of those clusters. For a favorable relative orientation of each cluster with respect to all its neighbors, a cluster solid is predicted and its structure appears to be stable at least up to 150 K, which is the highest temperature in our simulations. We have also studied the chemical bonding in the stoichiometric solid alloys PbA, where A is one of the alkali elements Na, K, Rb, or Cs. Those crystals exist in an ordered phase formed by tetrahedral Pb₄ clusters surrounded by the alkali atoms. The study of this family of natural cluster compounds reveals the coating role played by the cations, providing further insight into the favorable conditions required for the formation of cluster solids. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Solvation of sodium-chloride ion pair in water cluster at atmospheric conditions: grand canonical ensemble Monte Carlo simulation

Open statistical ensemble simulations are used to study the mechanism of nucleation of atmospheric water on sodium-chloride ion pair in a wide range of temperature and relative humidity values. The extended simple point-charge model is used for water molecules. Ions-water nonadditive interactions are taken into account by introducing the mutual polarization of ions and water in the field of each other. Gibbs free-energy variations are calculated from Na+-Cl- pair-correlation function and used as a criterion for determining the possible stable states of the cluster. In this relation, it was found that the dissociation of ion pairs in water clusters occurs even at vapor pressures of only a few millibars. In the conditions under consideration solvent-separated ion-pair states are found to be more probable than contact ion-pair configurations. The susceptibilities of water and ions are found to play an essential role in the stabilization of ions at large separations. The structure of ion-induced clusters is analyzed in terms of binary correlation functions. The non-pair interactions influence essentially the structure of ion solvation shells. The results of simulation show that the separation of the charges in water clusters containing simple ions can take place under atmospheric conditions.     

Transference numbers of sodium chloride in concentrated aqueous solutions and chloride conductances in several concentrated electrolyte solutions

Stable chlorine electrodes with low bias potentials have been developed by introducing 25% Ir+75% Pt electrodes and an improved gas line. With their use in cells with transference, cation constituent transference numbers have been measured at 25°C in NaCl solutions from 1.7 to 6 modal. These results agree well with four other sets of data in the literature but disagree with two further sets based on emf determinations with Ag/AgCl electrodes. A table of best NaCl transference numbers is proposed. The conductances of the chloride ion-constituent in concentrated NaCl, KCl, and HCl solutions are compared.     

Hydroxide ion hydration in aqueous solutions

Hydroxide ion hydration was studied in aqueous solutions of selected alkali metal hydroxides by means of Fourier transform infrared (FTIR) spectroscopy of HDO isotopically diluted in H2O. The quantitative difference spectra procedure was applied for the first time to investigate such systems. It allowed removal of bulk water contribution and separation of the spectra of solute-affected HDO. The obtained spectral data were confronted with ab initio calculated structures of small gas-phase and polarizable continuum solvation model (PCM) solvated aqueous clusters, OH-(H2O)n, n = 1-7, to establish the structural and energetic states of hydration spheres of the hydrated hydroxide anion. This was achieved by comparison of the calculated optimal geometries with the interatomic distances derived from HDO band positions. The energetic state of water in OH- hydration shells, as revealed by solute-affected HDO spectra, is similar to that of an isoelectronic F- anion. No evidence was found for the existence of stable hydroxide dimer, H3O2-, in an aqueous solution. Spectral data do confirm, however, existence of a weak interaction with a single water molecule at the hydrogen site of OH-.     

Structure of concentrated aqueous solutions of scandium chloride

It is shown via X-ray diffraction that aqueous solutions of scandium chloride form ionic associates in a wide range of concentrations. It is established that the Sc³⁺ ion coordination number increases upon dilution to 8.2 at an unchanged Sc³⁺–OH² distance of 0.215 nm. The second coordination sphere of the cation forms at an average distance of 0.420 nm. The number of solvent molecules in the sphere logically increases during dilution. It is concluded that the anion does not form its own sphere in highly concentrated solutions. This coordination sphere begins to form only in solutions with moderate concentrations at a distance of 0.315 nm, and it contains six water molecules in diluted solutions.     

Literature survey of on-line elemental speciation in aqueous solutions

The literature about the on-line speciation in water has been comprehensively studied. Critical examination of this subject reveals that a great deal of work has been performed in this area for inorganic metal species and some organic compounds. The topic studied mainly includes the speciation of chromium, selenium, copper, arsenic, lead, cadmium, mercury, iron, aluminium, nickel, tin, antimony, phosphorus, nitrogen and others. The present literature survey includes also comments about the possibilities and problems of speciation as a function of analytical steps, general strategies involved and methods proposed in the literature for each element or a series of elements.     

The surface structure of concentrated aqueous salt solutions

The surface-normal electron density profile rhos(z) of concentrated aqueous salt solutions of RbBr, CsCl, LiBr, RbCl, and SrCl2 was determined by x-ray reflectivity (XR). For all but RbBr and SrCl2 rhos(z) increases monotonically with depth z from rhos(z)=0 in the vapor (z<0) to rhos(z)=rhob of the bulk (z>0) over a width of a few angstroms. The width is commensurate with the expected interface broadening by thermally excited capillary waves. Anomalous (resonant) XR of RbBr reveals a depletion at the surface of Br- ions to a depth of approximately 10 A. For SrCl2, the observed rhos(z)>rhob may imply a similar surface depletion of Cl- ions to a depth of a few angstorms. However, as the deviations of the XRs of RbBr and SrCl2 from those of the other solutions are small, the evidence for a different ion composition in the surface and the bulk is not strongly conclusive. Overall, these results contrast earlier theoretical and simulational results and nonstructural measurements, where significant surface layering of alternate, oppositely charged, ions is concluded.     

Hydrogen bond structure and dynamics in aqueous electrolytes at ambient and supercritical conditions

Hydrogen bond (HB) connectivity in aqueous electrolyte solutions at ambient and supercritical conditions has been investigated by molecular dynamics techniques. Alkali metal and halides with different sizes have been considered. Modifications in the water HB architecture are more noticeable in the first ionic solvation shells and do not persist beyond the second shells. The coordination pattern is established between partners located in the first and second solvation shells. High-temperature results show dramatic reductions in the coordination number of water; at liquidlike densities the number of HBs is close to 2, while in steamlike environments water monomers are predominant. The addition of ions does not bring important modifications in the original HB structure for pure water. From the dynamical side, the lifetime of HBs shows minor modifications due to the simultaneous competing effects from a weaker HB structure combined with a slower reorientational dynamics of water induced by the Coulomb coupling with solute. At supercritical conditions, the overall dynamics of HB is roughly 1 order of magnitude faster than that at ambient conditions, regardless of the particular density considered.