Bulk Water Solution Research Articles

Structure and dynamics of water and aqueous solutions: The role of flexibility

The role molecular flexibility plays in the chemistry of bulk water and ionic solutions is evaluated using molecular dynamics simulations. For simplicity the flexible water model used here is the revised central force (CF) model of Stillinger and Rahman [J. Chem. Phys. 68, 666 (1977)]. A companion, rigid central force (RCF) model is invented to provide the most accurate average description of CF water. Discussion of the role of flexibility is divided into effects due to Coulombic and non-Coulombic parts of the potential energy. The Coulombic part provides the connection between flexibility and nuclear polarizability. A variety of thermodynamic and dynamical properties of the two models are compared. These include the static dielectric constant, orientational correlation functions, self-diffusion coefficient, pressure, intermolecular energy, and pair correlation functions. The effect of flexibility is greatest for dielectric properties, but in general it is found to be small. The dielectric constant of CF water is measured to be approximately 77, in good—perhaps fortuitous—agreement with the experimental value. A variety of ionic solution properties, including the free energy and dynamics of ionic association, are reported for model sodium chloride solutions at two concentrations. For these quantities also the effect of water flexibility is found to be small.

An application of the Poisson-Boltzmann equation inside a spherical cavity: Calculus of the equilibrium radius of reverse micelles in the presence of a saline solution

The equilibrium radius of reverse micelles for a microemulsion in equilibrium with a saline water solution is calculated by calculating the Gibbs energy change relative to the uptake of water and to the exchange of ions between the two phases. A major role is played by electrostatic effects. In order to take these effect into consideration, the Poisson-Boltzmann equation relative to the inside of the micelle has been considered. The solution is found in the form of a convergent series. The electroneutrality, the mass balance of the species and the thermodynamic equilibrium allow the problem to be resolved unambiguously. Once the potential profile inside the micelle is known, the electrostatic energy and entropy can easily be calculated. The entropic terms corresponding to ideal mixing of the species inside the micelle and in the bulk water solution, and to mixing of the micelles are also considered. The Gibbs energy is varied as a function of the radius and an equilibrium radius is found that corresponds to the minimum value of the Gibbs energy. The calculated values show good agreement with the experimental ones deduced from the water content of the microemulsion.

Isoelectric focusing in immobilized pH gradients in the pH 10-11 range.

With the synthesis of a new, strongly basic Immobiline (pK 10.3 at 10 degrees C) it has been possible to formulate a new pH 10-11 recipe for focusing very alkaline proteins, not amenable to fractionation with conventional isoelectric focusing in carrier ampholyte buffers. In this formulation, water is added as an acidic Immobiline having pK = 14 and a unit molar concentration (or with a pK = 15.74 and standard 55.56 molarity) since around pH 11 its buffering power becomes significant. The gel contains a 'conductivity quencher', i.e. a density gradient incorporated in the matrix, with the dense region located on the cathodic side (pH 11) for (a) smoothing the voltage gradient on the separation cell and (b) reducing the anodic electrosmotic flow due to the net positive charge acquired by the matrix at pH 11 (1 mM excess protonated amino groups to act as counterions to the 1 mm OH- groups in the bulk water solution generated by the local value of pH 11). Excellent focusing is obtained for such alkaline proteins as lysozyme (pI 10.55), So-6 (a leaf protein, pI 10.49), cytochrome c (pI 10.45) and ribonuclease (pI 10.12).

Electron spin resonance of the Mn2+ion in the study of the mobility of water adsorbed on silica gel and γ-alumina

The electron spin relaxation of Mn2+ in supercooled water adsorbed on silica gel and γ-alumina has been investigated. The correlation time for the modulation of the zero-field splitting tensor has been evaluated from the temperature and frequency dependence of the e.s.r. linewidth: τc= 1.4 ± 0.3 × 10–12 s at 25 °C has been extrapolated for bulk water solution. In the capillary adsorbed water, lineshape analysis suggests the existence of a distribution of crystal-field sites. The rate of interchange among differently relaxing sites is < 109 s–1. This suggests a long-range solid-water interaction which slows the mobility of water molecules up to 15–20 Å from the surface.

Effects of additives on the practical electrolytic separation of H and D isotopes. I. Pt and Fe

Electrolytic upgrading of D2O-enriched water is used for production of 99.8% D2O for CANDU-type nuclear reactor operation. Considerably enhanced values of the electrolytic H/D separation factorS can be achieved by employing additives such as urea or guanidine in the electrolyte and the increasedS values are consequently of practical significance for the overall operating economy of D2O cooled reactors. The effects of additions of urea, guanidine and cyanoguanidine to aqueous H2SO4 and KOH solutions on the hydrogen/deuterium separation factorS at Pt and Fe electrodes have been investigated. The adsorption of urea and guanidine was studied at the platinum electrode by means of H-electrosorption measurements using cyclic voltammetry. The additives, especially urea, enhanceS and values near the theoretically expected maximum for a free-proton transition state are found at Fe. The following possibilities for the origin of the effects of urea (or guanidine) are considered: (1) Urea causes a kinetically significant change of orientation and structure of water in the double-layer; (2) urea acts as an alternative proton source to H2O in the double-layer on account of its adsorption; (3) the adsorbed urea causes a local equilibrium enrichment of H w.r.t. D in comparison with the H/D ratio in the bulk water solution and hence gives rise to a change inS and (4) the OH-bonding frequencies in the H2O proton-transfer transition state are changed in the double-layer due to H-bonding with adsorbed urea or guanidine.