James K. Beattie

The surface of neat water is basic

Theoretical studies which conclude that the surface of neat water is acidic (with a pH < or = 4.8), due to the preferential adsorption of hydronium ions, are contrary to the available experimental evidence. Air bubbles in water have a negative charge, as do hydrophobic oil drops in water, and streaming potential measurements on inert surfaces such as Teflon in water show a similar negative surface charge. In each case the pH dependence of the zeta potential has an isoelectric point between pH 2-4. An isoelectric point of pH 4 implies a preference for hydroxide over protons of 10(6), the opposite of what was inferred from the theoretical simulations. Water behaves similarly at all inert hydrophobic interfaces with the preferential adsorption of hydroxide ions to give a negatively charged surface at neutral pH. The surface-charge density at the oil/water interface in mM salt solutions is -5 to -7 microC cm(-2), which corresponds to one hydroxide ion on every 3 nm2 of the surface. The homogenisation of an inert oil such as hexadecane in water in the absence of any salt or base still leads to formation of an emulsion. The hydroxide adsorbed on the large surface area of the emulsion greatly exceeds that present at 10(-7) M in neutral water; it is created by the increased autolysis of water, driven by the strong adsorption of hydroxide ions at the oil/water interface. These surfactant-free, salt-free emulsions are stable for some hours, with protons as the only counterions to the negative hydroxide surface.

Strong Specific Hydroxide Ion Binding at the Pristine Oil/Water and Air/Water Interfaces

Despite claims, based largely on molecular dynamics simulations, that the surface of water at the air/water interface is acidic, with a positive charge, there is compelling experimental evidence that it is in fact basic, with a negative charge due to the specific adsorption of hydroxide ions. The oil/water interface behaves similarly. The pH dependence of the zeta potentials of oil drops has been measured by two very different techniques: on a single drop in a rotating electrophoresis cell and on about 10(14) submicrometer drops in a 2 vol % emulsion by an electroacoustic method to give similar results with a sigmoidal pH dependence characterized by an isoelectric point at pH 2-3 and a half adsorption point about pH 5.5, or at 10(-8.5) M hydroxide ion. This indicates that hydroxide ion is absorbed much more strongly than other anions. The pH dependence of a single N(2) bubble has also been measured and has the same pH dependence, independently of whether HCl or HI is used to adjust the pH. These similarities between the pH dependences of the zeta potentials of air bubbles and oil drops, as well as those reported from streaming potentials on solid inert surfaces such as Teflon, indicate that water behaves similarly, with only subtle differences, at each of these low dielectric hydrophobic surfaces, with an isoelectric point of pH 2-4. In acidic solutions at pHs below the isoelectric point, the surface is indeed positive, consistent with spectroscopic observations of the adsorption of hydrogen ions.

Modeling proton binding at the goethite (α-FeOOH)–water interface

The basic charging behaviours of goethite particles with different surfaces area (23, 37 and 85 m2 g-1) in 0.003-2.0 M NaNO3 were interpreted using surface complexation theory with the basic Stern ...

Dynamics of spin equilibria in metal complexes

Publisher Summary This chapter examines the dynamics of spin-equilibrium processes, principally from studies in solutions. The properties of the complexes that are relevant to the dynamics studies are reviewed. The techniques used to observe these rapid processes are also described in the chapter. The chapter presents some aspects of solid-state dynamics and describes some implications for the description of intersystem crossing processes in excited states and for spin equilibria in heme proteins. Many of the complexes that occur in spin equilibrium possess ligands with complicated structures. Trivial abbreviations are used, with structural formulas presented in the table. Generally the complexes are of low symmetry, but in the description of their electronic structure idealized symmetries are assumed and the appropriate term symbols are used accordingly. Some of the concepts, which are used to describe intramolecular spin equilibria, can be extended to the description of these coordination-spin equilibria. Examples include equilibria among four-, five-, and six-coordinate nickel (II) complexes and equilibria involving coordination number changes in iron porphyrin complexes and in heme proteins. Anomalous magnetic susceptibility is the characteristic feature of spin-equilibrium complexes. The Evans nuclear magnetic resonance (NMR) method gives excellent results provided adequate care is taken. There have been very few reports of the Raman spectra of spin-equilibrium complexes.

The intrinsic charge on hydrophobic microfluidic substrates

A synthesis of the literature provides an explanation for the hitherto unexplained charge on hydrophobic microfluidic substrates. It is due to the adsorption of hydroxide ions created by the enhanced autolysis of water at these surfaces.

Structural studies on the caesium alums, CsMIII[SO4]2·12H2O

Structure determinations of the caesium alums CsMIII[SO4]2·12H2O, M = V, Cr, Mn, Fe, Co, Al, Ga, or In, have been carried out at 295(1) K by X-ray diffraction methods. The data obtained include (i) MIII–O distances for the metals in the [M(OH2)6]3+ species [V–O, 1.992(6); Cr–O, 1.959(3); Mn–O, 1.991(6); Fe–O, 1.995(4); Co–O, 1.873(5); Al–O, 1.877(3); Ga–O, 1.944(3); and In–O, 2.112(4)A], (ii) an estimate of 1.34A for the effective co-ordinated radius for the water molecule in the cation, and (iii) the proposal of a new criterion for the classification of the different alum types, based on the disposition of the water molecules about the univalent cation.

A vibrational spectroscopic 18O tracer study of pyrite oxidation

Pyrite was oxidised under 18O2 gas in H216O solutions, with and without added ferric ion, and the sulfate produced was analysed by vibrational spectroscopy to determine the relative amounts of sulfate isotopomers (S16On18O4−n2−) formed. At 70°C and pH 1, with no added Fe3+, the majority of the sulfate formed was that which derived all four oxygen atoms from water (i.e., S16O42−), but significant amounts of two other isotopomers, S16O318O2− and S16O218O22−, which derive one or two oxygen atoms from molecular oxygen were observed. When Fe3+ was added at the start under identical conditions, no S16O218O22− was observed. The major isotopomer formed was still S16O42−, with S16O318O2− present as a minor product. Experiments which were performed at initial pH 7 yielded similar results, as did others performed at 20°C, although the amounts of the minor isotopomers formed vary with temperature. All of the results were confirmed by performing identical experiments with the source of the oxygen isotopes reversed, that is, by oxidising pyrite under air in H218O solutions and obtaining the same products in isotopic reverse.

Preliminary in vitro assessment of erosive potential using the ultra-micro-indentation system

The aim of this study was to measure the change in hardness and modulus of elasticity of enamel and dentine in primary teeth using the ultra-micro-indentation system (UMIS) after exposure to potentially erosive beverages and to relate the changes to the chemical composition of the test beverages. Primary molar teeth were sectioned and polished. Indentations were made in enamel and dentine prior to and then after exposure to one of four beverages: distilled water (control), orange juice, Orange Cordial, Fanta® and a ‘toothfriendly’ Tropical Orange Cordial (Ribena®). Each beverage was then analysed. Fanta and Orange Cordial were the only test beverages to show a statistically significant reduction in the enamel hardness in comparison to the control and only Fanta showed a significant reduction in dentine hardness. None of the test beverages significantly reduced the modulus of elasticity of either enamel or dentine. Statistically the pH, phosphate and fluoride concentrations of the beverages were correlated with softening of enamel. Calcium concentration and titratable acidity were correlated with softening of dentine. The UMIS offers another method of measuring the basic mechanical properties of enamel and dentine. This is useful in predicting the erosive potential of substrates. It is likely that the pH, titratable acidity, fluoride, phosphate and calcium content of beverages are all important in determining the potential erosiveness of beverages.

Electroacoustic determination of size and zeta potential of casein micelles in skim milk

Measurements of the zeta potential and particle size of casein micelles in skim milk suspensions at natural and lower pH have been made using the technique of electroacoustics. This technique requires no dilution or change of environment of the casein micelles. The zeta potential obtained at natural pH for a commercial skim milk suspension was -18 mV; it became less negative with decreasing pH. The median particle size observed at natural pH for a commercial skim milk suspension was 0.2 μm, in good agreement with previously reported values. The particle size increased as the pH was decreased.

pH and the surface tension of water.

Despite the strong adsorption of hydroxide ions, the surface tension of water is almost independent of pH between pH 1 and 13 when the pH is adjusted by addition of HCl or NaOH. This is consistent with the Gibbs adsorption isotherm which measures the surface excess of all species in the double layer, if hydronium ions and hydroxide ions are adsorbed and sodium and chloride ions are not. The surface tension becomes pH dependent around pH 7 in millimolar NaCl or KCl solutions, for now sodium ions can replace hydronium ions as counterions to the adsorbed hydroxide ions.