Aqueous Solutions Of NaF Research Articles

Refining adsorption parameters of n-butyl alcohol at the interface of mercury electrode with aqueous NaF and Na2SO4 solutions by correcting the adsorbate concentration

As an adjunct to the regression analysis of differential capacitance curves, which allows refining the adsorption parameters, a program, which takes into account possible errors in volume concentrations of organic substances, is developed. Using this procedure, the earlier data on the differential capacitance of a mercury electrode in aqueous solutions of normal butanol (1-BuOH) containing either 0.1 M NaF or 0.05 and 0.5 M Na2SO4 as the supporting electrolyte are analyzed. This allows obtaining the most accurate values of adsorption parameters for the systems mentioned above within the framework of the model of two parallel capacitors and the Frumkin isotherm. It is shown that, when a linear dependence of the intermolecular interaction parameter on the electrode potential is taken into account, the standard deviation of experimental capacitance values from those calculated using the mentioned model is 6.8–8.8%, which points to very high accuracy of this phenomenological model.

純チタンおよびチタン合金の NaF 水溶液浸漬による水濡れ性の変化

Titanium is used as an implant material for dentures. However, the corrosion caused by fluoride and the low wettability of titanium by saliva are still problems. In this study, the corrosion behavior of pure titanium and its alloy was investigated in aqueous NaF solutions, and the macro-wettability change of pure water on these specimen surface after corrosion is discussed. The passive current density of pure titanium and its alloy tended to increase with increasing NaF concentration. The potential distribution nearby pure titanium surface slightly changed after immediate immersion in the solution, then the value soon became relatively stable. The contact angle ratio ψ, which is the ratio of the contact angle before immersion θ0 to that after immersion θi, decreased with increasing immersion time. Higher surface wettability was observed after more active corrosion, indicating that the progression of corrosion significantly influenced the wettability of these surfaces by pure water. The surface roughness Ra was larger after immersion in 2.0% NaF solution than in 0.1% solution, which might also have influenced its wettability by pure water.

Potential of zero free charge of Pd overlayers on Pt(1 1 1)

Differential capacitance measurements of Pd overlayers on a Pt(1 1 1) electrode in dilute aqueous NaF solutions have been performed as a function of film thickness in order to determine the potential of zero free charge (pzfc). The pzfc of the first, pseudomorphic Pd monolayer on Pt(1 1 1) is −0.21 V versus SCE. By increasing the amount of deposited Pd, a clear shift of the pzfc to more positive values is observed. After deposition of an equivalent of 10 monolayers, the value approaches that of a massive Pd(1 1 1) electrode (−0.12 V versus SCE). The pzfc's for the various Pd coverages are correlated with surface structure information, derived from STM images (R. Hoyer, L.A. Kibler, D.M. Kolb, Electrochim. Acta 49 (2003) 63). Variations in the pzfc are discussed in the context of an electronic modification by the underlying substrate and are compared with corresponding data for Pd overlayers on Au(1 1 1).

A Novel Sodium Silicate Fluoride Solution and a H2 Gas Formed by a Reaction Between Si and an Aqueous Solution of NaOH and NaF

Oxidation of Si in an alkali solution has attracted much attention to produce hydrogen gas from Si wastes. It was found that hydrogen gas was generated from Si in NaOH and NaF solution with more than 10 times reaction rate than in the NaOH solution. The reaction by‐product was a sodium silicate fluoride, Nax+zSiyO2y+x/2Fz solution, and it proved to transform to a glass foam by boiling. The Raman spectrum suggested that the silicon and oxygen atoms in the silicate anions were arranged in a sheet‐like structure. This may affect the behavior to form the glass foam.

Variation of the Surface Stress−Charge Coefficient of Platinum with Electrolyte Concentration

We report an experimental study of the variation of surface stress with surface charge density for nanoporous Pt immersed in aqueous solutions of NaF of various concentration. As the concentration is reduced, we find initially an increase in the magnitude of the surface stress-charge coefficient, followed by saturation at a value of -1.9 V. Since specific adsorption is expected to be reduced as the solution becomes more dilute, the results support the notion that changes in the bonding at the metal surface play a decisive role in determining the change in the surface stress during double-layer charging.

Molecular Dynamics Simulation of Aqueous NaF and NaI Solutions near a Hydrophobic Surface

We present results from molecular dynamics simulation of aqueous solutions of alkali halide salts (NaI and NaF) at the interface with hydrophobic objects. The primary objective of this study is to investigate the structural properties of the salt solutions at the hydrophobic surface. An alkane crystal has been taken as the parent model for a hydrophobic surface. A hexagonal hole was created on it, which was half a nm deep and 2.5 nm wide. The density distributions of different species (water, anions, and cations) are studied as a function of distance from the surface. While iodide prefers the interface, the fluoride ions stay inside the bulk water region. The higher concentration of iodide ions at the interface drags sodium counterions to the interface. It also decreases the water density at the interface because of steric effects of the iodide ions. The number of contacts between the surface carbons and water decreases in the case of NaI solutions but is unchanged for NaF solutions. The orientation of the water-ion and the water-water hydrogen bond vector orientations near the interface is discussed in detail.

Interfacial Processes in Nanoporous Platinum: Double-Layer Charging Versus Chemisorption

AbstractWhen conductive solids with nanoscale porosity are immersed in electrolyte and biased against a suitable counter electrode, the surface is charged and the properties of the matter at the surface vary reversibly as the function of the bias voltage. Due to the immense surface area at nanoscale pore size, the local changes are reflected in the macroscopic materials properties. Specifically, changes in the superficial interatomic bonding lead to a stress and strain of the material, with strain amplitude and strain energy density of sufficient magnitude to suggest possible application as actuator materials. We discuss an experiment which provides an insight into the underlying microscopic processes at the metal electrolyte interface: are changes in the surface bonding in the metal dominant, or does the effect arise mainly from the formation of chemical bonds with adsorbates? We studied the variation of the surface stress charge coefficient of Pt in weakly adsorbing aqueous solutions of NaF of various concentrations. As the concentration is reduced, we find initially an increase in the magnitude of the surface stress-charge coefficient, followed by saturation at a value of -1.9V. Since specific adsorption is expected to be reduced as the solution become more diluted, in favor of the capacitive charging, the results support the notion that changes in the bonding between the metal atoms control the variation in the surface stress during double-layer charging.

Chemical treatment effects of silicon surfaces in aqueous KF solution

The physical and chemical properties of Si(1 1 1), (1 1 0) and (1 0 0) surfaces treated in aqueous KF solution have been studied by means of spectroscopic ellipsometry (SE), ex situ atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and wettability measurements. The SE data indicate that the solution causes the removal of the silicon native oxide upon immersing the sample in the solution. The KF-treated Si(1 1 0) and (1 1 1) surfaces are nearly flat even after long-time etching. By contrast, the Si(1 0 0) surface is found to be considerably etched and roughened even if the etching time is shorter than the native oxide is completely etch-removed ( t < 30 min). The SE-estimated surface roughness is in reasonable agreement with the AFM rms value. The XPS data suggest that the KF-treated surface is cleaner than that etched in an HF solution. The as-degreased silicon surface is hydrophilic ( θ ∼ 35°), while the KF-treated surface is hydrophobic ( θ ∼ 80°). The properties of the KF-treated surface have also been discussed as compared to those obtained in aqueous NaF and HF solutions.

Transport of ions across peritoneal membrane

The electrical conductance of ions across the peritoneal membrane of young buffalo (approximately 18–24 months old) has been recorded. Aqueous solutions of NaF, NaNO 3, NaCl, Na 2SO 4, KF, KNO 3, KCl, K 2SO 4, MgCl 2, CaCl 2, CrCl 3, MnCl 2, FeCl 3, CoCl 2, and CuCl 2 were used. The conductance values have been found to increase with increase in concentration as well as with temperature (15 to 35 °C) in these cases. The slope of plots of specific conductance, κ, versus concentration exhibits a decrease in its values at relatively higher concentrations compared to those in extremely dilute solutions. Also, such slopes keep on increasing with increase in temperature. In addition, the conductance also attains a maximum limiting value at higher concentrations in the said cases. This may be attributed to a progressive accumulation of ionic species within the membrane. The κ values of electrolytes follow the sequence for the anions: SO 4 2−>Cl −>NO 3 −>F − while that for the cations: K +>Na +>Ca 2+>Mn 2+>Co 2+>Cu 2+>Mg 2+>Cr 3+>Fe 3+. In addition, the diffusion of ions depends upon the charge on the membrane and its porosity. The membrane porosity in relation to the size of the hydrated species diffusing through the membrane appears to determine the above sequence. As the diffusional paths in the membrane become more difficult in aqueous solutions, the mobility of large hydrated ions gets impeded by the membrane framework and the interaction with the fixed charge groups on the membrane matrix. Consequently, the membrane pores reduce the conductance of small ions, which are much hydrated. An increase in conductance with increase in temperature may be due to the state of hydration, which implies that the energy of activation for the ionic transport across the membrane follows the sequence of crystallographic radii of ions accordingly. The Eyring's equation, κ=( RT/ Nh)exp[−Δ H*/ RT]exp[Δ S*/ R], has been found suitable for explaining the temperature dependence of conductance in the said cases. This is apparent from the linear plots of log[ κNh/ RT] versus 1/ T. The results indicate that the permeation of ions through the membrane giving negative values of Δ S* suggest that there may be formation of either covalent linkage between the penetrating ions and the membrane material or else the permeation may not be the rate-determining step. On the one hand, a high Δ S* value associated with the high value of energy of activation, E a, for diffusion may suggest the existence of either a large zone of activation or loosening of more chain segments of the membrane. On the other hand, low value of Δ S* implies that converse is true in such cases, i.e., either a small zone of activation or no loosening of the membrane structure upon permeation.

Probing the Molecular Structure and Bonding of the Surface of Aqueous Salt Solutions

The surfaces of aqueous solutions of NaF, NaCl, NaBr, and NaI have been examined using vibrational sum-frequency spectroscopy. Spectra of these salts in mixtures of HOD/H2O/D2O have been used to provide insight into how simple salts alter the hydrogen bonding structure of water in the surface region. As the anion is changed, the observed interfacial hydrogen bonding also changes, indicating the presence of anions in the interfacial region. The isotopic dilution experiments performed on each solution enable separation of the contributions arising from interfacial water with differing degrees of hydrogen bonding. Frequency shifts in the peaks attributed to tetrahedrally coordinated water molecules within the interfacial region display the structure-making characteristics of F- and the structure-breaking characteristics of Cl-, Br-, and I-. However, water molecules residing in the topmost surface layer show minimal perturbation by the presence of these anions as probed by the characteristics of the donor OH mode of water molecules that straddle the air/water interface. These results indicate a significantly diminished population of the anions at the uppermost layer of the surface region.

Peculiarities of the electrical double layer structure on mechanically renewable electrodes of Ag–Sn alloys in aqueous solutions of surface-inactive electrolytes

The behavior of electrodes of Ag–Sn binary alloys (Sn content from 3 to 9 at.%; the range of solid solutions of Sn in Ag) renewable by cutting was studied in aqueous NaF solutions by using the impedance method. Upon the electrode surface renewal under potentiostatic conditions in the ideal polarizability range, quick changes in the differential capacitance of the electrical double layer were observed. These changes show that the content of tin atoms in the surface layer increases with the exposure of the renewed surface to the solution, i.e. tin is the surface-active component of the alloy. Within the framework of known phenomenological models of the structure of the electrode ∣ solution interface, a mechanism is put forward based on the analysis of the time effects observed and the experimental results obtained on the behavior of Ag 3Sn intermetallic compound in contact with aqueous NaF solutions. This mechanism allows the processes, which occur in the metal phase during the formation of the equilibrium alloy ∣ electrolyte interface, to be interpreted qualitatively. The mechanism involves two stages: (1) quick diffusion of the surface-active component of the alloy (Sn) to the surface and (2) a slower stage associated with the formation of an intermetallic-like compound Ag 3Sn in the surface layer.

Surface roughening of glass ionomer cements by neutral NaF solutions

The objective of this study was to investigate the effect of repeated applications of a neutral NaF solution on the surface roughness of four conventional glass ionomer cements (GIC) (ChemFil Superior encapsulated, Fuji Cap II, Ketac-Fil and Hi Dense), three resin-modified (RM-) GIC (Fuji II LC encapsulated, Photac-Fil and Vitremer) and one polyacid-modified composite resin (PAM-C) (Dyract). Matured specimens were four times alternately eluted in water and exposed to 2% neutral NaF aqueous solutions for 1 h. Control specimens were only subjected to elution in water for the same time period. After the treatment the surface roughness R a was determined using non-contact surface profilometry and selected samples were examined with SEM. Except for the PAM-C, R a increased drastically for the fluoride-treated samples compared to water-stored samples, the effect being most pronounced for the GIC. Surface roughening apparently is caused by a progressive disintegration or chemical erosion of the polysalt matrix of (RM-)GIC.

Specific features of the structure of electrical double layer on mechanically renewed Ag-Sn electrodes in aqueous surface-inactive electrolytes

The behavior of an Ag–Sn binary alloy (3–9 at. % Sn; region of solid solutions of Sn in Ag) in aqueous NaF solutions is studied with the aid of an impedance method. After the electrode surface renewal in potentiostatic conditions in the region of ideal polarizability, the EDL differential capacitance rapidly changes with time. These changes testify that the content of tin atoms in a surface layer increases with the duration of contact between a renewed surface and the solution, i.e. tin is a surface-active component of the alloy. On the basis of an analysis of the observed time effects in the framework of available phenomenological models for the electrode/solution interface and the experimental data obtained earlier for an Ag3Sn electrode in aqueous NaF solutions, a mechanism is suggested, which allows one to qualitatively interpret the formation kinetics of an equilibrium interface between alloys under study and the electrolyte, which occur in the metallic phase. The mechanism includes two stages: (1) a fast escape of atoms of the surface-active component of the alloy (Sn) into a surface layer, and (2) a slower stage of the formation of intermetallic compound Ag3Sn in a surface layer.

Specific Features of the Structure of the Electrical Double Layer on Intermetallic Compound Ag3Sn in Aqueous Surface-Inactive Electrolytes

The behavior of a polycrystalline electrode of intermetallic compound Ag3Sn in aqueous NaF solutions is studied using an impedance method. At potentials of –1.3 to –0.6 V (SCE), the electrode may be viewed as ideally polarizable. The Gouy–Chapman–Stern–Grahame model quite satisfactorily describes capacitance curves obtained at various electrolyte concentrations. Experimental capacitance curves for an Ag3Sn electrode are compared with those for polycrystalline Ag and Sn electrodes renewed by cutting. Characteristics of an Ag3Sn electrode are unique and its capacitance curves cannot be simulated from the data for Ag and Sn electrodes within the known phenomenological approaches.

Effects of the Starting Material and Hydrothermal Treatment Conditions on the Crystallization of Ultrafine Silica

The effects of the starting material and hydrothermal crystallization conditions on the phase composition and particle size of the resulting nanocrystalline silica powders were studied. Hydrothermal treatment of silica gels, xerogels, and Aerosils in water and aqueous NaF and NaOH solutions at temperatures in the range 250–500°C and pressures of up to 100 MPa causes gradual dehydration and densification of the materials, accompanied by the formation of crystalline SiO2 varieties, such as more or less ordered opaline silicas, cristobalite, keatite, and quartz. The extent and rate of these transformations, as well as the particle size of the crystallizing phases, depend on the starting material and the process parameters: temperature, pressure, duration, and environment. The best precursors to nanocrystalline silica with a particle size of 20–35 nm are silica gels and Aerosils treated in water at 250–300°C and 100 MPa.

Chemical Treatment Effects on Si(111) Surfaces in Aqueous NaF Solution

Chemically treated Si(111) surfaces in aqueous NaF solution have been investigated using spectroscopic ellipsometry (SE), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and wettability measurements. The SE data indicate that the solution causes the removal of the native oxide upon immersing the sample in the solution. After the native oxide is etched away completely, the SE data yield the spectrum of a slightly roughened surface. The SE-estimated roughness is ∼ 0.64 nm, which is considerably larger than the AFM determined rms value (∼ 0.26 nm); the difference is considered to be due to the SE technique being sensitive not only to the surface microroughness but also to the adsorbed chemical species. The XPS data support the fact that the native oxide is removed upon immersing the sample in the solution. It is also shown that the Si LMM signal at ∼ 1160 eV can provide direct information regarding the relative quality of surface regions prepared by different methods. The wettability measurements show that the as-degreased surface is hydrophilic (θ∼35°), while the NaF-etched surface is hydrophobic (θ∼70°).

In situ monitoring of the rising of aqueous solution meniscus for a partially immersed silver electrode during electrochemical reduction of oxygen

AbstractRising phenomena of aqueous solution meniscus were found for the silver electrode of a 5 MHz piezoelectric quartz crystal (PQC) partially immersed in Na2SO4, NaClO4, HClO4 and NaF aqueous solutions at oxygen reduction potentials, respectively. A detailed study revealed that a decrease in contact‐angle hysteresis (or a contact‐angle decrease) and a continued collection of the water product at the solid‐gas‐solution interface during oxygen reduction, rather than the electrocapillary effect and an agitation effect induced by the oscillation of PQC, are responsible for the meniscus‐rising phenomena. In addition, in situ determination of the immersed height of a partially immersed Ag electrode was studied on the basis of simultaneous measurements of the electroacoustic admittance and electrochemical impedance.

Enthalpic interactions of glycine in aqueous sodium halide solutions

Dilution enthalpies of glycine in aqueous NaF and NaI solutions of various molalities at 298.15 K have been determined, respectively, by mixing flow microcalorimetry. The results have been analyzed according to the modified McMillan–Mayer model to obtain homogeneous enthalpic interaction coefficients. The effect of the anions on enthalpic pair-wise interaction coefficients has been discussed from the point of view of electrostatic interaction and structural interaction.

Electroreduction of peroxodisulfate anion at Bi(111) single-crystal plane electrode

Electroreduction of peroxodisulfate anion on electrochemically polished face Bi(111) in aqueous NaF solution with different additions of Na2S2O8 is studied by rotating-disc voltammetry. The S2O 8 2- electroreduction rate depends on the electrode potential and base-electrolyte concentration, i.e. on the diffuse layer thickness. The kinetic current densities atE = const are obtained by the Koutecky-Levich method and used for the determination of the apparent rate constants of electroreduction of S2O 8 2- . The charge number of the reacting S2O 8 2- species is obtained; it depends on the surface-inactive electrolyte concentration and the electrode potential. The ψ0 potential values at the outer Helmholtz plane, obtained for various NaF + H2O systems according to Gouy-Chapman-Stern-Grahame (GCSG) model, are used for constructing corrected Tafel plots (CTP), which are linear atE- ψ0 < -0.9 V (SCE). The charge transfer coefficient oc for electrochemically polished Bi(111) somewhat decreases with the concentration of the base electrolyte (0.22 ≤α ≤0.27), but the value of α is practically independent of the S2O 8 2- concentration in solution if cNaF = const. However, CTP noticeably depend onc NaF.

Electroreduction of peroxodisulfate anion at a Cd(0001) single-crystal plane electrode

Electroreduction of peroxodisulfate anion on electrochemically polished face Bi(111) in aqueous NaF solution with different additions of Na2S2O8 is studied by rotating-disc voltammetry. The S2O82- electroreduction rate depends on the electrode potential and base-electrolyte concentration, i.e. on the diffuse layer thickness. The kinetic current densities atE = const are obtained by the Koutecky-Levich method and used for the determination of the apparent rate constants of electroreduction of S2O82-. The charge number of the reacting S2O82- species is obtained; it depends on the surface-inactive electrolyte concentration and the electrode potential. The ψ0 potential values at the outer Helmholtz plane, obtained for various NaF + H2O systems according to Gouy-Chapman-Stern-Grahame (GCSG) model, are used for constructing corrected Tafel plots (CTP), which are linear atE- ψ0 < -0.9 V (SCE). The charge transfer coefficient oc for electrochemically polished Bi(111) somewhat decreases with the concentration of the base electrolyte (0.22 ≤α ≤0.27), but the value of α is practically independent of the S2O82- concentration in solution if cNaF = const. However, CTP noticeably depend oncNaF.