Stern potential near gold and chromium surfaces in contact with aqueous and alcoholic ammonium salt solutions

The repulsive force between two solid surfaces immersed in electrolyte solution is created by overlapping of the electrical double layers adjacent to the solid surfaces when the separation is large enough so that solvation forces can be neglected. When the repulsive force is known, one can calculate the Sternpotential at the solid surfaces. The repulsive interactions in electrolyte solutions were directly measured between two high-grade polished fused silica plates, one planar, the other spherically curved. The surfaces were covered with thin gold or chromium layers and separated by aqueous and alcoholic solutions of NH₄Cl, NH₄Br and NH₄SCN in concentrations 10^–5–10^–2 M. The absolute value of the negative Stern potential derived from the repulsive forces increased when regarding the anions in the order

Simulating Unstable States during the Coumarin Adsorption on a Mercury Electrode

Dependences of differential capacitance C on potential E of a stationary electrode (hanging mercury drop) in aqueous 0.1 M NaF solutions containing 4.6 × 10^–4 to 3 × 10^–3 M C₉H₆O₂ are obtained using an automatic impedancemeter. At coumarin concentrations below 0.001 M and potential slowly scanned near –1.1 V (SCE) the capacitance is unstable, which results in differently-shaped C vs. E curves in this potential range. The obtained results are attributed to nonequilibrium phase transitions in the adsorption layer, during which the orientation of coumarin molecules at the electrode surface alters. These phenomena are explained semiquantitatively on the basis of a developed theory.     

Direct measurement of molecular interaction forces in foam films from lung surfactant fraction

Molecular interaction forces, operative in microscopic foam films obtained from the isolated hydrophobic fractions of porcine lung surfactant (AS-B) are investigated by monitoring film thickness h as a function of electrolyte concentration (C _el) and direct measurements of disjoining pressure/thickness (Π(h)) isotherms. The steep decrease of the common film thickness with the increase of C _el evidences the action of long-range electrostatic surface forces. The experimental h(C _el) curve indicates that non-Derjaguin-Landau-Verwey-Overbeek (DLVO) repulsive forces are operative at C _el where common black (CBF) and Newton black films (NBF) are obtained including the physiologically relevant C _el=0.14 mol dm⁻³ NaCl. The action of additional non-DLVO forces is corroborated by the comparison of the experimentally measured Π(h) isotherm with the DLVO theory. Considering the presence of proteins in AS-B and the formation of lipid-protein complexes it is inferred that steric type forces are operative in CBF and NBF.     

Determination of C4–C14 carboxylic acids by capillary zone electrophoresis: Application to the identification of diamide degradation products and partitioning studies

Capillary zone electrophoresis (CZE) was investigated for the determination of linear saturated carboxylic acid homologues ranging from C₄ to C₁₄. Separation conditions were optimised to overcome the problems of decreasing solubility and decreasing selectivity between successive homologues with increasing chain length. Separations were performed at 20°C, using a 20 kV separation voltage and a pH 8 electrolyte containing 30% methanol. A suitable chromophore (4-aminobenzoate) was added to ensure indirect UV detection of the analytes. Calibration curves and repeatability were established. Minimum detectable concentrations of 3·10⁻⁶ mol l⁻¹ were achieved. Resolution between successive homologues was better than 2. The electrophoretic mobility of each homologue (n=7–14) was assessed and a quasi-linear relationship between the mobility value and 1/n was observed. The quantitative analysis of a diamide degradation solution was performed and compared to potentiometric results. The CZE method was also applied to the determination of C₇–C₁₄ partitioning between an organic medium containing tributylphosphate in n-dodecane and different basic solutions. Their behaviour was established according to the chain length and the pH of the aqueous phase. For C₁₀–C₁₄ compounds, results were validated by comparison with gas chromatographic analysis of the organic phases.     

The Effect of Water Addition on the Electrosurface Properties of Quartz in Solutions of Alkali Metal Bromides in Ethanol

Electrosurface properties (the -potential and surface conductivity) of quartz particles in water–ethanol solutions of CsBr, NaBr, and LiBr with concentrations C = 10^–5–10^–2 M are studied. The (log C) dependences plotted from the results of electrophoretic measurements with allowance made for the particle surface conductivity demonstrate that, when water content in the aforementioned solutions increases from 4 to 40 vol %, the -potential of quartz becomes more negative and the isoelectric point shifts toward higher electrolyte concentrations, which increase in the following series: CsBr < NaBr < LiBr. This shift of the isoelectric point is explained by a decrease in the specific interaction of the alkali metal cations with the quartz surface because of a rise in the degree of their hydration (supersolvation).     

PE-CVD of Fluorocarbon/SiO Composite Thin Films Using C4F8 and HMDSO

Plasma copolymerization of hexamethyldisiloxane (HMDSO,(CH₃)₃-Si-O-Si-(CH₃)₃) and C₄F₈ was performed using an RF plasma enhanced chemical vapor deposition method for application to low dielectric constant intermetal dielectrics. Structure of the films was investigated by X-ray photoelectron spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. The film composition was controlled gradually from fluorinated carbon to organic siloxane by changing the mixing ratio of HMDSO/Ar. Dielectric constant of the films ranged from 2–3.3. Thermal stability of the films, which was characterized by intensity loss of IR absorbance peak around 1000–1500 cm^–1 corresponding to C-F _n , Si-O-Si and Si-(CH₂)_n-Si bonds, was inferior to that from C₂F₄/HMDSO/Ar. In situ gasphase FT-IR spectroscopy revealed that there was a marked difference between the gas phase of C₄F₈/HMDSO/Ar and that of C₂F₄/HMDSO/Ar discharges. The IR spectrum of the former combination plasma contained a peak at 1250 cm^–1 with full width at half maximum as large as 150 cm^–1, which suggests that fluorocarbon particles and/or dusts are formed in the plasma. This suggests also that deposition precursors are not only CF _n (n = 1, 2, and 3) but also larger precursors such as C _x F _y (x > 1, y < 2x + 2) in C₄F₈/HMDSO/Ar discharges, which is presumably the cause of difference in thermal stability of the films prepared from C₄F₈/HMDSO/Ar and C₂F₄/HMDSO/Ar mixtures.     

Electrode Properties of Tetrahedral Amorphous Carbon

Impedance spectra in a 2.5 M H₂SO₄ solution and kinetic characteristics of reactions in the Fe(CN) ₆ ^3−/4− redox system are measured for thin-film electrodes of tetrahedral amorphous carbon (ta-C). After an anneal in a vacuum at 700 to 900°C or implantation of C⁺ ions (10¹⁵ to 10¹⁷ ion/cm²), ta-C films acquire electrochemical activity, which can be explained by an increased content of sp ² carbon.__________Translated from Elektrokhimiya, Vol. 41, No. 7, 2005, pp. 866–871.Original Russian Text Copyright © 2005 by Evstefeeva, Pleskov, Kutsay, Bello.     

Formation of poly-p-phenylene thin films and their composites with fullerene by discrete evaporation in vacuum

The effect of parameters of discrete evaporation of poly-p-phenylene (PPP) and C₆₀ on quality of the films formed was studied, PPP and PPP/C₆₀ composite films and heterostructures based on these materials were prepared. It was established that the conjugation length of PPP macromolecules consists of –9–10 units, and they can be electrochemically transformed into the radical cation state at a potential of 2.10 V. Glassy carbon-PPP-C₆₀-Al heterostructures with a rectification ratio of 10⁶–10⁸ were formed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No, 12, pp, 2918–2921, December, 1996.     

Foam and Wetting Films from Aqueous Cetyltrimethylammonium Bromide Solutions: Electrostatic Stability1

Foam films and wetting films on quartz formed from aqueous solutions of cetyltrimethylammonium bromide (CTAB) are investigated in a wide range of surfactant concentrations in the presence of background electrolyte (5 × 10^–4 mol dm^–3 NaCl). Foam and wetting films are convenient models for the study of symmetric (free thin liquid films) and asymmetric (thin liquid films on solid substrate) films with the same air/solution interface. Microinterferometric methods of assessment of foam and wetting films are used which allow precise determination of the film thickness. Determined are the values of the potential ₀ of the diffuse electrical layer at the solution/air interface (applying the method of equilibrium foam films) and the potential ₁ at the solution/quartz interface (applying the method of capillary electrokinetics). These values are used to analyze the stability of the films studied in terms of the DLVO theory. A conclusion drawn is that both kinds of films studied are stabilized by electrostatic interaction forces. It is shown that with increasing CTAB concentration, a charge reversal occurs at both the solution/air and solution/quartz interfaces which determines the stability/instability conditions of the foam and wetting films. Concentration ranges where both kinds of films produce stable (equilibrium) films are found. There are also concentration ranges where the films either rupture or are metastable (quasi-equilibrium). The CTAB concentration ranges, which provide the formation of unstable (rupturing and metastable) and stable films, are different for symmetric (foam) and asymmetric (wetting) thin liquid films. It is only at high CTAB concentrations (higher that >2 × 10^–4 mol dm^–3) that both cases render formation of stable equilibrium films. These studies give direct experimental indications that the electrostatic interactions between identical or different interfaces can differ when the surfactant concentration is varied.     

Measuring Contact Angles at Copper Electrodes

Potential dependences of the angle of contact between perfluorodecaline C₁₀F₁₈ and copper, copper(I) sulfide, and copper telluride Cu₄Te₃ in 0.1 M CH₃COONa are measured. The data are in good agreement with the metal hardness measured in 1 M KOH. The uncharged-surface potentials (USP) for copper, copper(I) sulfide, and copper telluride equal 0.05–0.07, –0.02 to –0.04, and 0.05–0.07 V (NHE) in 0.1 M CH₃COONa. Studying the effect of Br^– ions shows that USP for copper and Cu₂S shift in, respectively, the negative and positive directions with an increase in [Br^–]. For copper telluride, USP shifts in the positive direction at low Br^– concentrations, and at NaBr concentrations in excess of 0.04 M, in the negative direction.     

A kinetic study of the oxidation of 3- and 4-pyridinemethanol,and 3- and 4-pyridinecarboxaldehyde by chromium(VI) in acidic aqueous media

Oxidation of 3-pyridinemethanol (3-pyol), 4-pyridinemethanol (4-pyol), 3-pyridinecarboxaldehyde (3-pyal) and 4-pyridinecarboxaldehyde (4-pyal) by Cr^VI was studied under pseudo-first-order conditions in the presence of a large excess of reductant and at various H_aq ⁺ concentrations; [Cr^VI] = 8 × 10^–4 M, [reductant] = 0.025–0.20 M, [HClO₄] = 1.0 and 2.0 M (I = 1.2 and 2.1 M) or 0.5–2.0 (I = 2.1 M). A linear dependence of the pseudo-first-order rate constant (k _obs) on [reductant] and a parabolic function of k _obs versus [H⁺] lead to the rate law: –d[Cr^VI]/dt = (a + b[H⁺]²)[reductant][Cr^VI], where a and b describe the reaction paths via HCrO₄ ^– and H₃CrO₄ ⁺ species respectively, and are composite values including rate constants and equilibrium constants. The apparent activation parameters were determined from second-order rate constants at 1.0 and 2.0 M HClO₄, at three temperatures within the 293–323 K range. The presence of chromium species with intermediate oxidation states – Cr^V, Cr^IV and Cr^II, was deduced based on e.s.r. measurements and the kinetic effects of Mn^II or O₂ (Ar), respectively. The alcohols were oxidized to the aldehydes, and carboxylic acids and the aldehydes to the carboxylic acids. Chromium(III) was in the form of the [Cr(H₂O)₆]³⁺ complex.     

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).     

Determination of Potassium and Sodium in Nonaqueous Solvents by Stripping Voltammetry in a Radio-Frequency Electromagnetic Field

The effect of a radio-frequency electromagnetic field on the parameters of the analytical signal of potassium and sodium in stripping voltammetry in nonaqueous solvents was studied. Sodium and potassium can be separately determined in a concentration range of 2 × 10^–5–2 × 10^–7 M (RSD = 10–17%) upon the irradiation of the solution with an electromagnetic field of a frequency of 55 MHz. The irradiation efficiency depends on the frequency and the nature of the solvent. The detection limits for potassium and sodium were 8.0 × 10^–8 and 1.0 × 10^–7 M, respectively, in the presence of 0.02 M [(C₄H₉)₄N]I in dimethylformamide at a field frequency of 55 MHz.     

Electrosurface and Ion-Selective Properties of Track-Etched Nanofilters: The Effect of Polyvalent Metal Adsorption

The effect of polyvalent metal adsorption on the performance and ion selectivity of poly(ethylene terephthalate) track-etched membranes with pores of 10 nm in diameter was studied. Membrane samples were prepared from the track-etched membranes with pores of 20 nm in diameter by thermal shrinkage. It was shown that an effective pore diameter decreases and selectivity of track-etched membranes increases upon filtration of Al(NO₃)₃ and Cr(NO₃)₃ solutions. The results obtained are explained by ion adsorption leading to the formation of complexes between polyvalent metals and carboxyl groups on the pore surface that is confirmed by IR spectroscopy data. The study of electrosurface properties of modified membranes and the dependence of ion selectivity of track-etched membranes on the concentration of Al³⁺ ions in 10^–2 M KCl solution indicates the decrease of membrane negative surface charge resulted from Al³⁺ adsorption and membrane charge reversal at Al³⁺ concentration in a solution higher than 10^–6 M. The dependences of the ion selectivity on pH and Al³⁺ concentration C _Al in a solution are similar. At pH < 3 and C _Al > 10^–6 M, the _1–2 > _1–1 > _2–1 ion selectivity series characteristic of the initial negatively charged membranes for the 1–1, 1–2, and 2–1 electrolyte solutions is reversed into the _2–1 > _1–1 > _1–2 series characteristic of positively charged membranes.     

Nonlinear Impedance of Liquid In–Ga Electrode in Aqueous Solutions of a Symmetrical Surface-Inactive Electrolyte

Experimental data related to the potential dependence of nonlinear characteristics of the electrical double layer at a liquid In–Ga electrode in aqueous solutions of a symmetrical surface-inactive electrolyte are obtained for the first time. It is shown that, as opposed to polycrystalline Cd and Pb electrodes, on a liquid (atomically smooth) In–Ga electrode, as on Hg, there is a clear intersection of the potential dependences of a nonlinear signal for different concentrations of a 1–1-valence surface-inactive electrolyte at one point. The intersection point exactly corresponds to the potential of zero charge of an electrode undistorted by specific adsorption of ions. It is established that, when estimating hydrophilicity of metals by a nonlinear impedance method, most information is provided by the region of average negative charges, rather than by the region near zero charge. It is shown that, as opposed to a linear impedance method, the nonlinear impedance method makes it possible to determine, directly from experiment, quantities that directly characterize the metal–solvent chemisorption interaction in a pure form; at the same time, these quantities are criteria of lyophilic nature of metals. Quantities that characterize the metal–solvent chemisorption interaction, obtained by the linear and nonlinear impedance methods are in good agreement, which confirms the validity of the approach we proposed earlier for separating the difference between reciprocal capacitances of the inner part of the electrical double layer on Hg and metal M, ^Hg _M C ^-1 _i= 1/C ^Hg _i– 1/C ^M _iinto physical (^Hg _M C ^–1)_physand chemical constituents. This coincidence also confirms correctness of numerical values obtained earlier for quantities (^Hg _M C ^–1)_phys.     

Derivatives of the closo-dodecaborate anion and their application in medicine

The paper presents a comparative analysis of the possibilities and characteristic features of the application of various polyhedral boron compounds, viz., the closo-decaborate anion [B₁₀H₁₀]^2–, the closo-dodecaborate anion [B₁₂H₁₂]^2–, the carba-closo-dodecaborate anion [CB₁₁H₁₂]^–, carboranes C₂B₁₀H₁₂, and the bis(dicarbollide) complexes [M(C₂B₉H₁₁)₂]^– (M = Fe, Co, or Ni), in boron neutron capture therapy (BNCT) for cancer. The requirements on compounds used in BNCT are formulated and the advantages of the application of the closo-dodecaborate anion are considered. The data on the synthesis of various derivatives of the closo-dodecaborate anion, which either already found use in BNCT or are most promising in this field, are summarized. The possibilities of the application of agents derived from the closo-dodecaborate anion in medical diagnostics are discussed.     

Solvent Extraction Separation of Th(IV) and U(VI) with PC-88A

Liquid-liquid extraction of Th(IV) and U(VI) has been investigated by commercial extractant PC-88A in toluene. The optimum conditions for extraction of these metals have been established by studying the various parameters like acid concentration/pH, reagent concentration, diluents and shaking time. The extraction of Th(IV) was found to be quantitative with 0.1–1.0M HNO₃ acid and in the pH range 1.0–4.0 while U(VI) was completely extracted in the pH range 1.0–3.5 with 2.5·10^–2M and 2.·10^–2M PC-88A in toluene, respectively. The probable extracted species have been ascertained by log D-log C plot as ThR₄·4HR and UO₂R₂·2HR, respectively. The method permits separation of Th(IV) and U(VI) from associated metals with a recovery of 99.0%.     

Kinetics and Mechanism of Electroreduction of Ammonia Complexes of Cobalt(II) on a Dropping Mercury Electrode

Effects of concentrations of ammonia (0.3–5.8 M) and supporting electrolytes (NaF, NaClO₄; 0.1–0.5 M) on the kinetics of electroreduction of ammonia complexes of cobalt(II) at a dropping mercury electrode are studied. Most experiments are performed with low concentrations of cobalt(II) complexes (1 × 10^–5 to 2 × 10^–5 M) in the absence of a polarographic maximum. The dependence of the half-wave potential of the reversible cathodic wave pertaining to the reduction of ammonia complexes of cobalt(II) on the concentration of ammonia molecules is obtained. It is found from the dependence that, at ammonia concentrations of 0.5–2.6 M, the slow electrochemical stage involves predominantly complexes Co(NH₃)₂ ²⁺. At higher ammonia concentrations, the stage involves complexes Co(NH₃) _k ²⁺ (k > 2), which form in preceding chemical stages from complexes Co(NH₃) _i ²⁺ (i = 3–6) that are predominant in solution. Values of the diffusion coefficients for complexes Co(NH₃) _i ²⁺, apparent transfer coefficients, and rate constant of the process of electroreduction of ammonia complexes of cobalt(II) are determined. The reasons for the complicating effect the insoluble products of reduction of cobalt(II) complexes have on the shape of polarographic waves are discussed.     

The character of interaction of organometallic derivatives of 4-nitrophenol and 4-nitrothiophenol with anions in different media

Interaction of R _n M-derivatives of 4-nitrophenol and thiophenol (R _n M= PhHG, PPh₃Au, Ph₃Sn, Ph₄Sb) with a set of inorganic and organic anions (Cl^–, Br^–, I^–, CN^–, [PhOCO]^–, [4-NO₂C₆H₄OCO]^–) was studied by IR and electronic spectroscopy in solvents with different polarities and coordinating properties (C₆H₆, CH₂Cl₂, DMSO). The dependence of the character of the interaction with the anions on the nature of the metal atom and heteroatom, the type of anions and the nature of the media was analyzed. Such interaction leads to ion-molecular complexes, and also to dissociation of the X-M bond (X = O, S, M = Au, Hg. Sb) with formation of the (4-NO₂C₆H₄X)^– anion even in weakly polar media. Only in the case of 4-NO₂C₆H₄OSnPh₃ does the reaction stop at the stage of complex formation. In other cases the role of complex formation with anions is less than with neutral coordinating reagents.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 7, pp. 1838–1841, July, 1996.     

A kinetic study on the iron(III)-promoted aquation of [Cr(C2O4)2(picolinato)]2− and [Cr(C2O4)2(2-pyridineacetato)]2− complexes

Two [Cr(C₂O₄)₂(AB)]^2– type complexes, obtained from the reaction of cis-[Cr(C₂O₄)₂(H₂O)₂]^– with the AB ligand, [AB = picolinic (pyac) or 2-pyridine-ethanoic acid (pyeac) anions], were converted into [Cr(C₂O₄)(pyac)(H₂O)₂]⁰ and [Cr(C₂O₄)(pyeac)(H₂O)₂]⁰ compounds, respectively via Fe^III-induced substitution of the oxalato ligand. The aquation products were separated chromatographically and their spectral characteristics and acid dissociation constants determined. The kinetics of the oxalato ligand substitution were studied with a 10–40 fold excess of Fe^III over [Cr^III] at [H⁺] = 0.2 M and at constant ionic strength 1.0 M (Na⁺, H⁺, Fe³⁺, ClO^– ₄). The reaction rate law is of the form: r = k _obs[Cr^III], where k _obs = kQ[Fe^III]/(1 + Q[Fe^III]). The first-order rate constants (k), preequilibria quotients (Q) and activation parameters derived from the k values have been determined. The reaction mechanism is discussed in terms of a Lewis acid catalyzed (induced) ligand substitution.