Electrochemical characterization of the surface of a small particle and nonequilibrium electric surface phenomena

By investigating the effects that originate from nonequilibrium double layer states, it is possible to discriminate between two types of surfaces respectively corresponding and not corresponding to the standard electrokinetic model (SEM). This goal is accomplished by measuring the induced dipole moment (i.d.m.) of a particle, which enables one to determine the specific surface conductivity κ^σ, mobile charge σ_m and efficient Stern potential Ψ_ef. Porous or rough surfaces are quite abundant, but they do not correspond to SEM which implies that Ψ_ef exceeds ζ. Since ion adsorption and the stability ratio depend exponentially on Ψ_ef, substitution of the ζ potential for Ψ_ef in these exponents results in a very large error if Ψ_ef is high. The investigation of particles with a porous or a rough surface or with an anomalous conductivity confined to Stern layer ions can be quantified through integrated measurements of the nonequilibrium electric surface phenomena (NESP) used in conjunction with the extension of DLVO theory over this more complicated type of surface.NESP which are used for measurement of κ^σ are characterized and experiments are discussed which have confirmed the correctness of the theory of NESP and of the procedure for κ^σ, σ_m and Ψ_d calculation and determination of the ζ-potential under polarization conditions. A comparative evaluation of the high- and low-frequency as well as of the stationary complexes of NESP measurements is carried out, and their advantages and disadvantages are shown as they reveal themselves in electrochemical characterization. The unsuitability of the Henry-Booth equation for the interpretation of low-frequency or stationary NESP is demonstrated as well as its apparent agreement with experiment and the complete inconsistency in the case of the nonstandard model. The software, which provides a correct extraction of information about the surface of a spherical particle from data of combined electric surface investigations of real disperse systems whose particles are usually polydisperse and do not correspond to SEM, is discussed. Attention is paid to efficient methods for measuring the i.d.m. κ^σ and Ψ_ef of a nonspherical particle, based on the anisotropy of conduction and on the electro-optic phenomena.The nonlinearity of high-voltage electrophoresis caused by surface diffusion of the adsorbed ions is predicted and described. It is proposed to use this new effect for studying the dynamic Stern layer and for measuring the adsorbed ion diffusivity. Such investigations are especially important for studying the electrostatic interaction of colloid particles which has a strong effect on the kinetics of slow coagulation according to studies made by Lyklema and Dukhin. Within the framework of the Lyklema-Klein model, the effect of gel layers on the electrostatic interaction of particles is considered and on this basis the importance is shown of identification of charged gel layers and of their electrochemical characterization.     

Protolytic equilibria on the surface of carboxyl-containing silica

Potentiometric titration has been used to study the protolytic equilibria on the surface of carboxyl-containing silica (CS) prepared by the reaction of silica (Silokhrom S-80, S_sp = 80 m²/g) with Cl₃SiCH₂CH₂COOCH₃, followed by hydrolysis with 30% sulfuric acid. The titration curve of the vacuum-dried sample is irreversible. The titration curve of its Na⁺ form with hydrochloric acid proceeds lower than the titration curve of its H⁺ form and coincides with the titration curve of the air-dried sample (the last curve is reversible). The titration curve of CS coincides with the titration curve of butyric acid at pH < 6. At pH > 6 the titration curve of CS passes below the titration curve of butyric acid; this is due to the participation of silanol groups on the silica surface in the protolytic equilibria. The pK_a of the grafted CS groups is equal to 4.80 which is close to the pK_a value of butyric acid (4.78). A method has been proposed for the determination of the amount of weak acid groups grafted to the silica. It has been shown that in the titration of CS the equilibrium is established much faster than in the case of the unmodified silica.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 243–247, March–April, 1986.     

Adsorption-calorimetric determination of the surface energy of dispersed oxides

A new method is proposed for the determination of the surface free energy σ_S and interfacial free energy σ_SL, based on their linear dependence on the specific heat of wetting of dispersed materials by water. The σ_S and σ_SL values of Silochrome, silica gel, and γ-aluminum oxide were determined first by this method. The obtained results are compared with data obtained from the similarity method and the empirical relationship between σ_S and the surface pressure. It is demonstrated that the methods can be used to determine σ_S for the investigated sorbents. Conclusions about the hydrophilicity-hydrophobicity of their surface were reached on the basis of the results. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 2, pp. 75–80, March–April, 2007.     

Investigation of chemisorption properties of the PbO surface

Chemisorption of O₂, Cl₂, and SO₂on the orthorhombic and tetragonal PbO samples was studied in the 225—550 °C temperature range. The main features of chemisorption are independent of the crystal modification of the sample. The change in the type of conductivity of PbO during chemisorption of O₂was found.     

Effect of hydrogen gas impurities on the hydrogen dissociation on iron surface

A small addition of oxygen to hydrogen gas is known to mitigate the hydrogen embrittlement (HE) of steels. As atomic hydrogen dissolution in steels is responsible for embrittlement, catalysis of molecular hydrogen dissociation by the steel surface is an essential step in the embrittlement process. The most probable role of oxygen in mitigating HE is to inhibit the reactions between molecular hydrogen and the steel surface. To elucidate the mechanism of such surface reaction of hydrogen with the steel in the presence of oxygen, hydrogen, and oxygen adsorption, dissociation, and coadsorption on the Fe(100) surface were investigated using density functional theory. The results show that traces of O₂ would successfully compete with H₂ for surface adsorption sites due to the grater attractive force acting on the O₂ molecule compared to H₂. The H₂ dissociation would be hindered on iron surfaces with predissociated oxygen. Prompted by the notable results for H₂ + O₂, other practical systems were considered, that is, H₂ + CO and CH₄. Calculations were performed for the CO chemisorption and H₂ dissociation on iron surface with predissociated CO, as well as, CH₄ surface dissociation. The results indicate that CO inhibition of H₂ dissociation proceeds via similar mechanism to O₂ induced inhibition, whereas CH₄ traces in the H₂ gas have no effect on H₂ dissociation. © 2014 Wiley Periodicals, Inc.     

Adsorption of atomic hydrogen on the surface of the boron-carbon nanotubes

Results of theoretic studies of geometrical, electronic, and energy composition of monolayer boron-carbon BC₃ nanotubes of the zig-zag type (n, 0) and the cylindrical symmetry are presented. The mechanism of adsorption of atomic hydrogen on the outer surface of the boron-containing BC₃-nanotubes of the (6, 0) type is studied. The calculations are carried out on the basis of models of the ion-incorporated covalent-cyclic cluster using the semiempiric MNDO scheme and density functional (DFT) methods.     

Specific ion effects on the surface tension and surface potential of aqueous electrolytes

The phenomena of electrolytes affecting the surface tension of aqueous solutions and producing measurable surface potentials are reviewed in the light of recent studies of them. The factual information presented includes the molar ionic surface tension increments k_i = lim(c_i → 0)(dσ / dc_i) of many ions and the surface potential increments ∆ χ = χ_E − χ_W of electrolytes involving the cations H⁺, Na⁺, K⁺, and NH₄⁺ and various anions. Gaps in the data that invite filling and inconsistencies in reported data are pointed out. Correlations of k_i with several properties of the ions that should be relevant to their specific effects: their sizes, quantities representing their polarizabilities, their effects on the structure of the water and the binding of water molecules by them, are presented. Correlations of the surface potential increment ∆ χ with the electrolyte surface tension increments and with the differences between the cation and anion increments are shown. Models recently proposed for the rationalization of the observed phenomena and relevant theoretical developments are shown and discussed. The paradox of hydrogen ions not promoting significant charge separation at the interface but yielding large surface potentials is emphasized.     

Electrophoresis of large colloidal particles with surface charge layers. Position of the slipping plane and surface layer thickness

A theory is proposed for the electrophoresis of a large colloidal particle with a surface charge layer. The slipping plane is assumed to be located within the surface layer but may not be located at the boundary between the surface layer and the particle core. In previous studies, the depth of the slipping plane is assumed to coincide with the surface layer thickness. The present theory makes it possible to examine the separate dependence of the electrophoretic mobility on the position of the slipping plane and on the surface layer thickness. It is shown that, at constant amount of particle-fixed charges in the surface layer, the mobility increases as the depth of the slipping plane (d _s ) increases, while it decreases as the surface layer thickness (d _c ) increases, causing a mobility maximum in some cases ifd _s =d _c . Several approximate analytic expressions for the mobility are presented.     

Nature of the centers on the surface of oxide catalysts and the forms of adsorbed toluene

IR spectroscopy has been used to analyze the forms of toluene adsorbed on various oxides, differing in the nature of their surface centers: TiO₂, CuO, V₂O₅/Al₂O₃, V₂O₅. The form of adsorption is found to depend on the nature of the surface centers of the oxide. When coordinationally saturated cations are involved complexes of toluene are formed, while if surface oxygen is involved benzoate structures are produced. The temperature at which the latter begin to be formed depends on the strength of the bond between oxygen and the surface. It was shown that the presence of acidic hydroxyl groups on the surface of the oxide leads to proton transfer to the aromatic compound to give carbonium ions. The nature of the interaction is then determined by the mobility of the proton on the surface and the basic strength of the adsorbed molecule. The possibility of identifying such compounds spectroscopically is discussed.Translated from Teoreticheskie i Éksperimental'naya Khimiya, Vol. 26, No. 6, pp. 710–715, November–December, 1990.     

Characterisation of the surface structure and bioactivity of glass and glass ceramics using surface topography

The present paper reports the results of the relationship between the surface topography, microstructure and the in vitro bioactivity of samples with and without fluorapatite content in simulated body fluid. Glasses and glass ceramics belonging to the Li₂O-SiO₂-CaO-P₂O₅-CaF₂ system were prepared by using conventional melting technique following by heat treatment to obtain glass ceramics. This current study demonstrates the benefits of combining two microscopic methods for better investigation of the surface structure. The formation of apatite layer on the surface and the increase in surface roughness proved that the glasses and glass ceramics with bioactive fluorapatite content could satisfy to the requirements for biomaterial applications. The results also showed that the roughness of apatite layer formed after immersion in body fluid on the surface of glasses with fluorapatite was more pronounced than that of equivalent glass ceramic samples cured under the same conditions.      

Photostimulated exoemission from the surface of magnesia

Photostimulated exoemission (PSE) from magnesia was studied along with the effect of the adsorption of active gases (H₂, O₂, CO₂, and water vapors) on the intensity and kinetics of the emission decay. Based on the literature data, it was shown that PSE was excited from the local surface centers including a pair of low-coordinated ions Mg_1c²⁺-O_1c²⁻ and V_s hole centers, which were also active centers of photosorption and photocatalytic oxidation of methane and some other hydrocarbons.     

Thermal analysis of the polymerization process on the surface of inorganic fillers

The thermal polymerization of pentabromobenzyl (mono)acrylate (PBB-MA) on the surface of the inorganic fillers Mg(OH)₂ and CaCO₃ was studied. FTIR spectroscopy and extraction of the polymer in bromobenzene show that polypentabromobenzyl acrylate (PBB-PA) was mostly grafted on the surface of Mg(OH)₂. Thermal analysis (TG, DSC, isothermal DSC (IDSC)) demonstrated an increase in polymerization starting temperature, and differences in polymerization enthalpy and apparent activation energy when an inorganic filler is added. These differences depend on the chemical composition of the filler used.The authors acknowledge valuable discussions with Prof. S. Yariv. The authors are also grateful to Berecha Foundation (Geneva) for the financial support of this work.     

Oxygen vacancy O-terminated surface: The most exposed surface of hexagonal WO3 (001) surface

It is known that exposed surface determines material’s performance. WO₃ is widely used in gas sensing and its working surface is proposed to control its sensitivity. However, the working surface, or most exposed surface with detailed surface structure remain unclear. In this paper, DFT calculation confirmed that oxygen vacancy O-terminated surface is the most exposed hexagonal WO₃ (001) surface, judging from competitive adsorption of CO and O₂, working surface determination for CO sensing and comparison of oxygen vacancy formation energies on different h-WO₃ (001) surfaces. It is found that DFT can be a useful alternate for exposed surface determination. Our results provide new perspectives and performance explanations for material research.     

Interaction of cadmium nitrate with the surface of functionalized organosilicas

The adsorption of Cd^II cations on the surface of amorphous macroporous silicas chemically modified by β-cyclodextrin and its functional derivatives was studied. The adsorption of Cd^II cations was shown to follow the equation of the Freundlich isotherm for the heterogeneous surface. Analysis of the adsorption kinetic curves showed that two parallel processes occurred on the surface of β-cyclodextrin-containing silicas. A substantial increase in the adsorption of Cd(NO₃)₂ is a result of the formation of uncharged supramolecular structures on the surface of silica adsorbents. The composition of these structures depends on the polarizability of functional substituents of β-cyclodextrins.     

Mechanism of the decomposition of the surface oxide film on polycrystalline palladium

The decomposition of thin surface oxide films on polycrystalline palladium Pd(poly) at 500–1300 K was investigated by mathematical modeling. This process was analyzed in terms of a model including O₂ desorption from the chemisorbed oxygen layer (O_ads) and the passage of oxygen inserted under the surface layer of the metal (O_abs) and oxygen dissolved in metal subsurface layers (O_dis) to the surface. O₂ desorption was modeled on a surface with a square lattice of adsorption sites, with account taken of the energy of the lateral repulsive interactions between adjacent O_ads atoms (ε_aa). At ε_aa = 10 kJ/mol and when the activation energy of O₂ desorption for a chemisorbed-oxygen surface coverage of θ ≈ 0 is E_des⁰ = 230 kJ/mol, the calculated spectra are in agreement with the oxygen temperature-programmed desorption (TPD) spectra obtained for Pd(poly) at θ ≤ 0.5. The passage of O_abs and O_dis atoms to the surface was calculated using a first-order equation, with account taken of the activation energy for these atoms coming out to the surface (E₂ and E₃, respectively). As the oxide film is heated, O₂ desorption is accompanied by the passage of O_abs and then O_dis to the surface, which leads to an increase in the O_ads surface coverage and, accordingly, to a buildup of lateral surroundings in the adsorbed layer. Owing to this fact and to the repulsive interactions between O_ads atoms, the bonds between O_ads and the surface weaken and E_des decreases. As a consequence, the O₂ desorption rate increases and a low-temperature peak with T_max ≈ 710 K, which is due to the passage of O_abs atoms to the surface, and then a high-temperature peak with T_max ≈ 770 K, which is due to the passage of O_dis atoms to the surface, appear in the TPD spectrum. At ε_aa = 10 kJ/mol, E_des⁰ = 230 kJ/mol, E₂ = 145 kJ/mol, and E₃ = 160 kJ/mol and when the number of inserted oxygen monolayers is θ_abs ≤ 0.3 and the number of oxygen monolayers dissolved in subsurface layers is θ_dis ≤ 10, the TPD spectra calculated for the given model are in agreement with the O₂ TPD spectra that are observed for Pd(poly) and are due to the decomposition of surface oxide films.     

Thermokinetic studies of the groups on TiO2 surface

The properties of groups on TiO₂ surface were evaluated by infrared (IR) spectra and thermal analysis. It could be found that water and ethanol are removed mainly between 343 and 363 K and the process is accompanied by an endothermic change. The results also show that ethanol could be removed completely below 673 K and the physically bonded or hydrogen‐bonded water could be removed almost completely above 873 K. The exothermic peaks ranging from 573 to 773 K are caused by the changes that chemisorbed water (Ti‐O‐H) on the surface condenses and evolves into water. There appears broad and unstable endothermic process in all temperature ranges due to the adhesions or transformations among the crystal particles. Thermogravimetry (TG) results show that with heat treatment at increasing temperatures, the groups on TiO₂ surface decreases gradually. The half peak breadth of X‐ray diffractometer (XRD) results suggest that TiO₂ crystallize better with higher temperature calcination. The photo‐catalytic efficiency was evaluated by UV‐Vis spectrophotometer. TiO₂ with heat treatment at higher temperature shows more effective photo‐catalytic property. Copyright © 2009 John Wiley & Sons, Ltd.     

Humic substances in surface waters of the Ukraine

The results of studies of humic substances in water bodies of different types (lakes, rivers, and reservoirs) are given. It is shown that concentration of these natural organic compounds varies in a wide range of values (from 1.2 to 126.5 mg L^?1) due to different sources of their formation. The highest concentrations of humic substances are characteristic for rivers of the Pripyat’ River basin flowing through the wetland. As we move from the north to the south, the content of humic substances is reduced. So, in the Kakhovka Reservoir, closing the Dnieper cascade of reservoirs, the concentration of humic substances is almost thrice as low as in the Kiev Reservoir, which is at the head of the cascade. Seasonal changes of humic substances concentration and the reasons for these changes are discussed. The prevailing fraction in the composition of humic substances is represented by fulvic acids, the content of which reaches 80.8–94.8% of the total. The results of studies of the molecular weight distribution of humic substances and the reasons for changes in the ratio of their individual fractions, depending on the detection method (spectrophotometric and fluorescence methods), are considered. The values of the number average (M _n) and weight average (M _w) molecular weight of humic substances and the degree of polydispersity are calculated. It is shown that M _w varies seasonally. In spring and summer it is lower, but significantly increased in autumn. The reason for this phenomenon is degradation of high-molecular fractions of humic substances under the influence of UV light of solar radiation and increased microbiological activity during the summer season. As a result of these processes high-molecular fractions of humic substances are transformed into fractions with lower molecular weight, which become predominant.     

Effect of long-range interactions on the surface tension

The effect of long-range interactions on the surface tension at a liquid-gas interface was considered. An analytical expression for the correction to the surface tension for the cutoff of the particle interaction potential at the distance r _c was derived based on a step density profile. For the Lennard-Jones fluid, this correction was calculated numerically from the results of computer simulations of the density profiles. It was established that, in the vicinity of the triple point, the correction is as great as ～6% at the potential cutoff radius r _c=6.78 molecular diameters, a quantity insensitive to the form of the density profile in the interfacial layer.     

Physicochemistry of the surface of and exoemission from magnesium oxide

To optimize the processes of obtaining MgO with the highest possible exoemission intensity and time of decay, the influence of dehydration conditions and the subsequent adsorption of active gases (H₂, O₂, and H₂O vapor) on its parameters was investigated. The nature of adsorption centers and exoemission is discussed, based on the results obtained using the latest literature data on dehydration processes, as are the formation of defects on the MgO surface (coloring centers) and their interaction with hydrogen and oxygen. Due to the introduction of exoemissionally active MgO in the manufacturing of plasma displays, the literature data dealing with the exposure of extremely active grains of oxide monocrystals containing exoemission centers (OH-groups) are examined.