Non-superaromatic reference for carbon nanotube as a quasi-one-dimensional π-bonding model for graphite

A non-superaromatic reference defined for an armchair tubule is proposed as a simple model for the graphite -electron system. This kind of reference structure is easy to deal with because of its quasi-one-dimensional character. We found that even the non-superaromatic reference for a relatively thin armchair tubule has essentially the same -binding energy per carbon atom as graphite. The accurate -binding energy per carbon atom of graphite turned out to be 1.57459724.     

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.     

An investigation of micro-brownian motions in polydimethylsiloxane by complementary incoherent-neutron-scattering and nuclear-magnetic-resonance experiments below room temperature

Two different experimental methods, neutron scattering and nuclear magnetic resonance are used to investigate the random methyl group and segmental rotational jump motions in polydimethylsiloxane. It is shown that nuclear magnetic relaxation and line width experiments are complementary to incoherent neutron scattering fixed-window experiments, the principle of the fixed-window experiments being discussed in somewhat detail. Satisfactory agreement of these experimental techniques is achieved as to the determination of the two model parameters of the motional processes in question, i. e. the activation energy and the preexponential factor of the Arrhenius ansatz made for the correlation time or jump time, respectively.Contribution partly presented during the meeting of the Macromolecular Club, June 8–10, 1983 in Uppsala, the spring conference of the Deutsche Physikalische Gesellschaft, March 12–17, 1984 in Münster and the Fourth International Seminar on Polymer Physics, October 22–26, 1984 in Eyba (GDR).     

Wetting of Solids by Aqueous Solutions of Surfactant Binary Mixtures: 2. Wetting of High-Energy Surface

Wetting of glass by aqueous solutions of binary mixtures of cationic and nonionic surfactants was studied in the range of overall concentrations c ₀ = 10^–8–10^–2 M at the molar fraction of cationic surfactant = 0.2, 0.5, and 0.8. It was established that the character of glass wetting is determined by the presence of cationic surfactant in the mixture: contact angle isotherms (c ₀) exhibit maximum, as in the case of individual cationic component solutions. Maximal values are virtually independent of the nature of cationic surfactant and its molar fraction in the mixture. It was shown that the synergistic effect in glass wetting is controlled by the chemical structure of surfactant cation.     

Preconcentration technique for nonylphenol using cellulose cotton with homogenous liquid–liquid extraction for liquid chromatographic analysis

A powerful preconcentration method for nonylphenol (NP) has been developed for liquid-chromatography by combining the use of cellulose cotton (solid-phase extraction) with homogeneous liquid–liquid extraction. A 100 ml of sample solution was preconcentrated using cotton, and the eluate obtained (acetonitrile; 5 ml) was further preconcentrated to 50 l within 10 min using a homogeneous liquid–liquid extraction method (volume ratio, 2,000-fold; 100 ml 50 l). The sample concentration increases from preconcentration was 1,599-fold, and NP was extracted into the sedimented phase at 80%. The proposed method was applied to high performance liquid chromatography with fluorescence detection (FL/HPLC); the lowest determination limit obtained was 1.0×10^–9 mol l^–1.     

Liquid–Gas Adsorption Chromatography

The history of the discovery, physical and chemical properties, and analytical potentialities of chromatographic methods using the stationary gas phase, such as liquid–gas and liquid–gas adsorption chromatography, are considered. Taking into account the retention of the stationary gas phase in the pores of a hydrophobic support, one can optimize the conditions for the separation and extraction of volatile substances from aqueous solutions on hydrophobic adsorbents in solid-phase extraction and reversed-phase liquid chromatography. Scientific progress is the progress of a method. M.S. Tswett     

Transition from Incomplete to Complete Wetting of Solid Surface in Polymer–Solvent System: 3. The Regime of Weak Adsorption

The transition from incomplete to complete wetting occurring near the critical temperature of a two-phase polymer–solvent system at the substrate surface that weakly adsorbs macromolecules was studied using the Cahn–de Gennes model. It was shown that, depending on the force of attraction between the segments and the wall, the energy of interaction between the segments in the surface layer, as well as on the length of chain, the wetting transition can occur as the first- or second-order phase transitions, or as the tricritical wetting transition. Near the temperatures of these transitions, we determined the character of the variations in the difference of the surface concentrations that are established at the boundaries between the substrate and semidilute or dilute polymer solutions, as well as in the difference between the interfacial tensions and of the cosine of contact angles. It was shown that the temperature of each transition varies inversely to the square root of the molecular mass of polymer, and its deviation from the critical temperature is determined by the type of transition. At the first-order transitions at the SP-regime, the deviation is proportional to the energy of attraction between the chain units and the wall and is independent of polymer chain length, whereas at the critical wetting it is proportional to the squared energy of attraction between the segments and the substrate and increases with polymer chain length according to the N ^1/2law. At the considered asymptotic regime, which corresponds to the substrates that weakly attract polymer chain units, the type of the wetting transition can be regulated by varying only the length of polymer chain at the same energy characteristics of a substrate. The possibility of observing the critical wetting transitions using the solutions of high-molecular-mass compounds is discussed.     

A constitutional diagram of the system TiC−HfC−“MoC”

The system TiC–HfC–MoC was investigated by means of melting point, differentiothermoanalytical, X-ray diffraction and metallographic techniques on hotpressed as well as melted alloy specimens. A constitutional diagram from 1500°C through the melting range was established.Investigation of the (Hf, Mo)C system (isopleth: HfC_0.98–MoC_1.0) showed a small miscibility gap within the cubic monocarbide solution () [Tc=1630°C, (HfC)_0.45(MoC)_0.55]. The miscibility gap interacts with the solvus curve with a monotectoid-like decomposition reaction at 1575°C, (HfC)_0.35(MoC)_0.65.At temperatures below 1630°C, phase equilibria within TiC–HfC–MoC are characterized by a large miscibility gap connecting the TiC–HfC and HfC–MoC boundary systems. Additions of MoC to TiC–HfC alloys decrease the critical temperature (1780°C); additions of TiC to HfC–MoC alloys raise the critical temperature (1630°C). No maximum type ternary critical point or saddle point was found to occur.Isothermal sections were prepared at 1500°C and 1650°C. At temperatures above 1960°C (-MoC+C-MoC) a complete solid solution (-B 1) is formed within TiC–HfC–MoC. The melting behaviour (liquidus projection of TiC–HfC–MoC) shows flat melting temperatures in the MoC corner but extremely heterogeneous melting near the TiC–HfC boundary.Isothermal sections have been calculated assuming regular solutions.With 5 Figures     

The Quadrature Discretization Method (QDM) in comparison with other numerical methods of solution of the Fokker–Planck equation for electron thermalization

The determination of the relaxation of electrons in atomic gases continues to be an important physical problem. The main interest is the determination of the time scale for the thermalization of electrons in different moderators and the nature of the timedependent electron energy distribution. The theoretical basis for the study of electron thermalization is the determination of the electron distribution function from a solution of the Lorentz–Fokker–Planck equation. The present paper considers a detailed comparison of different numerical methods of solution of the Lorentz–Fokker–Planck equation for the electron distribution function. The methods include a pseudospectral method referred to as the Quadrature Discretization Method (QDM) which is based on nonstandard polynomial basis sets, a finitedifference method, and a Lagrange interpolation method. The Fokker–Planck equation can be transformed to a Schrödinger equation, and methods developed for the solution of either equation apply to the other.     

Structure-reactivity problem in cycloaddition reactions to form heterocyclic compounds

The steric and electronic effects of substituents in both diene and dienophile components have been considered as the main factors that control the electronic character of (4+2)-cycloaddition leading to transitions between normal, neutral, and inverse Diels-Alder reactions.Institut für Organische Chemie der Universität Regensburg, Universitätsstr. 31, D-93040 Regensburg, Germany. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1307–1322, October, 1995. Original article submitted August 15, 1995.     

A Study of the Linear Tension Effect on the Polystyrene Microsphere Wettability with Water

A technique aimed at measuring contact angles for microparticles and suitable for determining the linear tension is developed. The technique is based on determining the equilibrium position of the microsphere (where the sum of forces acting on the microsphere is zero) at the liquid–gas interface. This position is unambiguously determined from the experimental force–distance curve obtained for the microparticle–interface interaction. The measurements are performed using an original setup based on the operating principle of the atomic force microscope. The advancing and receding contact angles of water are measured for individual polystyrene microspheres with radii from 1 to 5 m. The contact angles are shown to increase with a decreasing microsphere radius, whereas the hysteresis of the contact angle decreases. The results indicate the existence of a negative linear tension in the case of the rough surface of polystyrene microspheres, which is deformed along the three-phase contact line.     

Adsorptive Catalytic Voltammetry of Physcion in the Presence of Dissolved Oxygen at a Carbon Paste Electrode

A novel electroanalytical method for the determination of physcion is described for the first time. Physcion yields an adsorption catalytic voltammetric peak at –0.74V (vs. SCE) in 0.4molL^–1 NH₄Cl–NH₃·H₂O buffer solution (pH 10.5) at a carbon paste electrode (CPE). The experimental results indicated that physcion is efficiently accumulated at a CPE by adsorption. In the subsequent potential scan, physcion was reduced to a homologous anthrahydroquinone compound. The compound was then immediately oxidized to physcion by the dissolved oxygen in the solution, and then physcion was again reduced at the CPE. As a result, a cyclic catalytic reaction was established. The second-order derivative peak current is proportional to the physcion concentration in the ranges of 2.0×10^–104.0×10^–9molL^–1 (accumulation 90s) and 4.0×10^–92.0×10^–8molL^–1 (accumulation 60s). The limit of detection is 8×10^–11molL^–1 (S/N=3) for a 120s accumulation time. The method was applied to the direct determination of physcion in the medicinal plant polygonum multiflorum Thumb with satisfactory results.     

Mössbauer study of iron in the grain boundary of aluminium

Iron was diffused into aluminium along grain boundaries and studied by Mössbauer spectroscopy. The Mössbauer spectrum of the investigated sample contained one doublet with isomer shift =0.360/5/ mm s^–1 /relative to -iron/ and quadrupole splitting =0.84/l/ mm s^–1. These results suggest that the iron atoms in the grain boundaries of aluminium have environment very similar to that which is present in amorphous iron /III/ oxides.     

Trace priority pollutant phenols enrichment from water by ion chromatography

Summary The efficiency of ion chromatography columns packed with styrene-divinylbenzene copolymer containing quaternary ammonium groups to preconcentrate phenols at g l^–1 levels has been established. Retention is carried out in acidic medium based on non-polar interactions between the column and phenols. Pure acetonitrile was used for one step elution. Enrichment factors of at least 100 times are achieved. Regeneration of the column can easily be accomplished with a mixture of 60 mM nitric acid: methanol (199). Recoveries from a river water sample for nine priority pollutant phenols were in the range 92–105%. Other retained organic substances do not disturb liquid chromatographic determination of the phenols studied.     

Kinetics of Free Radical Generation in the Catalytic Oxidation of Methanol

The formation of free radicals over the surface of platinum-containing catalysts in the methanol oxidation reaction depending on the temperature, the composition of the reaction mixture, and the procedure used for introducing platinum was studied by the matrix isolation method technique. The nature and transformations of surface intermediates depending on the temperature and the presence of oxygen in the gas phase were studied by Fourier transform IR spectroscopy. The main surface intermediate was the methoxy group. The following three types of these groups were stabilized in alumina-based catalysts: (I) CH₃O–Al_oct (_s(–) = 2806 cm^–1), (II) CH₃O–Al_tetr (_s(–) = 2825 cm^–1), and (III) CH₃O < (Al)₂ (_S(–) = 2845 cm^–1, _s(–) = 1460 cm^–1, _s(–) = 1440 cm^–1, r _|| (₃) = 1185 cm^–1, and (–) = 1095 cm^–1). At the same time, isolated methoxy groups (_as(–) = 2997 cm^–1, _as(–) = 2959 cm^–1, _s(–) = 2857 cm^–1, and (₃) = 1450 cm^–1) and hydrogen-bonded groups ((–) = 3400–3550 cm^–1), which resulted from chemisorption at siloxane bridges, were stabilized in silica gel–based catalysts. It was found that CH₃O and CH₃OO radicals were formed only over the surfaces of pure supports (SiO₂ and Al₂O₃) and their mechanical mixtures with platinum. The total concentration of radicals was described by an extremal function of the composition of reactants, whereas the relative concentration depends on the nature of the support. This is conceivably due to the effect of coordinatively unsaturated cations of the support, which are formed by dehydroxylation in the course of catalyst pretreatment. An increase in the rate of formation of gas-phase radicals on mixed catalysts was explained by special properties of the platinum/support interface region, at which surface intermediates were formed in superequilibrium concentrations under reaction conditions.     

Photosensitivity of paramagnetic centers in radiolyzed solid hydrogen cyanide

The photolysis of solid hydrogen cyanide and the effects of UV light on⁶⁰Co--irradiated HCN at 77 K were studied using an ESR technique. As in the case of radiolysis, the HZ₂C=N radical formed due to sticking of the H atom to the triple bond of the HCN molecule is the main radical product of low-temperature HCN photolysis. The C=N^– radicals are accumulated at 77 K in insignificant amounts (3 %). It was established that radical and ionic products stabilized in y-irradiated HCN possess photochromism and under the action of UV light enter photochemical reactions leading to their decomposition. Upon photobleaching, the concentration of H₂C=N^– radicals first increases two- to threefold because of the decomposition of H₂C=N^– ions and then decreases. The presence of radicals and ions formed upon the low-temperature radiolysis of HCN broadens the optical absorption band of the system, and the boundary of the action spectrum shifts from 280 nm (for nonirradiated HCN) to the visible region at 400–440 nm.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 859–863, April, 1996.     

On the size variation of the primary solvation sphere of Nd3+ and Tm3+ in mixed system of methanol and water

The stability constants, ₁, of each monochloride complex of Nd(III) and Tm(III) have been determined in the mixed system of methanol and water with 1.0 mol·dm^–1 ionic strength using a solvent extraction technique. The values of ₁ of Nd(III) and Tm(III) increase as the mole fraction of methanol in the mixed solvent system (X _s) increases. However, the variation mode of ₁ againstX _s in the region of 0.00X _s0.40 differs from each other, a concave curve for the Nd(III) and a convex curve for the Tm(III). The LnCl²⁺ formed is present as a solvent-shared ion-pair. Since Cl^– is a structure breaking ion, it was assumed that the primary solvation sphere of Ln³⁺ directly contacted with Cl^–. Calculation of Ln³⁺–Cl^– distance using Bom-type equation revealed the followings: (1) for Tm³⁺ with coordination number 8, the estimated distance between Tm³⁺ and Cl^– increases linearly withX _s in 0.00X _s0.40. The results mean an increase of the primary solvation sphere size of Tm³⁺ withX _s. (2) For Nd³⁺, the distance between Nd³⁺ and Cl^– decreases linearly withX _s in 0.00X _s<0.13, where both coordination numbers of 9 and 8 coexist, while it increases withX _s in 0.13<X _s0.40. The results mean a decrease of the primary solvation-sphere size of Nd³⁺ withX _s in 0.00X _s<0.13 and an increase of that withX _s in 0.13<X _s0.40.     

Acylation of alkynes by α-bromobutyroyl tetrafluoroborate

Conclusions The acylation of alkynes by -bromobutyroyl tetrafluoroborate under nonnucleophilic conditions takes place as a reaction involving the conjugated addition of the acylium cation and the bromide anion from the acyl residue over the triple bond by means of a 1, 4 shift.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1077–1080, May, 1978.     

Determination of Heavy Metal Ions in Chinese Herbal Medicine by Microwave Digestion and RP-HPLC with UV-Vis Detection

A new method for the simultaneous determination of heavy metal ions in Chinese herbal medicine by microwave digestion and reversed-phase high-performance liquid chromatography (RP-HPLC) has been developed. The Chinese herbal medicine samples were digested by microwave digestion. Lead, cadmium, mercury, nickel, copper, zinc, and tin ions in the digested samples were pre-column derivatized with tetra-(4-chlorophenyl)-porphyrin (T₄-CPP) to form the colored chelates which were then enriched by solid phase extraction with C₁₈ cartridge and eluted from the cartridge with tetrahydrofuran (THF). The chelates were separated on a Waters Xterra RP₁₈ column by gradient elution with methanol (containing 0.05molL^–1 pyrrolidine-acetic acid buffer salt, pH=10.0) and THF (containing 0.05molL^–1 pyrrolidine-acetic acid buffer salt, pH=10.0) as mobile phase at a flow rate of 0.5mLmin^–1 and detected with a photodiode array detector in the range of 350–600nm. In the original samples the detection limits of lead, cadmium, mercury, nickel, copper, zinc and tin are 4ngL^–1, 3ngL^–1, 6ngL^–1, 5ngL^–1, 2ngL^–1, 6ngL^–1, and 4ngL^–1, respectively. This method was applied to the determination of lead, cadmium, mercury, nickel, copper, zinc and tin in Chinese herbal medicine samples with good results.