Temperature-induced changes in the composition of floatglass surfaces

Floatglass is an important kind of commercial glass, comprising the main body of modern flat glass used in buildings and vehicles. The stoichiometry of the surface layer differs from that of the bulk and will usually change due to subsequent high-temperature process steps or attack by water or humidity. Glass samples have been investigated by means of ion beam analysis. Using resonant nuclear reaction analysis (¹⁵N technique), hydrogen concentration profiles have been determined. Profiles of the heavier glass constituents, especially sodium and tin, have been obtained by RBS. Changes in the composition of the floatglass surfaces caused by subsequent temperature treatment (up to 700^°C) and by controlled hydration treatment are reported. Possible mechanisms of hydrogen uptake and release are discussed.     

Redetermination of the Crystal Structure of the 1,2,4,7-anti-tetramethylbicyclo[2.2.1]heptan-2-yl cation at 110 K

The structure of the 1,2,4,7-anti-tetramethylbicyclo[2.2.1]heptan-2-yl cation ( 1 ) was redetermined by X-ray crystal structure analysis of its Sb₂F₁₁ salt at 110 K (P2₁/c, R1 = 5.76%). The most important structural features of 1 are: C(1)? C(2) = 1.409(9), C(1)? C(6) = 1.710(8), and C(2)…C(6) = 2.113(9) Å and C(2)? C(1)? C(6) = 84.7(4)°. These results agree with those obtained earlier by other methods for the rapidly equilibrating, partially s?-delocalized 1,2-dimethylbicyclo[2.2.1]heptan-2-yl cation ( 2 ). The detailed experimental procedure for the preparation of crystalline 1 · Sb₂F₁₁ and the crystal selection and mounting are described.     

Five-component food-grade microemulsions: structural characterization by SANS

In this paper we present the structural characterization of a five-component food-grade microemulsion containing Tween 80, R(+)-limonene, ethanol, glycerol, and water. Our main approach to investigating the microstructure of dense microemulsions, and how it can be influenced by the various components, was to employ small-angle neutron scattering and the new evaluation technique for dense, interacting systems, the Generalized Indirect Fourier Transformation. We started our investigation with the impact of glycerol and ethanol on Tween 80 micelles in water. We found that glycerol increases the aggregation number and withdraws the hydrating agents from the headgroup region of the surfactant, resulting in a higher packing density of molecules in a micelle at slightly increasing size. The same trend holds when the micelles are oil swollen and/or ethanol is present. Ethanol, on the other hand, redistributes mainly between water and the interface-headgroup region of the surfactant. Part of it replaces surfactant molecules in the micelles, which increases the available interface and results in a higher number of micelles with shrinking size. The same trend holds when the micelles are oil swollen and/or glycerol is present in the aqueous phase. We also investigated samples along the dilution of a mixture of surfactant and oil phase (R(+)-limonene and ethanol), which can be diluted with aqueous phase (mixture of water and glycerol) without the occurrence of phase separation. In some samples of this dilution most probably bicontinuous structures are present. To elucidate this point, we also employed dynamic light scattering, viscosity, and conductivity measurements.     

Development of a robust technique for sampling volatile metal(loid)s in wetlands

The formation of volatile organic and inorganic metals and metalloids in aquatic environments is a known, but not very intensively investigated, process. Several techniques have been developed over the past 10 years to determine these trace components. These techniques are of limited use in wetland environments, where samples have to be taken from the soil-water interface, and require an immediate sample analysis due to thermodynamic instabilities of the volatile metal(loid)s. This paper presents an innovative sampling technique for total concentrations of volatile metal(loid)s in wetlands, based on an in situ gas-water separation via a porous PTFE membrane and stabilising the volatile metal(loid)s in a liquid sorbent (NaOCl solution). Samples may thus be collected even at remote sites, where longer storage times have to be accounted for. The sampling system was tested by means of a laboratory facility simulating the generation of arsine and dimethyl arsine under abiotic conditions as well as under field conditions. Results for sampling efficiency, reproducibility, and long-term storage are presented. Application of the sampling system in the field is shown.     

Matroid bases with cardinality constraints on the intersection

Mathematical Programming - Given two matroids $$\mathcal {M}_{1} = (E, \mathcal {B}_{1})$$ and $$\mathcal {M}_{2} = (E, \mathcal {B}_{2})$$ on a common ground set E with base sets $$\mathcal...     

Photocycloaddition of Cyclohex‐2‐enones to Penta‐1,2,4‐triene

Dihydrothiinone 9a undergoes photocycloaddition regioselectively to all three C?C bonds of penta‐1,2,4‐triene ( 10 ), the relative stabilities of the biradical intermediates determining the product distribution. In contrast, cyclohexenone 9b and dihydropyranone 9c afford more complex mixtures of bicyclo[4.2.0]octanones, which also turn out to be less stable on chromatographic workup, reflecting the higher strain due to the shorter bond lengths (C? O and C? C vs. C? S) in the six‐membered rings, respectively.     

Total synthesis of two 12-nordrimanes and the pharmacological active sesquiterpene hydroquinone yahazunol

The synthesis of two 12-nordrimanes and yahazunol was achieved via 8-oxo-12-nordrimanic acid methyl ester. The cytotoxic activity of yahazunol and seven other sesquiterpene hydroquinones and sesquiterpene quinones has been determined.     

Cathodic stripping voltammetric determination of selenium(IV) at a thin-film mercury electrode in a thiocyanate-containing electrolyte

The well-known method for the determination of selenium(IV), which is based on the cathodic stripping voltammetry of copper(I) selenide, has been adapted for application at the thin-film mercury electrode on glassy carbon (TFME). Insufficient reproducibility and sensitivity have been overcome by using a 0.1 mol/L HClO₄ electrolyte solution containing 0.02 mol/L thiocyanate ions. Thiocyanate ions have been found to increase the peak height of the selenium response and shift it to more positive potentials. This behaviour is explained by an adsorption of SCN^– at the interface glassy carbon/Cu₂Se and its action as an electron transfer catalyst between glassy carbon and copper(I) selenide. A 3σ-detection limit of 75 ng/L Se^(IV) has been achieved. The relative standard deviation is 5.2% at 5 μg/L selenium(IV). The influence of cadmium(II), arsenic(III), zinc(II), iron(III) and lead(II) ions on the selenium response has been studied. In case of lead ions, a new signal occurred at more negative potentials than the reduction of Cu₂Se. This signal, which is probably due to the reduction of PbSe, can also be used for the determination of selenium(IV). Received: 13 November 1996 / Revised: 19 December 1996 / Accepted: 24 December 1996