A microspectroscopic study of the electronic homogeneity of ordered and disordered Sr2FeMoO6

Disordered Sr₂FeMoO₆ shows a drastic reduction in saturation magnetization compared to highly ordered samples, moreover magnetization as a function of the temperature for different disordered samples shows qualitatively different behaviours. We investigate the origin of such diversity by performing spatially resolved photoemission spectroscopy on various disordered samples. Our results establish that e_xtensive electronic inhomogeneity, arising most probably from an underlying chemical inhomogeneity in disordered samples, is responsible for the observed magnetic inhomogeneity. It is further pointed out that these inhomogeneities are connected with composition fluctuations of the type Sr₂Fe_1+x Mo_1-x O₆ with Fe-rich (x > 0) and Mo-rich (x < 0) regions. Dedicated to Prof J Gopalakrishnan on his 62nd birthday.     

The role of variable surface charge and surface complexation in the adsorption of humic acid on magnetite

The ionic strength dependence of humic acid (HA) adsorption on magnetite (Fe₃O₄) was investigated at pH 5, 8 and 9, where variable charged magnetite is positive, neutral and negative, respectively. The adsorption studies revealed that HA has high affinity to magnetite surface especially at lower pH, where interacting partners have opposite charges. However, in spite of electrostatic repulsion at pH 9 notable amounts of humate are adsorbed. Increasing ionic strength enhances HA adsorption at each pH due to charge screening. The dominant interaction is probably a ligand-exchange reaction, nevertheless the Coulombic contribution to the organic matter accumulation on oxide surface is also significant under acidic condition. The results from size exclusion chromatography demonstrate that the smaller size HA fractions enriched with functional groups are adsorbed preferentially on the surface of magnetite at pH 8 in dilute NaCl solution.     

Reflection spectroscopy in microstructured light scattering materials

Summary A theoretical model is derived that describes the influence of lateral light diffusion in a scattering medium on the absorptivity of an absorber spot on top of the substrate. The model uses the lateral resolved reflectivity under point irradiation that has been analyzed experimentally with a scanning-micro-laser-reflectometer. The model allows quantification of the absorptivity by one single equation that contains only the mean radial diffusion length of light and the spot area. Experiment and theory are applied to typical substrates for thin layer chromatography (alumina, silica, cellulose). The diffusion lengths in these substrates are given and the absorptivities of the spots are calculated as a function of the spot area.     

Enhancement effect of iron addition for the decomposition of organic mercury as studied by continuous flow analysis with cold vapor atomic absorption spectrometric detection

Summary A continuous flow analysis is described for the determination of total mercury by cold vapor atomic absorption spectrometry. Organic mercury compounds such as methylmercury(II) chloride, ethylmercury(II) chloride and phenylmercury(II) chloride were decomposed by potassium peroxodisulphate with addition of ferric chloride as catalytic reagent. The reducing reagent used was tin(II) chloride in sodium hydroxide solution. With 1,000 mg Fe/l added in the decomposition process, we found that methylmercury(II) chloride and ethylmercury(II) chloride gave response signals similar to those of mercury(II) chloride. The proposed method was applied to the analysis of total mercury in waste water. Permanent address: Department of Chemistry, Faculty of Mathematics and Natural Sciences, Andalas University, Padang, West Sumatra, Indonesia     

Foams and surface rheological properties of β-casein, gliadin and glycinin

Interfacial rheological properties and their suitability for foam production and stability of two vegetable proteins were studied and compared to β-casein. Proteins used ranged from flexible to rigid/globular in the order of β-casein, gliadin and soy glycinin. Experiments were performed at pH 6.7. Network forming properties were characterised by the surface dilational modulus (determined with the ring trough) and the critical falling film length (L_still) at which a stagnant protein film will break. Gliadin had the highest dilational modulus, followed by glycinin and β-casein, whereas glycinin formed the strongest film against fracture in the overflowing cylinder. The rate of decrease in the surface tension was studied at the air–water (Wilhelmy plate method) and the oil–water interface (bursting membrane) and the dynamic surface tension during compression and expansion in the caterpillar. Gliadin had the lowest equilibrium interfacial tensions and β-casein the lowest dynamic surface tension during expansion. Hardly any foam could be formed at a concentration of 0.1 g/l by shaking. At a concentration of 1.4 g/l most foam was formed by β-casein, followed by gliadin and glycinin. It seems that in the first place the rate of adsorption is important for foam formation. For the vegetable proteins, adsorption was slow. This resulted in lower foamability, especially for glycinin.     

Substitution of calcium by strontium within selected calcium phosphates

Sr incorporation in the molecules of amorphous calcium phosphate, apatitic tricalcium phosphate, hydroxyapatite, octacalcium phosphate and dicalcium phosphate dihydrate was investigated. The concentration of Sr ranged from 225 to 1010 μ g / g, i.e. it overlapped with the physiological range of Sr concentrations in human bone. The leading experimental technique was extended X-ray absorption fine structure (EXAFS) at the Sr K edge. Results of these studies demonstrated the following: (1) Sr incorporation in the calcium phosphates is compound-dependent, (2) the coordination of incorporated Sr atoms in the Ca-P molecules is similar to that of Ca atoms, but interatomic distances are ≈0.015 nm larger, (3) in apatitic tricalcium phosphate, hydroxyapatite and octacalcium phosphate lattices Sr atoms may occupy selected Ca sites, which was not the case for dicalcium phosphate dihydrate, (4) in the apatite lattice Sr atoms are coordinated by 6 PO₄ tetrahedrals and (5) EXAFS spectra at the K edge of the incorporated Sr may be used to distinguish the structures of amorphous calcium phosphate, dicalcium phosphate dihydrate as well as apatite and its derivatives (apatitic tricalcium phosphate, octacalcium phosphate).