The interaction of H2, CO, CO2, H2O and NH3 on ZnO surfaces: an Oniom Study

We have used the Oniom method with three layers in order to study the interaction of CO, H₂, H₂O, NH₃ and CO₂ molecules with the ZnO surfaces using a (ZnO)₃₄₈ cluster model. The layers are divided into the high layer at the CCSD level, the medium layer at the RHF level and the low level layer using the UFF force field method. The orbital and binding energies of the adsorbed molecules, Mulliken and ChelpG charges as well as geometrical parameters were analyzed and compared with the available experimental and theoretical data.     

Nonequilibrium molecular dynamics of the rheological and structural properties of linear and branched molecules. Simple shear and poiseuille flows; instabilities and slip

Nonequilibrium molecular-dynamics simulations are performed for linear and branched chain molecules to study their rheological and structural properties under simple shear and Poiseuille flows. Molecules are described by a spring-monomer model with a given intermolecular potential. The equations of motion are solved for shear and Poiseuille flows with Lees and Edward's [A. W. Lees and S. F. Edwards, J. Phys. C 5, 1921 (1972)] periodic boundary conditions. A multiple time-scale algorithm extended to nonequilibrium situations is used as the integration method, and the simulations are performed at constant temperature using Nose-Hoover [S. Nose, J. Chem. Phys. 81, 511 (1984)] dynamics. In simple shear, molecules with flow-induced ellipsoidal shape, having significant segment concentrations along the gradient and neutral directions, exhibit substantial flow resistance. Linear molecules have larger zero-shear-rate viscosity than that of branched molecules, however, this behavior reverses as the shear rate is increased. The relaxation time of the molecules is associated with segment concentrations directed along the gradient and neutral directions, and hence it depends on structure and molecular weight. The results of this study are in qualitative agreement with other simulation studies and with experimental data. The pressure (Poiseuille) flow is induced by an external force F(e) simulated by confining the molecules in the region between surfaces which have attractive forces. Conditions at the boundary strongly influence the type of the slip flow predicted. A parabolic velocity profile with apparent slip on the wall is predicted under weakly attractive wall conditions, independent of molecular structure. In the case of strongly attractive walls, a layer of adhered molecules to the wall produces an abrupt distortion of the velocity profile which leads to slip between fluid layers with magnitude that depends on the molecular structure. Finally, the molecular deformation under flow depends on the attractive force of the wall, in such a way that molecules are highly deformed in the case of strong attracting walls.     

Synthesis and characterization of Zn1−xNixFe2O4 spinels prepared by a citrate precursor

Nanocrystalline single-phase samples of Zn_1−xNi_xFe₂O₄ ferrites (0<x<1) have been obtained via a soft-chemistry method based on citrate-ethylene glycol precursors, at a relatively low temperature (650 °C). The influence of the nickel and zinc contents as well as that of heat treatments were investigated by means of X-ray powder diffraction, Brunauer-Emmett-Teller (BET) surface area, scanning electron microscopy (SEM) and Fourier Transform Infrared (FTIR) Spectroscopy. Higher Ni content increases the surface areas, the largest one (∼20 m²/g) being obtained for NiFe₂O₄ annealed at 650 °C for 15 h. For all compositions, the surface area decreases for prolonged annealing at 650 °C and for higher annealing temperatures. Those results were correlated to the particle size evolution; the smallest particles (∼50 nm) observed in the NiFe₂O₄ sample (650 °C, 15 h) steadily increase as Ni ions were replaced by Zn, reaching ∼100 nm in the ZnFe₂O₄ sample (650 °C, 15 h). For all the Zn_1−xNi_xFe₂O₄ samples and, whatever the heat treatments was, the FTIR spectra show two fundamental absorption bands in the range 650-400 cm⁻¹, characteristics of metal vibrations, without any superstructure stating for cation ordering. The highest ν₁-tetrahedral stretching, observed at ∼615 cm⁻¹ in NiFe₂O₄, shifts towards lower values with increasing Zn, whereas the ν₂-octahedral vibration, observed at 408 cm⁻¹ in NiFe₂O₄, moves towards higher wavenumbers, reaching 453 cm⁻¹ in ZnFe₂O₄.     

Persistence of layer motifs in the crystal structures of [1,4-bis-(2-methoxy-2-carbonylethyl)piperazinium (2’)] chloride and [1,4-bis-(2-amidoethyl)piperazinium (2’)] perchlorate

Two structures of the closely related piperazinium salts, [1,4-bis-(2-methoxy-2-carbonylethyl)piperazinium (2+)] chloride (1) and [1,4-bis-(2-amidoethyl)piperazinium (2+)] perchlorate (2) reveal the persistence of a layer structure, interspersed with anions, held in place by N–H···Cl, C–H···O and C–H···Cl interactions in the case of 1, and N–H···O and C–H···O interactions in the case of 2. Compound 1 crystallizes in the triclinic space group P-1 with a=6.7667(6) ?, b = 6.9467(6) ?, c=9.3880(18) ?, α=105.845(9)°, β=99.896(16)°, γ=103.600(9)°, and Z=1. Compound 2 crystallizes in the monoclinic space group P2₁/c with a = 9.0930(8) ?, b=10.0525(11) ? c=10.2104(13) ?, β=114.279(9)°, and Z=2.     

Physicochemical,Structural, Thermal and Rheological Properties of Flour and Starch Isolated from Avocado Seeds of Landrace and Hass Cultivars

The objective of this study was to obtain and characterize flours and starches from the avocado seeds of Hass and landrace cultivars. The morphological, physical-chemical, structural, thermal and rheological characteristics were evaluated. The flour yield of the Hass and landrace cultivars was 41.56 to 46.86% (w/w), while for starch, it was 35.47 to 39.57% (w/w) (cv. Hass and landrace, respectively). Scanning electron microscopy (SEM) revealed the presence of oval starch granules and other particles in flour, in contrast to flours, starches showed lower ash, proteins and lipids content. However, the amylose content was higher in starches (42.25–48.2%). Flours showed a higher gelatinization temperature (T_p = 73.17–73.62 °C), and their starches presented greater gelatinization enthalpy (∆H_gel = 11.82–13.43 J/g). All samples showed a B-type diffraction pattern, and the crystallinity was higher in the flours. The rheological analysis (flow curves and viscoelastic tests) evidenced a pseudoplastic (n = 0.28–0.36) behavior in all samples analyzed, but the consistency index (k) was higher in starches. In general, the flours and starches from avocado seeds presented interesting proximal, thermal and functional properties for possible application in food systems, and these findings could contribute to the revaluation of this by-product.     

Electrorheology of nematic liquid crystals in uniform shear flow

A hydrodynamic model for the electrorheological effect in a polymeric nematic confined in a rectangular cell is studied. The competition between a constant electric and a uniform shear flow is explicitly considered. For the final stationary state where the induced reorientation of the director has already occurred, we show that the averaged viscosity is enhanced. For this same state several rheological properties such as the first normal stress difference and the force between the cell plates are also analytically calculated as a function of position, the applied field, and Reynolds' number. These results are compared with those obtained previously for a pressure driven flow. The scope and limitations of the model and methods employed are discussed. Received: 23 November 1999/Accepted: 13 June 2000     

Towards a loop representation for quantum canonical supergravity

We study several aspects of the canonical quantization of supergravity in terms of the Ashtekar variables. We cast the theory in terms of a GSU(2) connection and we introduce a loop representation. The solution space is similar to the loop representation of ordinary gravity, the main difference being the form of the Mandelstam identities. Physical states are in general given by knot invariants that are compatible with the GSU(2) Mandelstam identities. There is an explicit solution to all the quantum constraint equations connected with the Chern-Simons form, which coincides exactly with the Dubrovnik version of the Kauffman polynomial. This provides for the first time the possibility of finding explicit analytic expressions for the coefficients of that knot polynomial.     

Mueller matrix determination for one-dimensional rough surfaces: Four reduced measurement equivalent sets

There are several methods for the experimental determination of the Mueller matrix (MM) for a general scattering system. For the case of scattering in the plane-of-incidence, we apply the direct method of Mueller matrix ellipsometry to determine the MM associated to a one-dimensional scattering system. We show the MM for a 1-D surface can be determined with four different measurements from four equivalent sets consisting of four Stokes vector inputs and four analyzers. We show all the possible measurements can be reduced to a single set of four different intensity measurements.     

Asymmetric synthesis of serinol-monoesters catalyzed by amine transaminases

The asymmetric synthesis of serinol-derivatives was investigated employing different amine transaminases as biocatalysts. Under the optimized conditions conversions up to 92% and excellent enantiomeric excesses up to 99% ee were obtained providing access to both, the (R)- and (S)-configurations of the serinol-monoester (2-amino-3-hydroxypropyl hexanoate).     

Kinetic Modeling of Solvent-Free Lipase-Catalyzed Partial Hydrolysis of Palm Oil

This work reports the experimental data and kinetic modeling of diacylglycerol (DAG) production from palm oil using a commercial immobilized lipase (Lipozyme RM IM) in a solvent-free medium. The experiments were performed in batch mode, at 55?°C and 400?rpm, and the effects of enzyme concentration (0.68?C2.04?wt% related to the mass of substrates), initial water concentration (5?C15?wt% related to the mass of oil), and reaction time were evaluated. A novel kinetic model is presented based on the ordered-sequential bi?Cbi mechanism considering hydrolysis and esterification steps, in which a correlation between water-in-oil solubility and surfactant molecules concentration in the oil allowed the model to describe the induction period in the beginning of the hydrolysis reaction. Moreover, mass transfer limitations related to the enzyme concentration in the system were also taken into account. The proposed model presented a very satisfactory agreement with the experimental data, thus allowing a better understanding of the reaction kinetics. The best conditions obtained for the product (partially hydrolyzed palm oil) in terms of DAG yield (35.91?wt%) were 2.87?wt% enzyme/substrate, 2.10?wt% water/oil, and 72?h of reaction.