Cinchonidine adsorption on gold and gold-containing bimetallic platinum metal surfaces: an attenuated total reflection infrared and density functional theory study

Adsorption of cinchonidine on monometallic Au and bimetallic Pt-Au and Pd-Au thin model films prepared by physical vapor deposition has been investigated with attenuated total reflection infrared (ATR-IR) spectroscopy. On Au the alkaloid forms an adsorbed layer that shows higher stability against desorption than the corresponding adsorption on Pt. In this adsorption layer the intermolecular interactions dominate over metal-adsorbate interactions as indicated by the absence of the spectroscopic features attributed to strongly flat adsorbed species. This behavior is further supported by Density Functional Theory (DFT) calculations indicating that flat and tilted orientations of the quinoline ring have comparable adsorption energy on Au but lower (7-10 kcal/mol) compared to adsorption on Pt (ca. 40 kcal/mol). As a consequence, the creation of a metal surface with isolated chiral sites is prevented by formation of an adsorbed structure formed by intermolecularly bound cinchonidine molecules on Au. While the binding to Pt is due to the formation of sigma-bonds to surface atoms, such aggregates are bound to Au mainly by van der Waals forces. Given this different nature of bonding of cinchonidine to Au and Pt, addition of Au to Pt and Pd films could be used to probe the changes of fractional coverage of the different adsorbed species of cinchonidine on the platinum metals. It is demonstrated that the lowering of the domain size of the platinum group metal by Au can simulate the effect of particle size on the distribution of the surface conformations of the alkaloid on a metal surface.     

Impact of Zeolite Framework Composition and Flexibility on Methanol-To-Olefins Selectivity: Confinement or Diffusion?

The methanol-to-olefins reaction catalyzed by small-pore cage-based acid zeolites and zeotypes produces a mixture of short chain olefins, whose selectivity to ethene, propene and butene varies with the cavity architecture and with the framework composition. The product distribution of aluminosilicates and silicoaluminophosphates with the CHA and AEI structures (H-SSZ-13, H-SAPO-34, H-SSZ-39 and H-SAPO-18) has been experimentally determined, and the impact of acidity and framework flexibility on the stability of the key cationic intermediates involved in the mechanism and on the diffusion of the olefin products through the 8r windows of the catalysts has been evaluated by means of periodic DFT calculations and ab initio molecular dynamics simulations. The preferential stabilization by confinement of fully methylated hydrocarbon pool intermediates favoring the paring pathway is the main factor controlling the final olefin product distribution.     

Three-State Opinion Model on Complex Topologies

We investigate opinion diffusion on complex networks and the interplay between the existence of neutral opinion states and non-trivial network structures. For this purpose, we apply a three-state opinion model based on magnetic-like interactions to modular complex networks, both synthetic and real networks extracted from Twitter. The model allows for tuning the contribution of neutral agents using a neutrality parameter. We also consider social agitation, encoded as a temperature, that accounts for random opinion changes that are beyond the agent neighborhood opinion state. Using this model, we study which topological features influence the formation of consensus, bipartidism, or fragmentation of opinions in three parties, and how the neutrality parameter and the temperature interplay with the network structure.     

A comparison of extensional viscosity measurements from various RME rheometers

The transient uniaxial extensional viscosity η _e of linear low density polyethylene (LLDPE) has been measured using the commercial Rheometric Scientific RME and the Münstedt Tensile Rheometer in an effort to compare the performance of available extensional rheometers. The RME indicated a significant strain hardening of the LLDPE, especially at a strain rate of 1 s⁻¹. In contrast, the Münstedt rheometer showed the LLDPE to be only slightly strain hardening. This artificial strain hardening effect in the RME resulted from the strain rate applied to the sample, determined from the sample deformation, being up to 20% less than the set strain rate. These results initiated a round-robin experiment in which the same LLDPE was tested on several RMEs in various locations around the world. All but one of the RMEs indicated a deviation between set and applied strain rates of at least 10%, especially at strain rates above 0.1 s⁻¹. The strain rate deviation was found to depend strongly on the value of the basis length L ₀ , and may result from the upper pair of belts not properly gripping the sample during extension. Thus visual inspection of the sample deformation is necessary to determine the applied strain rate. The most accurate measurements of η _e with respect to the strain rate deviation were obtained when the correct L ₀ value and belt arrangement were used. A list of recommendations for running an RME test is provided. Future work focusing on the fluid mechanics during the test may identify fully the cause of the strain rate deviation, but from a practical point of view the problem can be corrected for in the determination of η _e . Received: 27 September 2000/Accepted: 5 February 2001     

Differential pulse polarographic determination of traces of iron in solar-grade silicon

Iron is determined, after volatilization of the matrix as hexafluorosilicic acid, by means of the polarographic iron(III) wave in a 0.1 M triethanolamine—0.1 M potassium bromate—0.5 M sodium hydroxide medium. Differential pulse polarography provides a detection limit of about 0.15 μg g^-1 with a precision of 1–2% and linear calibration graphs up to 0.5 μg Fe(III) ml^-1.     

Perceptually weighted optical flow for motion-based segmentation inMPEG-4 paradigm

In the MPEG-4 paradigm, the sequence must be described in terms of meaningful objects. This meaningful, high-level representation should emerge from low-level primitives such as optical flow and prediction error which are the basic elements of previous-generation video coders. The accuracy of the high-level models strongly depends on the robustness of the primitives used. It is shown how perceptual weighting in optical flow computation gives rise to better motion estimates which consistently improve motion-based segmentation compared to equivalent unweighted motion estimates     

Surface-pressure isotherms of monolayers formed by microsize and nanosize particles

The thermodynamic model of a 2D solution developed earlier for protein monolayers at liquid interfaces is generalized for monolayers composed of micro- and nanoparticles. Surface pressure isotherms of particle monolayers published in the literature for a wide range of particles sizes (between 75 microm and 7.5 nm) are described by the theoretical model with one modification. The calculations of surface pressure pi on area A provide satisfactory agreement with the experimental data. The theory also yields reasonable cross-sectional area values of the solvent molecule water in the range between 0.12 and 0.18 nm2, which is almost independent of particle size. Also, the area per particle in a closely packed monolayer obtained from the theory is quite realistic.     

High-performance liquid chromatography determination of phenyllactic acid in MRS broth

Phenyllactic acid (PLA) is an organic acid produced by some strains of lactic acid bacteria (LAB) and concentrations higher than 7.5 mg/ml inhibit growth of moulds and yeasts. Since PLA can be used to select LAB, a rapid, simple and cheap method for its determination is desirable. Typical methods for its analysis in broth are time-consuming, analytically complicated, and have poor recoveries. Herein we propose a simple and rapid method that does not require extraction, but only microfiltration of broth before injection in HPLC. The improved chromatographic conditions allow separation and quantification of PLA with a recovery of 98.7%. The method is highly reproducible with an intraday repeatability of the total peak area of 2.00%, while an interday repeatability of 2.69%.     

Structural characterization of complexes between iminodiacetate blocked on styrene-divinylbenzene matrix (Chelex 100 resin) and Fe(III), Cr(III), and Zn(II) in solid phase by energy-dispersive X-ray diffraction.

Local structure of Fe(III), Cr(III), and Zn(II) cations has been determined on the amorphous sample by means of the difference method used for liquid systems. We recorded energy-dispersive X-ray diffraction spectra of a chelating resin (Chelex 100), containing paired iminodiacetate ions coupled to a styrene-divinylbenzene support, in several ionic forms. Coordination geometry of Fe(III), Cr(III), and Zn(II) metal cations with Chelex 100 resin ligand sites, and conformation of the ligand groups have been determined.