Surface effects in maghemite nanoparticles

A consistent model is presented for the variation of saturation magnetization with particle size in maghemite nanoparticles, based on the existence of a magnetically disordered layer with a constant thickness of 1 nm. For particles smaller than 3 nm, layer thickness increases rapidly, and M_S is already zero for 2.5 nm particle size. Magnetization measurements have been performed on maghemite–polymer nanocomposites with low size dispersion and a regular distribution of particles in the matrix. A representative number of samples have been studied with a diameter size in the range from 1.5 to 15 nm and ±10% of size dispersion.     

Characterization of natural cellulosic fibers from Yucca aloifolia L. leaf as potential reinforcement of polymer composites

Cellulose - In this work lignocellulosic fibers were obtained from Yucca aloifolia L. leaves and their chemical, morphological, thermal and mechanical properties were studied. The fibers were...     

Layered stable equilibria of a reaction-diffusion equation with nonlinear Neumann boundary condition

In this work we investigate the existence and asymptotic profile of a family of layered stable stationary solutions to the scalar equation ut=ε²Δu+f(u) in a smooth bounded domain Ω⊂R³ under the boundary condition εν_∂u=δεg(u). It is assumed that Ω has a cross-section which locally minimizes area and limε→0εlnδε=κ, with 0?κ<∞ and δε>1 when κ=0. The functions f and g are of bistable type and do not necessarily have the same zeros what makes the asymptotic geometric profile of the solutions on the boundary to be different from the one in the interior.     

A Monte Carlo study of finite temperature λφ 2, 3 4 ⋆

Zeitschrift für Physik C Particles and Fields - We study the finite temperature behaviour of λφ4 theory in two and three dimensions, using Monte Carlo simulations. We renormalize the...     

On optimal zero-preserving corrections for inconsistent linear systems

This paper addresses the problem of finding an optimal correction of an inconsistent linear system, where only the nonzero coefficients of the constraint matrix are allowed to be perturbed for reconstructing a consistent system. Using the Frobenius norm as a measure of the distance to feasibility, a nonconvex minimization problem is formulated, whose objective function is a sum of fractional functions. A branch-and-bound algorithm for solving this nonconvex program is proposed, based on suitably overestimating the denominator function for computing lower bounds. Computational experience is presented to demonstrate the efficacy of this approach.     

Surfactant/oil/water system for the determination of selenium in eggs by graphite furnace atomic absorption spectrometry

An oil-in-water formulation has been optimized to determine trace levels of selenium in whole hen eggs by graphite furnace atomic absorption spectrometry. This method is simpler and requires fewer reagents when compared with other sample pre-treatment procedures. Graphite furnace atomic absorption spectrometric (GF AAS) measurement was carried out using standard addition calibration and Pd as a modifier. The precision, expressed as relative standard deviation, was better than 5% and the limit of detection was 1 µg L^− 1. The validation of the method was performed against a standard reference material Whole Egg Powder (RM 8415), and the measured Se corresponded to 95.2% of the certified value. The method was used for the determination of the Se level in eggs from hens treated with Se dietary supplements. Inorganic and organic Se sources were added to hen feed. The Se content of eggs was higher when hens were fed with organic Se compared to the other treatments. The proposed method, including sample emulsification for subsequent Se determination by GF AAS has proved to be sensitive, reproducible, simple and economical.     

Cross-sections for rotational excitations of C3H4 isomers by electron impact

We report elastic (rotationally summed) and rotationally resolved cross-sections for scattering of low-energy electrons by the C₃H₄ isomers allene, propyne, and cyclopropene, which belong to the D_2d, C_3v, and C_2v groups, respectively. We employed the Schwinger multichannel method with pseudopotentials at the static-exchange approximation, combined with the adiabatic-nuclei-rotation (ANR) approximation to calculate the rotational excitation cross-sections for energies ranging from 5 to 30 eV. Our rotational resolved cross-sections show the isomer effect more strongly related to scattering potentials of different molecular geometries and to transition selection rules than to differences in mass distribution which account for the energy spacing in the rotational spectra of the molecules.     

Solvent effect on the tautomers' stabilities of protonated N,N‐dimethylnitrosamine: The role of hydrogen bonds network

DFT calculations have been applied in order to study the free energies of the structures corresponding to the three different protonation sites of N,N‐dimethylnitrosamine (DMNA). The solvent effect has been taken into account through the study of clusters consisting of protonated DMNA and up to four explicit water molecules, either in the absence or in the presence of a continuum (CPCM) solvation model. Addition of water molecules has been done by a careful screening procedure through which all important hydrogen bonds are likely to be considered. Protonation of DMNA makes all their lone pairs no longer available for hydrogen bond formation with water molecules, such that hydrogen bonds have been observed, for almost all structures, only between water molecules and between one water molecule and the protonated DMNA, in this latter case intermediated by the proton. The stabilities of the solvated structures are governed not only by the number of hydrogen bonds but also by the positions of the water molecules involved in these bonds, as well as by which of them donate or accept H atoms. Our results indicate that oxygen protonation is the most favorable one, regardless of the presence of water molecules. In vacuum protonation at the N‐amino ( 2a ) is approximately as favorable as protonation at the N nitroso ( 2c ). However, in water the former protonation is by far the less favorable one. Our best estimates for the ΔG values in bulk solvent are: ΔG( 2a ) ≈ 17.9, ΔG( 1c ) ≈ 4.3, and ΔG( 2c ) ≈ 4.9 kcal/mol.     

Electrochemical Assay to Quantify the Hydroxyl Radical Scavenging Activity of Medicinal Plant Extracts

A new electrochemical assay has been developed which is based on quantifying the attack of OH^. radicals on a self‐assembled thiol monolayer on electrodes in the presence and absence of antioxidants. The OH^. radicals were produced in Fenton solutions. The radical scavengers diminish the concentration of the OH^. radicals in the Fenton solution, and the extent of thiol layer destruction is monitored with the help of a redox probe in solution, the signal of which is only generated at SAM‐free electrode surface. The results of the electrochemical assay were compared with those of the standard DPPH^. assay. In case of each plant extract, an excellent linear correlation was observed between the radical scavenging activity and the concentration of plant extracts; however, the scavenging properties of different plants strongly differ in the electrochemical and the DPPH^. assay. Since the proposed new method probes the reactivity of radical scavengers towards OH^., one of the most important reactive oxygen radicals in tissues, it is supposed to provide much more relevant information than the DPPH^. or other assays.     

Thermal characterization of usnic acid/collagen-based films

The purpose of this study was to evaluate the physical–chemical properties of collagen (CL) and usnic acid/collagen-based (UAC) films, using differential thermal analysis (DTA), thermogravimetry (TG/DTG), infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Both films were prepared by casting process using polyethylene glycol 1500 (PEG 1500) as plasticizer. In the spectrum of UAC, similar bands of the usnic acid are observed, indicating that the polymerization (film formation) did not affect the stability of the drug. Distinctly, DTA curve of UAC did not show an endothermic peak at 201 °C, indicative that the drug was incorporated into the polymeric system. These results were corroborated by the scanning electron microscopy (SEM). The TG/DTG curves of UAC presented a different thermal decomposition profile compared to the individual compounds and CL. These findings suggest the occurrence of molecular dispersion or solubilization of the drug in the collagen film.