Density Of Shell Research Articles

Local and global Casimir energies for a semitransparent cylindrical shell

The local Casimir energy density and the global Casimir energy for a massless scalar field associated with a λδ-function potential in a (3 + 1)-dimensional circular cylindrical geometry are considered. The global energy is examined for both weak and strong coupling, the latter being the well-studied Dirichlet cylinder case. For weak coupling, through , the total energy is shown to vanish by both analytic and numerical arguments, based both on Green's-function and zeta-function techniques. Divergences occurring in the calculation are shown to be absorbable by renormalization of physical parameters of the model. The global energy may be obtained by integrating the local energy density only when the latter is supplemented by an energy term residing precisely on the surface of the cylinder. The latter is identified as the integrated local energy density of the cylindrical shell when the latter is physically expanded to have finite thickness. Inside and outside the δ-function shell, the local energy density diverges as the surface of the shell is approached; the divergence is weakest when the conformal stress tensor is used to define the energy density. A real global divergence first occurs in , as anticipated, but the proof is supplied here for the first time; this divergence is entirely associated with the surface energy and does not reflect divergences in the local energy density as the surface is approached.

High-precision measurements of cementless acetabular components using model-based RSA: An experimental study

Background In RSA, tantalum markers attached to metal-backed acetabular cups are often difficult to detect on stereo radiographs due to the high density of the metal shell. This results in occlusion of the prosthesis markers and may lead to inconclusive migration results. Within the last few years, new software systems have been developed to solve this problem. We compared the precision of 3 RSA systems in migration analysis of the acetabular component.Material and methods A hemispherical and a nonhemispherical acetabular component were mounted in a phantom. Both acetabular components underwent migration analyses with 3 different RSA systems: conventional RSA using tantalum markers, an RSA system using a hemispherical cup algorithm, and a novel modelbased RSA system.Results We found narrow confidence intervals, indicating high precision of the conventional marker system and model-based RSA with regard to migration and rotation. The confidence intervals of conventional RSA and model-based RSA were narrower than those of the hemispherical cup algorithm-based system regarding cup migration and rotation.Interpretation The model-based RSA software combines the precision of the conventional RSA software with the convenience of the hemispherical cup algo-rithm-based system. Based on our findings, we believe that these new tools offer an improvement in the measurement of acetabular component migration.

Formation and morphological characteristics of selenium-containing nanostructures based on rigid-chain cellulose derivatives

Nanostructures arising from the reduction of ionic selenium by a selenite-ascorbate redox system in aqueous solutions of oxyethyl cellulose, methyl cellulose, and carboxymethyl cellulose have been studied by using a set of optical methods (flow birefringence and static and dynamic light scattering) and viscometry. The adsorption of a substantial amount of macromolecules (up to 3200) on selenium nanoparticles has been experimentally discovered. This effect leads to the formation of superhigh-molecular-mass spherical nanostructures with a high density of the polymer shell. The thermodynamic state of solutions of nanostructures has been characterized. In the region of occurrence of stable dispersions, the values of the free energy of macromolecule-selenium nanoparticle interaction have been calculated for polymer nanostructures. Radii of amorphous selenium nanoparticles occurring in the nuclei of nanostructures and the thickness of the polymer shell have been estimated. Given the fixed molecular mass and comparable rigidity of a polymer matrix, the structure of the monomer unit of the cellulose derivative defines the morphology of the nanostructure being formed.

Cover Picture: Dendritic Encapsulation – “Postsynthetic” Functionalizations of a Single Benzophenone Shielded by Shape‐Persistent Polyphenylene Dendrons (Eur. J. Org. Chem. 11/2006)

AbstractThe cover picture shows shape‐persistent first‐ and second‐generation polyphenylene dendrimers grown from a single benzophenone unit as core. To investigate the influence of the size and the density of the dendritic shell upon the reactivity of the core, “postsynthetic” chemical functionalizations with reagents of different size as well as alkali‐metal reduction of the encapsulated benzophenone were performed. Details are discussed in the article by K. Müllen et al. on p. 2523 ff.

Dendritic Encapsulation – “Postsynthetic” Functionalizations of a Single Benzophenone Shielded by Shape‐Persistent Polyphenylene Dendrons

AbstractFirst‐ and second‐generation polyphenylene dendrimers were synthesized starting from benzophenone as core. Variation of the size and the density of the dendrimer shell resulted in different isolation of the cores. To investigate the effect of shielding upon the reactivity of the core, chemical functionalizations as well as the alkali‐metal reduction of the encapsulated benzophenone core were performed. The herein presented synthetic concept opens the way to spatially well‐defined spherical nanoparticles bearing a single isolated function in the center. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Release behavior of microspheres from cross-linked N-methylated chitosan encapsulated ofloxacin

Abstract A series of hollow microspheres encapsulated with ofloxacin have been successfully prepared, using cyclohexane droplets as a template and the N-methylated chitosan (NMC) cross-linked with glutaraldehyde as the shell. The structure and morphology were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The swelling and releasing behaviors of the microspheres having different weight-average molecular weights (Mw) and the degree of quaternization of NMC at pH 1.2 and 7.4 media were investigated. The results revealed that microspheres exhibited a very smooth surface, and electrostatic interaction existed between cross-linked NMC and ofloxacin. Ofloxacin encapsulated in the microspheres was rapidly released into phosphate buffer solution (pH 7.4), whereas it was only slowly released in 0.1 M HCl (pH 1.2). The degree of swelling of microspheres at pH 7.4 was higher than that at pH 1.2. Moreover, ofloxacin was released more slowly from the microspheres having high Mw of NMC than that with low Mw. The release mechanism of the hollow microsphere was proposed to be a non-Fickan diffusion through the swollen microspheres, and to be controlled by the Mw and the cross-linking density of shell.

XPS Characterization of Au (Core)/SiO2 (Shell) Nanoparticles

Core-shell nanoparticles with ca. 15-nm gold core and 6-nm silica shell were prepared and characterized by XPS. The Au/Si atomic ratio determined by XPS is independent of the electron takeoff angle because of the concentric spherical shape of the nanoparticles. The formula given by Wertheim and DiCenzo (Phys. Rev. B 1988, 37, 844) for spherical nanoparticles and the modified one by Yang et. al. (J. Appl. Phys. 2005, 97, 024303) for core-shell nanoparticles are used to correlate the XPS-derived composition with the geometry of the nanoparticles only after significantly modifying either the bulk density of the silica shell or the attenuation length of the photoelectrons.

Experimental study of gas-puff z-pinch plasma

A small neon gas-puff z-pinch device was constructed, and the plasma implosion and its radiation characteristics were studied experimentally. The plasma implosion was investigated using a three-frame Mach–Zehnder interferometer (TFMZI) that is capable of taking three pictures (5 ns exposure and 13 ns interpicture delay) within a single z-pinch shot. Thermopiles were also used in the experiments with the TFMZI to study the correlation between the total energy of x-ray emission and the plasma implosion. The ion beams from the z-pinch plasma were analysed using a compact Thomson spectrometer. Based on the experiments, the imploding velocity and electron density of the plasma shell were obtained, a conclusion that a nearly uniform and symmetric imploding plasma shell would emit a higher x-ray yield than an asymmetric one was drawn and the charge state resolved energy spectra of ion beams emitted from the z-pinch plasma were plotted.

Molecular Dynamics Simulations of Staphylococcal Nuclease: Properties of Water at the Protein Surface

We report on a molecular dynamics study of fully hydrated, native staphylococcal nuclease (SNase). Besides the global structural properties and fluctuations of the protein structure, a detailed analysis of the structural properties of the water at the protein surface (density profile, coordination numbers, hydrogen-bond distribution) was carried out. Taking into account the excluded volume effect, we found that the average density of the first hydration shell of SNase is about 0.3−0.6% larger than that of the bulk solvent. The perturbation of the water hydrogen-bonded network extends roughly two or three water layers from the protein surface at ambient temperature (300 K), and a bimodal density profile of water is observed at the protein interface. The data clearly show that the first peak of the water density profile arises from water molecules that are bonded to polar atoms (N and O) at the surface of SNase, whereas the second peak corresponds to the localization of water molecules near apolar atoms (C)...

The nature of the Ly$\mathsf{\alpha}$-emission region of FDF-4691

In order to study the origin of the strong Lya emission of high-redshift starburst galaxies we observed and modeled the rest-frame UV spectrum and the Lya emission of the z = 3.304 galaxy FDF-4691 (rest-frame EW Lyα = 103 A). The observations show that FDF-4691 is a young starburst galaxy with a (for this redshift) typical metallicity. The broad, double-peaked profile of the Lya emission line can be explained assuming a highly turbulent emission region in the inner part of the starburst galaxy, and a surrounding extended shell of low-density neutral gas with a normal dust/gas ratio and with Galactic dust properties. The detection of the Lya emission line is explained by the intrinsic broad Lya emission and a low HI column density of the neutral shell. A low dust/gas ratio in the neutral shell is not needed to explain the strong Lya line.

Partial Molar Volumes of mRNA 5′ Cap Analogues

AbstractFor Abstract see ChemInform Abstract in Full Text.

Partial Molar Volumes of mRNA 5′ Cap Analogues

Partial molar volumes in aqueous solution of eleven selected 7-methylguanine cap-analogues and their guanine counterparts were determined by means of density measurements. Hydrophobicity of the investigated compounds regarding their structural features was analysed within the framework of the solute-solvent interaction model, based on the relative density of the molecular solvation shell.

TRAPPING PHOTONS BY A LINE SINGULARITY

We present cylindrically symmetric, static solutions of the Einstein field equations around a line singularity such that the energy momentum tensor corresponds to infinitely thin photonic shells. Positivity of the energy density of the thin shell and the line singularity is discussed. It is also shown that thick shells containing mostly radiation are possible in a numerical solution.

Glass-Coated, Analyte-Tagged Nanoparticles: A New Tagging System Based on Detection with Surface-Enhanced Raman Scattering

Glass-coated, analyte-tagged nanoparticles (GANs) are core−shell particles where a nanometer-scale Au or Ag core is functionalized with Raman active molecules and encapsulated in a glass shell. The glass shell provides the particle with mechanical and chemical stability. Specifically, the glass coating renders the particle amenable to use in many solvents without altering the Raman spectral response and makes agglomeration a nonfactor. The density and thickness of the glass shell are controllable through synthetic conditions; thus, the rate of diffusion through the silica network can be tuned and the metal cores kept sequestered from any exterior reaction. This will allow for the attachment of biomolecules to the glass shell without altering the Raman response. GANs can be identified by the Raman spectrum of the attached Raman tag, and two differently labeled samples are unambiguously identified. The scattering from the Raman tag is amplified through surface-enhanced Raman scattering. The narrow bandwidth (∼20 cm-1) of the Raman peaks and fingerprint-like spectra will allow multiple Raman tags to be simultaneously evaluated with a single excitation source.

Volumetric investigations of solute–solvent interactions of cytosine and their methyl-, methoxy-, and hydroxy-derivatives in methanol and N, N-dimethylformamide at 25 °C

Experimentally determined densities of cytosine and their methyl-, methoxy-, and hydroxy-derivatives at 25 °C in methanol and N, N-dimethylformamide are presented. The apparent molar volumes and then the partial molar volumes V 2 0 were calculated. Together with previously obtained V 2 0 for aqueous solutions of compounds studied: (1) the transfer functions Δ V 2,tr 0 water–methanol, water– N, N-dimethylformamide; (2) the contributions to V 2 0 value which correspond to substitution on cytosine moiety of a hydrogen atom on a N, O, C atom by CH 3 group and replacement of a hydrogen on N 4 by an OH group were determined and discussed. Structural parameters: molecular accessible surface S M areas and their atomic partition S N, S O, S H(N,O), molecular volumes V 2 M of compounds studied were calculated. Its enables to determine the correlations between volumetric properties and structural parameters according to the model which is based on the assumptions that density of solvation shell α depends on polarity P and the structure of the hydrophobic compounds. The transfer functions Δ V 2,tr 0 water–methanol, water– N, N-dimethylformamide were also calculated and discussed.

Volumetric investigations of solute-solvent interactions of some alkylated derivatives of uracil in methanol, and N,N-dimethylformamide at 25°C

Densities of uracil, thymine, 1,3-dimethyluracil, 1,3,6-trimethyluracil, 1,3-dimethyl-6-ethyluracil, 1,3-dimethyl-6-propyluracil, 1,3-diethylthymine in methanol and N,N-dimethylformamide solutions at 25°C by means of Anton Paar DMA 60/602 digital densimeter were determined, then the partial molar volumes V 2 0 were evaluated. The contribution of CH 2 groups present at the molecules studied in the V 2 0 value, using the simple additivity scheme, were calculated. The linear correlations V 2 0= f( n CH 2 ) were found. The model based on the assumption that the density of the solvation shell depends on the structure and polarity P of the solute molecule was applied. The structural parameters of the compounds investigated: molecular volumes V 2 M , volumes of solvation shell V 1, solv and molecular surface areas S M, and their atomic partitions S N, S 0, S H(N) were calculated. The determinations of P values were made assuming that both acceptors and donors of protons in aqueous solutions are involved into the specific solute-solvent interactions. Methanol more active as a donor than acceptor of protons interacts with the oxygen and nitrogen atoms, whereas DMF interacts solely with the hydrogen atom of the solute molecule. The transfer functions of ΔV 2,tr 0 water-methanol, water-N,N-dimethylformamide were also calculated.

X-ray photoelectron spectroscopic studies of zinc–tellurite glasses

Zinc–tellurite glasses with chemical composition [(ZnO) x (TeO 2) 100− x ], where x=25, 30 and 35 have been prepared and investigated by X-ray photoelectron spectroscopy (XPS). Zn 2p peaks shift by about 0.25 eV towards higher binding energy in the zinc–tellurite glasses in comparison to its value in ZnO powder, while the Te 3p, Te 3d, and O 1s core levels for the glasses remain essentially unchanged from those of TeO 2 powder. Each O 1s spectrum is deconvoluted into two peaks, in which the higher energy one is due to oxygen atoms in the hydroxides covering the sample surface. Although both bridging oxygen (BO) and non-bridging oxygen (NBO) atoms should certainly exist in these tellurite glasses, the electrons are delocalised in the NBO–Te–BO bonds which equalize the electronic density of the valence shell between BO and NBO atoms and make it difficult to distinguish.

X-Ray and neutron scattering analyses of hydration shells: a molecular interpretation based on sequence predictions and modelling fits

Solution scattering is a low resolution diffraction method that provides important structural data on proteins. The ability to model scattering curves by recourse to known crystal structures for proteins under study significantly improves the resolution (and the utility) of the method because of the strict constraints that the crystal structures impose. For these structure determinations, a molecular description of the effect of hydration shells is needed. In calibration studies used for X-ray scattering curve modelling, it has been reproducibly found that a hydration shell is required. In molecular terms, this results from the higher electron density of the hydration shell compared to that of bulk water, which then becomes similar to that of the protein. This is well represented by a level of 0.3 g H 2O/g glycoprotein and a water molecule volume of 0.0245 nm 3. Procedures for the addition of a hydration shell to a sphere model of a protein are described. For neutron scattering fits, it is not necessary to incorporate a hydration shell, as to a good approximation this is not detectable. In molecular terms, this apparent absence of the neutron hydration shell results from the effect of proton exchange on the scattering densities of bulk water and bound water which causes these to be similar but different from that of the protein.

Photochemical instability of CdSe nanocrystals coated by hydrophilic thiols.

The photochemical instability of CdSe nanocrystals coated by hydrophilic thiols was studied nondestructively and systematically in water. The results revealed that the photochemical instability of the nanocrystals actually included three distinguishable processes, namely the photocatalytic oxidation of the thiol ligands on the surface of nanocrystals, the photooxidation of the nanocrystals, and the precipitation of the nanocrystals. At first, the thiol ligands on the surface of a nanocrystal were gradually photocatalytically oxidized using the CdSe nanocrystal core as the photocatalyst. This photocatalytic oxidation process was observed as a zero-order reaction in terms of the concentration of the free thiols in the solution. The photogenerated holes in a nanocrystal were trapped onto the thiol ligands bound on the surface of the nanocrystal, which initiated the photooxidation of the ligands and protected the nanocrystal from any photooxidation. After nearly all of the thiol ligands on the surface of the nanocrystals were converted into disulfides, the system underwent several different pathways. If the disulfides were soluble in water, then all of the disulfides fell into the solution at the end of this initial process, and the nanocrystals precipitated out of the solution without much variation over their size and size distribution. When the disulfides were insoluble in water, they likely formed a micelle-like structure around the nanocrystal core and kept it soluble in the solution. In this case, the nanocrystals only precipitated after severe oxidation, which took a long period of time. If the system contained excess free thiol ligands, they replaced the photochemically generated disulfides and maintained the stability and solubility of the nanocrystals. The initiation stage of the photooxidation of CdSe nanocrystals themselves increased as the thickness and packing density of the ligand shell increased. This was explained by considering the ligand shell on the surface of a nanocrystal as the diffusion barrier of the oxygen species from the bulk solution into the interface between the nanocrystal and the surface ligands. Experimental results clearly indicated that the initiation stage of the photooxidation was not caused by the chemical oxidation of the system kept in air under dark conditions or the hydrolysis of the cadmium-thiol bonds on the surface of the nanocrystals, both of which were magnitudes slower than the photocatalytic oxidation of the surface ligands if they occurred at all. The results described in this contribution have already been applied for designing new types of thiol ligands which dramatically improved the photochemical stability of CdSe nanocrystals with a ligand shell that is as thin as approximately 1 nm.

The peak CT number of profiles perpendicular to the vertebral cortical shell may be a useful indicator of the integrity of the cortical shell

Limited spatial resolution affects the ability of clinical CT systems to directly measure the thickness and density of the vertebral cortical shell. In such cases the peak CT number ( CT p) of a profile through the cortical shell is approximately proportional to the product of thickness and density and hence an average of several of these values ( CT p ) may be useful as an indicator of the integrity of the cortical shell. Values of CT p were determined around the anterior two-thirds of the cortical shell of the L3 and L4 lumbar vertebrae of 96 Kuwaiti women (mean age±SD=50.0±8.5). There was no significant difference between the two vertebrae in terms of CT p and trabecular BMD values. There was a significant correlation between CT p and trabecular BMD for each vertebra ( r=0.597, p<0.0001), and between CT p and patient age ( r=0.624, p<0.001), suggesting that both cortical and trabecular bone may be subject to similar thinning mechanisms during ageing and osteoporosis. The variation of CT p around the cortical shell, represented by the standard deviation of nine CT p measurements, was significantly correlated with the mean value ( r=0.503, p<0.0001) and may be useful as a descriptor of cortical structure.