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Diffusion Experiments (diffusion + experiment)

Distribution by Scientific Domains

Chemistry	46%
Medical Sciences	26%
Polymers and Materials Science	20%

Selected Abstracts

Reverse osmosis of nonaqueous solutions through porous silica-zirconia membranes

AICHE JOURNAL, Issue 2 2006
Toshinori Tsuru
Abstract Porous silica-zirconia membranes with pore diameters from 0.8 to 2 nm were prepared by a sol-gel process, and applied to the separation of alcohols (hexanol, octanol, decanol) and alkanes (hexane, decane, tetradecane) in ethanol solutions by reverse osmosis over the temperature range from 25 to 60° C. A silica-zirconia membrane with a pore diameter of 1 nm showed a molecular weight-cut-off (MWCO) of 200 in ethanol solutions. Rejection increased with the applied pressure, for both alcohol and alkane solutes. However, the rejection of alcohols was found to decrease with temperature, while that for alkanes remained nearly constant. The separation characteristics were examined for the following membrane parameters: solvent permeability, Lp, reflection coefficient, ,, and solute permeability, P, based on the Spiegler-Kedem equation. The viscosity of solutions and the diffusivity of alkanes and alcohol solutes in nano-sized pores were found to show a larger temperature dependency than in bulk. The diffusivity of alkane solutes showed the same temperature dependency as the viscosity of ethanol in nano-sized pores, while the diffusivity of alcohol solutes showed a larger temperature dependency than the viscosity of ethanol, probably because of a larger interaction between alcohol solutes and the hydrophilic surface of silica-zirconia membranes. Diffusion experiments were carried out to confirm the temperature dependency of the diffusivities in nano-sized pores. A bilayer model verified that solute permeabilities by reverse osmosis and diffusion experiments were consistent with each other. © 2005 American Institute of Chemical Engineers AIChE J, 2006 [source]

Neutron Reflectometry: A Tool to Investigate Diffusion Processes in Solids on the Nanometer Scale,

ADVANCED ENGINEERING MATERIALS, Issue 6 2009
Harald Schmidt
Abstract The investigation of self-diffusion for the characterization of kinetic process in solids is one of the most fundamental tasks in materials science. We present the method of neutron reflectometry (NR), which allows the detection of extremely short diffusion lengths in the order of 1,nm and below at corresponding low self-diffusivities between 10,25 and 10,20,m2 s,1. Such a combination of values cannot be achieved by conventional methods of diffusivity determination, like the radiotracer method, secondary ion mass spectrometry, quasielastic neutron scattering, or nuclear magnetic resonance. Using our method, the extensive characterization of materials which are in a non-equilibrium state, like amorphous or nanocrystalline solids becomes possible. Due to the small experimentally accessible diffusion length microstructural changes (grain growth and crystallization) taking place simultaneously during the actual diffusion experiment can be avoided. For diffusion experiments with NR isotope multilayers are necessary, which are chemical homogeneous but isotope modulated films. We illustrate the basic aspects and potential of this technique using model systems of different classes of materials: single crystalline germanium, amorphous silicon nitride, and nanocrystalline iron. [source]

Nanocarving of Titania as a Diffusion-Driven Morphological Instability,

ADVANCED FUNCTIONAL MATERIALS, Issue 3 2008
Doh-Kwon Lee
Abstract Under strongly reducing conditions at high temperatures titania develops a specific surface morphology, comprising a regular array of fibers with a diameter in the sub-micrometer range. By a chemical diffusion experiment in a defined oxygen potential gradient it is shown that this surface structuring is caused by a diffusion-driven morphological instability of an advancing reaction front (surface). The kinetics of the process is analyzed in terms of linear transport equations. The conditions for the occurrence of the surface instability are discussed and the required materials properties are analyzed. The observed surface structuring is not restricted to titania, rather it has to occur in all nonstoichiometric compounds with predominant cation mobility. [source]

NMR diffusion measurements under chemical exchange between sites involving a large chemical shift difference

CONCEPTS IN MAGNETIC RESONANCE, Issue 2 2010
S. Leclerc
Abstract This study concerns the thallium-205 cation in aqueous solution in the presence of a calixarene molecule. Although the measurement of the self-diffusion coefficient of pure thallium (without calixarene in the aqueous solution) does not pose any particular problem, major difficulties are encountered with the standard method using gradient strength increment as soon as thallium is partly complexed by calixarene. With static magnetic field gradients, the NMR signal is so weak that it prevents any reliable measurement, whereas radio frequency (rf) field gradients lead to an unrealistic value of the diffusion coefficient. This failure is explained by the fact that thallium is in fast exchange between two sites (complexed and free thallium) thus exhibiting a single NMR signal although, in the course of the experiment, two signals, with an important difference in resonance frequencies (due to the large thallium chemical shift range), are effectively involved. With the objective to understand these quite unexpected observations, the theory underlying NMR diffusion experiments is first reviewed, and criteria of fast exchange are discussed for three parameters: chemical shifts, relaxation rates, and diffusion coefficients. It turns out that off-resonance effects are responsible for unwanted defocusing due to rf pulses in the static magnetic field gradient method and for time-dependent gradients in the rf field gradient method. Concerning the latter, a remedy is proposed which consists in applying the stronger gradient and incrementing the gradient pulse durations. After correction for relaxation, the expected value of the diffusion coefficient is retrieved. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part A 36A: 127,137, 2010. [source]

Suppression of background gradients in (B0 gradient-based) NMR diffusion experiments

CONCEPTS IN MAGNETIC RESONANCE, Issue 5 2007
Gang Zheng
Abstract Artifacts arising from background gradients are very common in NMR diffusion (i.e., PGSE) experiments involving B0 gradients because of the unavoidable magnetic susceptibility differences and B0 inhomogeneity within and around the sample. This article presents the general methodology to develop PGSE sequences with background gradient suppression. Most of the available methods which can be used for the suppression of the effects of background gradients are discussed. And two newly developed methods are presented in detail: frequency analysis of spin-dephasing, which assumes the artifacts due to background gradients come from the resonance between the spin-dephasing caused by applied gradients and background gradients, and asymmetric bipolar stimulated-echo-based PGSE, which can suppress the effects of nonconstant background gradients. © 2007 Wiley Periodicals, Inc.Concepts Magn Reson Part A 30A: 261,277, 2007. [source]

Effects of (,)-carveol and HPMC on the in vitro ocular transport and the in vivo intraocular pressure lowering effects of dorzolamide formulations in normotensive New Zealand rabbits

DRUG DEVELOPMENT RESEARCH, Issue 3 2009
Mohsen I. Afouna
Abstract The objective of the current study was to maximize the ocular bioavailability of the carbonic anhydrase inhibitor, dorzolamide hydrochloride (DZD) via (a) enhancement of DZD corneal transport using terpene enhancers, (b) reducing pre-corneal loss of the installed dose via increased formulation viscosity, and (c) assessment of the in vivo intraocular pressure (IOP) lowering effects of test formulations using rabbit. DZD was formulated as a 2% ophthalmic solution containing different concentrations of HPMC as a viscosity improving agent (VIA), and (,)-carveol as a corneal penetration enhancer. The transport of DZD from test formulations was quantitatively determined using in vitro diffusion experiments, the permeability parameters were mathematically calculated, and the in vivo IOP lowering effects were assessed using a Tono-Pen XL® tonometer. The results revealed a good correlation between the in vitro permeability parameters and the in vivo ,IOP. The magnitude of the DZD-IOP lowering effects and durations of actions for DZD formulations were dependent on (a) the concentration of (,)-carveol, and (b) the contact period with ocular tissue which was found to be a single-valued function of the HPMC as VIA. Drug Dev Res 70, 2009. © 2009 Wiley-Liss, Inc. [source]

Neutron Reflectometry: A Tool to Investigate Diffusion Processes in Solids on the Nanometer Scale,

Reverse osmosis of nonaqueous solutions through porous silica-zirconia membranes

G,s protein C -terminal ,-helix at the interface: does the plasma membrane play a critical role in the G,s protein functionality?

JOURNAL OF PEPTIDE SCIENCE, Issue 10 2005
Stefania Albrizio
Abstract The heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins, G,,,) mediate the signalling process of a large number of receptors, known as G protein-coupled receptors. The C -terminal domain of the heterotrimeric G protein ,-subunit plays a key role in the selective activation of G proteins by their cognate receptors. The interaction of this domain can take place at the end of a cascade including several successive conformational modifications. G,s(350,394) is the 45-mer peptide corresponding to the C -terminal region of the G,s subunit. In the crystal structure of the G,s subunit it encompasses the ,4/,6 loop, the ,6 ,-sheet segment and the ,5 helix region. Following a previous study based on the synthesis, biological activity and conformational analysis of shorter peptides belonging to the same G,s region, G,s(350,394) was synthesized and investigated. The present study outlines the central role played by the residues involved in the ,4/,6 loop and ,6/,5 loops in the stabilization of the C -terminal G,s,-helix. H2O/2H2O exchange experiments, and NMR diffusion experiments show interesting evidence concerning the interaction between the SDS micelles and the polypeptide. These data prompt intriguing speculations on the role of the intracellular environment/cellular membrane interface in the stabilization and functionality of the C -terminal G,s region. Copyright © 2005 European Peptide Society and John Wiley & Sons, Ltd. [source]

Human skin permeation of ,8 -tetrahydrocannabinol, cannabidiol and cannabinol

JOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 3 2004
Audra L. Stinchcomb
ABSTRACT The purpose of this study was to quantify the in-vitro human skin transdermal flux of ,8 -tetrahydrocannabinol (,8 -THC), cannabidiol (CBD) and cannabinol (CBN). These cannabinoids are of interest because they are likely candidates for transdermal combination therapy. Differential thermal analysis and in-vitro diffusion studies with human tissue were completed for the compounds. Heats of fusion, melting points and relative thermodynamic activities were determined for the crystalline compounds, CBD and CBN. Flux, permeability, tissue concentration and lag times were measured in the diffusion experiments. CBN had a lower heat of fusion and corresponding higher calculated relative thermodynamic activity than CBD. Ethanol concentrations of 30 to 33% significantly increased the transdermal flux of ,8 -THC and CBD. Tissue concentrations of ,8 -THC were significantly higher than for CBN. Lag times for CBD were significantly smaller than for CBN. The permeabilities of CBD and CBN were 10-fold higher than for ,8 -THC. Combinations of these cannabinoids with ethanol will be further studied in transdermal patch formulations in vitro and in vivo, as significant flux levels of all the drugs were obtained. CBD, the most polar of the three drugs, and other more polar cannabinoids will also be the focus of future drug design studies for improved transdermal delivery rates. [source]

A pulsed field gradient NMR diffusion investigation of enkephalin peptide-sodium dodecyl sulfate micelle association

MAGNETIC RESONANCE IN CHEMISTRY, Issue 6 2006
Brandon A. Begotka
Abstract Pulsed field gradient NMR (PFG-NMR) diffusion experiments were used to investigate the binding of leucine and methionine enkephalin peptides to anionic sodium dodecyl sulfate (SDS) micelles. The study was undertaken to investigate the mechanism of interaction between enkephalin peptides and SDS micelles and to determine if NMR-derived association constants, Keq, can predict the elution order in electrokinetic chromatography (EKC). In EKC, peptides are separated on the basis of their interactions with micelles. The Leu-enkephalin peptide,micelle association constant increased from 130 ± 8 to 1459 ± 57 and 1744 ± 64 M,1, respectively, when an Arg or Lys was added to the C -terminus. The association constant of Leu-enkephalinamide was approximately equal to that of Leu-enkephalin-Arg. Substitution of Phe4 with a Trp or Gly2 with an Ala in the Leu-enkephalin peptides also increased the micelle binding affinity. These results confirm that the interaction of Leu-enkephalin peptides with SDS micelles is largely electrostatic and that the non-polar amino acid side chains interact with the hydrophobic micelle core. The peptide,micelle association constants for the cationic Met-enkephalin peptides were also greater than their zwitterionic counterparts. For example, the Met-enkephalin Keq value was 162 ± 9 M,1, while the association constants for Met-enkephalin-Arg, Met-enkephalin-Lys, and Met-enkephalinamide were, respectively, 674 ± 31, 426 ± 23, and 453 ± 27 M,1. In both Met-enkephalin and Met-enkephalinamide, replacing Gly2 with an Ala did not significantly increase the association constant. These results confirm that with the Met-enkephalin peptides, there was little or no interaction of the amino acid side chains with the micelle core. For both the Leu-enkephalin and Met-enkephalin peptides, the association constants were consistent with EKC results, in that the peptides with smaller Keq values were found to elute before those with larger association constants. Copyright © 2006 John Wiley & Sons, Ltd. [source]

NMR methods applied to anisotropic diffusion

MAGNETIC RESONANCE IN CHEMISTRY, Issue 13 2002
István Furó
Abstract The methodology of NMR experiments intended to measure anisotropic diffusion is reviewed. Experiments of this kind preferably require oriented samples and/or orientation-dependent spin coupling and/or magnetic field gradients in different directions. One strategy of diffusion experiments in anisotropic systems with broad NMR lines employs line narrowing techniques, thereby allowing for efficient gradient encoding/decoding. Depending on the nuclei, spin couplings and samples, the preferred methods vary from decoupling through echo techniques to magic angle sample orientation and spinning. Another avenue to efficient gradient encoding/decoding is through very strong magnetic field gradients. Either way, anisotropic diffusion reveals new structural features as illustrated by a few selected examples in liquid crystals and in biological tissues. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Identification of bound exciton complexes in ZnO

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2004
M. Strassburg
Abstract An identification of shallow bound exciton centers in ZnO is presented based on magneto-optical measurements and diffusion experiments. The thermalization behavior of the Zeeman split components confirms that the I4, I6, I8 and I9 exciton lines stem from donor bound exciton complexes. The results are supported by theoretical analysis of shallow bound exciton complexes revealing the ,7 symmetry of the upper valence band. The presence of two-electron satellites related to the respective transitions is further evidence for the donor bound complexes and enabled the determination of donor binding energies. Hydrogen, aluminum, gallium and indium were identified to origin the I4, I6, I8 and I9 lines by doping, diffusion and annealing experiments combined with photoluminescence and secondary ion mass spectrometry. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Conformations of Betanova in aqueous trifluoroethanol,

BIOPOLYMERS, Issue 10 2010
Danny P. Chagolla
Abstract Conformations of the designed peptide Betanova in 42% trifluoroethanol/water (v/v) were explored. Circular dichroism (CD) observations provided no evidence for the presence of significant amounts of ,-structures in water, in TFE/water, or in ethanol/water. Nuclear magnetic resonance (NMR) diffusion experiments showed no significant difference in the hydrodynamic radius of the peptide in water and in 42% TFE/water. However, calculations indicated that the hydrodynamic radii of the triple-stranded ,-sheet, originally proposed for Betanova by Kortemme et al. (Science 1998, 281, 253-256), and a variety of partially folded forms of Betanova would be similar and likely could not be convincingly distinguished by diffusion experiments. Temperature coefficients (,,/,T) of the peptide NH chemical shifts are similar in water and 42% TFE/water, implying that most of these protons are highly solvent exposed in both solvents and likely do not participate in intramolecular hydrogen bonding interactions. Possible exceptions to this conclusion are the Lys9 and Lys15 residues, where a more positive coefficient may indicate that these residues are involved to some extent in local turn structures. Peptide proton,solvent fluorine intermolecular nuclear Overhauser effect (NOE)s at 25°C were consistent with the presence of a mixture of conformations, which could include the triple-stranded ,-sheet structure as a minor component. At 0°C, peptide-TFE NOEs indicated that TFE interacts strongly enough with many protons of Betanova that alcohol-peptide interactions persist for times of the order of nanoseconds, appreciably longer than the encounter time characteristic of mutual diffusion of TFE and the solute. © 2010 Wiley Periodicals, Inc. Biopolymers 93: 893,903, 2010. [source]

Effective Diffusivities and Convective Coefficients for CaO-CaSO4 and CaO-CaCl2 Pellets

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 1 2004
C. Akosman
Abstract Diffusion and convective flow in the pores of pellets formed by compressing mixtures of calcined limestone and CaSO4/CaCl2 powders have been studied experimentally by using the single pellet moment technique. The experiments were conducted in a diffusion cell by flowing nitrogen gas (carrier) through both faces of the pellet. Limestone powder was calcined in an atmosphere of N2 at 800,°C and mixed with CaSO4/CaCl2 for diffusion experiments. Effective diffusivity of helium has been estimated by exposing the upper face of the pellet to a pulse of and matching the response peak on the lower face of the pellet with theoretical expressions. The values of the effective diffusivities increased with temperature, but decreased with increasing CaSO4/CaCl2 content in the pellet. The convective flow contribution to the diffusion flux was found to increase with increasing pressure drop across the pellet. [source]