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Diffusion Coefficient D (diffusion + coefficient_d)
Selected AbstractsDiffusion and distribution of element-labelled surfactants in human hairINTERNATIONAL JOURNAL OF COSMETIC SCIENCE, Issue 2 2004F.-J. Wortmann Synopsis To directly follow the diffusion process of cosmetically relevant agents into human hair, a specific methodological approach is presented and elucidated for selected surfactants. For this, practically relevant anionic and cationic surfactants were synthesized with a chlorine atom at the end of their alkyl chain. The property changes of the surfactants through the modification are corresponding to an extension of the alkyl chain by about two methylene groups, thus representing a moderate increase of hydrophobicity. After the application of a modified surfactant to hair, it can be localized and quantified through its chlorine atom in cross-sections by scanning electron microscopy combined with micro X-ray fluorescence analysis. The determination of the diffusion coefficient D is realized through the application of the Matano-equation to element intensity profiles. Values for D vary within the chosen range of pH and temperature between 10,14 and 10,16 m2 s,1. The diffusion coefficients for the anionic surfactants increase with decreasing pH and increasing temperature, The temperature dependence follows in all cases the Arrhenius relationship with activation energies EA of 50,100 kJ mol,1, which decrease with pH. The pH-related effects, with comparable values for D and EA, are opposite for the cationic surfactant. These observations are consistently interpreted on the basis of ionic and hydrophobic interactions in hair. Résumé Afin de suivre directement le processus de diffusion d'agents cosmétiques pertinents dans le cheveu humain, une approche méthodologique spécifique est présentée et approfondie pour des tensio actifs sélectionnés. En pratique, des tensio actifs pertinents, anioniques et cationiques ont été synthétisés en introduisant un atome de Chlore à l'extrémité de leur chaîne alkyle. Les changements de propriétés de ces tensio actifs, via cette modification, correspondent à un allongement de la chaîne alkyle d'environ deux groupes méthylène, représentant ainsi une augmentation modérée de leur caractère hydrophobe. Suite à l'application sur le cheveu d'un tensio actif modifié, il peut être localisé et quantifié par l'atome de Chlore dans des coupes transverses par microscopie électronique à balayage couplée à l'analyse par micro Fluorescence X. La détermination du coefficient de diffusion D est effectuée par l'application de l'équation de Matano aux profils de l'intensité de l'élément. Les valeurs de D varient, selon l'échelle de pH et de température, entre 10,14 et 10,16 m2 s,1. Les coefficients de diffusion pour les tensio actifs anioniques augmentent avec des pH décroissants et des températures croissantes. La dépendance vis à vis de la température suit, dans tous les cas, la relation d'Arrhenius avec des énergies d'activation EA de 50 à 100 kJ mol,1, qui décroît avec le pH. Les effets liés au pH, avec des valeurs comparables de D et EA, sont opposés pour le tensio actif cationique. Ces observations sont constamment interprétées par les interactions de types ioniques et hydrophobes dans le cheveu. [source] ESR Imaging of Conduction Electrons in Organic ConductorsISRAEL JOURNAL OF CHEMISTRY, Issue 1 2008Elmar Dormann In organic conductors built from stacks of planar aromatic hydrocarbons the mobility of the conduction electrons perpendicular to the radical cation stacks is more than four orders of magnitude smaller than the respective along-the-stack value. Thus the macroscopic properties of real single crystals of such radical cation salts are strongly influenced by the distribution of inevitable defects. Therefore, spatial resolution is required for a meaningful interpretation of the ESR data. Various schemes for 2D and 3D ESR imaging have been realized in the radio frequency and microwave frequency range with a resolution down to 10 m,m. Spatial distribution of spin density, T1, T2, and spin diffusion coefficient D were recorded, often as a function of temperature. The conducting-chain length distribution was observed. Our first attempts for the imaging of the integral and the spatially distributed carrier motion caused by an applied electric current were successful. [source] Diffusion-controlled growth of wollastonite rims between quartz and calcite: comparison between nature and experimentJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2002R. Milke Abstract Growth rates of wollastonite reaction rims between quartz and calcite were experimentally determined at 0.1 and 1 GPa and temperatures from 850 to 1200 °C. Rim growth follows a parabolic rate law indicating that this reaction is diffusion-controlled. From the rate constants, the D,,-values of the rate-limiting species were derived, i.e. the product of grain boundary diffusion coefficient D, and the effective grain boundary width, ,. In dry runs at 0.1 GPa, wollastonite grew exclusively on quartz surfaces. From volume considerations it is inferred that (D,CaO,)/(D,SiO2,),1.33, and that SiO2 diffusion controls rim growth. D,SiO2, increases from about 10,25 to 10,23 m3 s,1 as temperature increases from 850 to 1000 °C, yielding an apparent activation energy of 330±36 kJ mol,1. In runs at 1 GPa, performed in a piston-cylinder apparatus, there were always small amounts of water present. Here, wollastonite rims always overgrew calcite. Rims around calcite grains in quartz matrix are porous and their growth rates are controlled by a complex diffusion-advection mechanism. Rim growth on matrix calcite around quartz grains is controlled by grain boundary diffusion, but it is not clear whether CaO or SiO2 diffusion is rate-limiting. D,, increases from about 10,21 to 10,20 m3 s,1 as temperature increases from 1100 to 1200 °C. D,SiO2, or D,CaO, in rims on calcite is c. 10 times larger than D,SiO2, in dry rims at the same temperature. Growth structures of the experimentally produced rims are very similar to contact-metamorphic wollastonite rims between metachert bands and limestone in the Bufa del Diente aureole, Mexico, whereby noninfiltrated metacherts correspond to dry and brine-infiltrated metacherts to water-bearing experiments. However, the observed diffusivities were 4 to 5 orders of magnitude larger during contact-metamorphism as compared to our experimental results. [source] Chain stiffness of heteropolysaccharide from Aeromonas gum in dilute solution by dynamic light scatteringBIOPOLYMERS, Issue 6 2002Xiaojuan Xu Abstract Dynamic light scattering measurements have been made on 15 fractions of aeromonas (A) gum, an extracellular heteropolysaccharide produced by the strain Aeromonas nichidenii, with dimethylsulfoxide containing 0.2M lithium chloride as the solvent at 25°C. Data for the translational diffusion coefficient D covering a molecular weight range from 4.5 × 105 to 2.1 × 106 and ratios of the z -average radius of gyration ,s2,z1/2 to the hydrodynamic radius RH (calculated with previous ,s2,z data) suggest that the polymer behaves like a semiflexible chain in this solvent similar to the stiffness of cellulose derivatives. Thus the D data are analyzed on the basis of the Yamakawa,Fujii theory for the translational friction coefficient of a wormlike cylinder by coarse-graining the heteropolysaccharide molecule. Excluded-volume effects are taken into account in the quasi-two-parameter scheme, as was done previously for ,s2,z and [,] (the intrinsic viscosity) of A gum in the same solvent. The molecular weight dependence of RH is found to be explained by the perturbed wormlike chain with a persistence length of 10 nm, a linear mass density of 1350 nm,1, an excluded-volume strength parameter of 1.3 nm, and a chain diameter of 2.8 nm. These parameters are in substantial agreement with those estimated previously from ,s2,z and [,] data, demonstrating that the solution properties (D, ,s2,z, and [,]) of the heteropolysaccharide are almost quantitatively described by the current theories for wormlike chains in the molecular weight range studied. © 2002 Wiley Periodicals, Inc. Biopolymers 65: 387,394, 2002 [source] Insulin adsorption into porous charged membranes: Effect of the electrostatic interactionBIOTECHNOLOGY PROGRESS, Issue 4 2009Shaoling Zhang Abstract Insulin adsorption into a series of porous charged membranes was investigated by batch adsorption experiments, and the experimental results were analyzed by the homogeneous diffusion model. The membranes used in this study were prepared by pore-surface modification of porous poly(acrylonitrile) (PAN) membranes by grafting with weak acidic and basic functional groups. The amount of insulin adsorbed into the membrane was determined from the material balance of insulin. The insulin partition coefficient K between the membrane and solution was estimated from the equilibrium adsorption amount, and the effective diffusion coefficient D was estimated by matching the model with the experimental data as a fitting parameter. The dependence of K and D on the charge properties of the insulin and membrane is observed and discussed. The partition coefficient K increased when the insulin and the membrane carried opposite charges, on the other hand, the effective diffusion coefficient D was reduced. These results indicate that the electrostatic interaction between the insulin and the membranes played an important role in the insulin adsorption. © 2009 American Institute of Chemical Engineers Biotechnol. Prog. 2009 [source] Measurement of Spin Diffusion Coefficients in Glassy Polymers: Failure of a Simple Scaling LawMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 2 2008Bernard Meurer Abstract For a series of polymers, the spin diffusion coefficients D do not scale as predicted from a simple expression based on regularly spaced nuclei. We compare D for PVA and polystyrene with their side group either protonated or deuterated. For polystyrene, D is considerably reduced from 600,800 to 65 nm2,·,s,1. For PVA, D is already small for the fully protonated chain and is only slightly reduced from 170 to 130 nm2,·,s,1. This indicates that the rapidly rotating methyl group does not contribute appreciably to spin diffusion between neighboring chains and confirms that the mean proton density is not the pertinent parameter to control D. [source] Monte Carlo study of cycloamylose: Chain conformation, radius of gyration, and diffusion coefficientBIOPOLYMERS, Issue 2 2002Yasushi Nakata Abstract Cyclic (1 , 4)-,- D -glucan chains with or without excluded volume have been collected from a huge number (about 107) of linear amylosic chains generated by the Monte Carlo method with a conformational energy map for maltose, and their mean-square radii of gyration ,S2, and translational diffusion coefficients D (based on the Kirkwood formula) have been computed as functions of x (the number of glucose residues in a range from 7 to 300) and the excluded-volume strength represented by the effective hard-core radius. Both ,S2,/x and D in the unperturbed state weakly oscillate for x < 30 and the helical nature of amylose appears more pronouncedly in cyclic chains than in linear chains. As x increases, these properties approach the values expected for Gaussian rings. Though excluded-volume effects on them are always larger in cycloamylose than in the corresponding linear amylose, the ratios of ,S2, and the hydrodynamic radius of the former to the respective properties of the latter in good solvents can be slightly lower than or comparable to the (asymptotic) Gaussian-chain values when x is not sufficiently large. An interpolation expression is constructed for the relation between the gyration-radius expansion factors for linear and cyclic chains from the present Monte Carlo data and the early proposed asymptotic relation with the aid of the first-order perturbation theories. © 2002 Wiley Periodicals, Inc. Biopolymers 64: 72,79, 2002 [source] | ||||||||||