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PubMed2003

Electrochemical characterization of binderless, recompressed exfoliated graphite electrodes: electron-transfer kinetics and diffusion characteristics.

Exfoliated graphite (EG) is prepared by the thermal exfoliation of graphite intercalation compounds at different temperatures. Surface and bulk physicochemical properties of EG are followed by spectroscopic and analytical methods and are observed to be a function of exfoliation temperature. EG particles can be recompressed without any binder and used as surface-renewable electrodes. Surface preparation is accomplished by either polishing or roughening the electrode surface using emery sheets. Effects of exfoliation temperature and the surface preparation on the electron-transfer kinetics and on the diffusion characteristics have been followed by electrochemical methods using several benchmark redox systems. It is found that the electron-transfer kinetics and the diffusion of K(4)[Fe(CN)(6)] are affected by the nature of the EG surface while that of iron(II)(1,10-phenanthroline)(3) and cobalt(II)(1,10-phenanthroline)(3) are not affected by the surface preparation. The redox systems are classified into different groups according to their kinetic sensitivity. Diffusion of electroactive species toward the EG electrodes is found to nonlinear. Current-time plots suggest that the recompressed EG electrodes can be modeled as fractals.

Journal Article↗
PubMed1993

How does a virus bud?

How does a virus bud from the plasma membrane of its host? Here we investigate several possible rate-limiting processes, including thermal fluctuations of the plasma membrane, hydrodynamic interactions, and diffusion of the glycoprotein spikes. We find that for bending moduli greater than 3 x 10(-13) ergs, membrane thermal fluctuations are insufficient to wrap the viral capsid, and the mechanical force driving the budding process must arise from some other process. If budding is limited by the rate at which glycoprotein spikes can diffuse to the budding site, we compute that the budding time is 10-20 min, in accord with the experimentally determined upper limit of 20 min. In light of this, we suggest some alternative mechanisms for budding and provide a rationale for the observation that budding frequently occurs in regions of high membrane curvature.

Animals↗
PubMed2002

Passive sampling in combination with thermal desorption and gas chromatography as a tool for self-assessment of chemical exposure.

Diffusive samplers for monitoring of air quality are user-friendly devices that can normally be operated by the user himself. Hence these samplers are suitable for self-assessment. Practical and work organisational aspects of self-assessment of chemical exposure were studied in different occupational settings. It was found that the diffusive sampler used in these studies, the Perkin-Elmer tube in combination with thermal desorption, worked well for the purpose and could be correctly handled by the individuals using it. The results from self-assessments agreed well with expert measurements carried out by an occupational hygienist. However, in order to obtain a sustainable system of self-assessment strong organizational support is needed.

Adsorption↗
PubMed1993

Steady-state kinetics of ubiquinol-cytochrome c reductase in Saccharomyces cerevisiae mitochondria: effects of fluidity changes obtained by different growth temperatures.

We have investigated the steady-state kinetics of ubiquinol-cytochrome c reductase using either ubiquinol-1 or -2 as the donor substrate in mitochondria from Saccharomyces cerevisiae either grown at 36 degrees C or brought from 36 to 10 degrees C at the end of the exponential phase ("thermal shock"). The fatty acid composition of the mitochondrial lipids varied under the two conditions, with an increase of the unsaturation degree in the thermally shocked cells. The fluorescence polarization of the membrane probe diphenylhexatriene incorporated into mitochondria was decreased in a wide temperature range in the thermally shocked cells, in accordance with the fatty acid changes. Also the motion and order parameters of two fatty acid spin labels revealed significant changes in the thermally shocked cells, interpreted as an increase of membrane fluidity brought about by the thermal shock. This increase agrees with the finding that the diffusion coefficient of the ubiquinone homolog Q3 in the membrane, investigated by collisional fluorescence quenching of the membrane probe 12-(9-anthroyl) stearic acid, was consistently increased in the mitochondria from the thermally shocked cells. In spite of the physical changes of the membrane and the increase in the diffusion coefficients of the ubiquinone homolog, the steady-state kinetics of ubiquinol cytochrome c reductase showed no change to be interpreted in terms of a diffusion-limited component. An increase of Vmax of the enzyme in the thermally shocked cells was the result of an increased content of the bc1 complex; both the Km for ubiquinols and the turnover number were almost identical under the two growth conditions, so that the kcat/Km ratio, which approaches the second-order rate constant of substrate interaction with the enzyme active site, was unmodified. This is a demonstration that the ubiquinol cytochrome c reductase activity in S. cerevisiae mitochondria is not diffusion-controlled in the interaction of the donor substrate with the enzymatic protein.

Cytochromes↗
PubMed2005

Influence of thermal fluctuations on interfacial electron transfer in functionalized TiO2 semiconductors.

The influence of thermal fluctuations on the dynamics of interfacial electron transfer in sensitized TiO2-anatase semiconductors is investigated by combining ab initio DFT molecular dynamics simulations and quantum dynamics propagation of transient electronic excitations. It is shown that thermal nuclear fluctuations speed up the underlying interfacial electron transfer dynamics by introducing nonadiabatic transitions between electron acceptor states, localized in the vicinity of the photoexcited adsorbate, and delocalized states extended throughout the semiconductor material, creating additional relaxation pathways for carrier diffusion. Furthermore, it is shown that room-temperature thermal fluctuations reduce the anisotropic character of charge diffusion along different directions in the anatase crystal and make similar the rates for electron injection from adsorbate states of different character. The reported results are particularly relevant to the understanding of temperature effects on surface charge separation mechanisms in molecular-based photo-optic devices.

Journal Article↗
PubMed2003

Silica-alginate composites for microencapsulation.

Optimisation of membrane properties of alginate microcapsules is a key factor for the application of microencapsulation techniques to bioartificial organ elaboration. Coacervation and layer-by-layer processes involving additional biopolymers have been extensively studied. Recently, the use of silica as a membrane-forming agent was investigated. This approach was rendered possible by the development of biocompatible routes to silica formation. The composites exhibit enhanced mechanical and thermal stability as well as suitable diffusion properties. Moreover, encapsulated enzymes and cells retain their biological activities. Similarly, silica can be associated to many other biopolymers, opening a promising route for new biocomposites design and biotechnology applications.

Alginates↗
PubMed2000

Attenuation of fission neutrons by some hydrogeneous shield materials and the exponential dependence of the attenuated total neutron dose rate on the shield thickness.

This work deals with the attenuation of fission neutrons by some hydrogeneous shield materials. The attenuated fission neutrons are described by the energy groups (fast, epithermal and thermal). The exponential decrease in the fast flux is represented by the removal cross section concept. Each of the epithermal and thermal fluxes is expressed using the diffusion equation including a pair of arbitrary constants to be determined using the corresponding boundary conditions. The solution obtained for the required arbitrary constants is then approximated in a simplified form such that it may easily replace the corresponding exact solution. The attenuation values, by which the neutron dose rate distributions are exponentially decreased through certain thicknesses are also determined for the given materials. They are compared to the corresponding experimental and theoretical data. The results obtained for the total neutron dose rate distributions in terms of a suitable range of layer thicknesses are then used to determine--for each material--an average value for the total neutron dose rate representing the exponential decrease during passage through the considered range of layer thicknesses.

Journal Article↗
PubMed2005

Physicochemical properties and structures of room temperature ionic liquids. 2. Variation of alkyl chain length in imidazolium cation.

The alkyl chain length of 1-alkyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([Rmim][(CF(3)SO(2))(2)N], R = methyl (m), ethyl (e), butyl (b), hexyl (C(6)), and octyl (C(8))) was varied to prepare a series of room-temperature ionic liquids (RTILs), and the thermal behavior, density, viscosity, self-diffusion coefficients of the cation and anion, and ionic conductivity were measured over a wide temperature range. The self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity change with temperature following the Vogel-Fulcher-Tamman equation, and the density shows a linear decrease. The pulsed-field-gradient spin-echo NMR method reveals a higher self-diffusion coefficient for the cation compared to that for the anion over a wide temperature range, even if the cationic radius is larger than that of the anion. The summation of the cationic and anionic diffusion coefficients for the RTILs follows the order [emim][(CF(3)SO(2))(2)N] > [mmim][(CF(3)SO(2))(2)N] > [bmim][(CF(3)SO(2))(2)N] > [C(6)mim][(CF(3)SO(2))(2)N] > [C(8)mim][(CF(3)SO(2))(2)N], which greatly contrasts to the viscosity data. The ratio of molar conductivity obtained from impedance measurements to that calculated by the ionic diffusivity using the Nernst-Einstein equation quantifies the active ions contributing to ionic conduction in the diffusion components, in other words, ionicity of the ionic liquids. The ratio decreases with increasing number of carbon atoms in the alkyl chain. Finally, a balance between the electrostatic and induction forces has been discussed in terms of the main contribution factor in determining the physicochemical properties.

Journal Article↗
PubMed2005

Molecular dynamics simulations of atmospheric oxidants at the air-water interface: solvation and accommodation of OH and O3.

A comparative study of OH, O3, and H2O equilibrium aqueous solvation and gas-phase accommodation on liquid water at 300 K is performed using a combination of ab initio calculations and molecular dynamics simulations. Polarizable force fields are developed for the interaction potential of OH and O3 with water. The free energy profiles for transfer of OH and O3 from the gas phase to the bulk liquid exhibit a pronounced minimum at the surface, but no barrier to solvation in the bulk liquid. The calculated surface excess of each oxidant is comparable to calculated and experimental values for short chain, aliphatic alcohols. Driving forces for the surface activity are discussed in terms of the radial distribution functions and dipole orientation distributions for each molecule in the bulk liquid and at the surface. Simulations of OH, O3, and H2O impinging on liquid water with a thermal impact velocity are used to calculate thermal accommodation (S) and mass accommodation (alpha) coefficients. The values of S for OH, O3, and H2O are 0.95, 0.90, and 0.99, respectively. The approaching molecules are accelerated toward the liquid surface when they are approximately 5 angstroms above it. The molecules that reach thermal equilibrium with the surface do so within 2 ps of striking the surface, while those that do not scatter into the gas phase with excess translational kinetic energy in the direction perpendicular to the surface. The time constants for absorption and desorption range from approximately 35 to 140 ps, and the values of alpha for OH, O3, and H2O are 0.83, 0.047, and 0.99, respectively. The results are consistent with previous formulations of gas-phase accommodation from simulations, in which the process occurs by rapid thermal and structural equilibration followed by diffusion on the free energy profile. The implications of these results with respect to atmospheric chemistry are discussed.

Air↗
PubMed2005

Stationary fronts due to weak thermal effects in models of catalytic oxidation.

We analyze the possible existence of an infinite number of stationary front solutions in a microkinetic model of a catalytic reaction coupled with weak enthalpy effects in the domain of kinetics bistability. The kinetic model incorporates three steps: dissociative oxygen adsorption, reactant adsorption and desorption, and surface reaction. The infinitude of stationary front solutions emerges due to the lack of intercrystallites communication of surface species in supported catalysts; thermal conductions and gas-phase diffusion are the only means of interaction. Incorporation of surface species diffusion leads to a very slow front motion. We complement this analysis with simulations of stationary states on one- (wire and ring) and two-dimensional (disk) systems which may be subject to control or to fluid flow. These results account for certain experimental results and may have implications for various technological problems.

Journal Article↗
PubMed2002

Dynamic and correlation properties of solid supported smectic-A films.

Dynamic properties and layer displacement-layer displacement correlation functions of the smectic-A films on a substrate are investigated. The eigenfrequencies spectrum and eigenmotions of the film are calculated within the framework of a discrete model. It was found that the static as well as dynamic properties of freely standing and solid supported smectic-A films differ significantly. In particular, the acoustic mode is absent in the films on a substrate and the surface tension is not essential for the film dynamics. The correlation length of the spatial layer displacement correlation functions is finite for films on a substrate in contrast to the free standing films. An effect of thermal fluctuations on the specular and diffuse x-ray scattering was analyzed.

Journal Article↗
PubMed2004

Nonlinear response of the trap model in the aging regime: exact results in the strong-disorder limit.

We study the dynamics in the one-dimensional disordered trap model with a broad distribution of trapping times p(tau) approximately 1/tau(1+mu), when an external force is applied from the very beginning at t=0, or only after a waiting time t(w), in the linear as well as in the nonlinear response regime. Using a real-space renormalization procedure that becomes exact in the limit of strong disorder mu-->0, we obtain explicit results for many observables, such as the diffusion front, the mean position, the thermal width, the localization parameters and the two-particle correlation function. In particular, the scaling functions for these observables give access to the complete interpolation between the unbiased case and the directed case. Finally, we discuss in detail the various regimes that exist for the average position in terms of the two times and the external field.

Journal Article↗
PubMed2006

Nitrogen diffusion in amorphous silicon nitride isotope multilayers probed by neutron reflectometry.

Amorphous silicon nitride is a model system for a covalently bound amorphous solid with a low atomic mobility where reasonable values of self-diffusivities are still lacking. We used neutron reflectometry on isotope enriched Si3 14N4/Si3 15N4 multilayers to determine nitrogen self-diffusivities ranging from 10(-24) to 10(-21) m2/s between 950 and 1250 degrees C. Time dependent diffusivities observed at 1150 degrees C indicate the presence of structural relaxation. For long annealing times (relaxed state) the diffusivities follow an Arrhenius law with an activation enthalpy of (3.6 +/- 0.4) eV. The results are indicative of a direct diffusion mechanism without the involvement of thermal point defects.

Journal Article↗
PubMed1999

Climate change as a regulator of tectonics on Venus.

Tectonics, volcanism, and climate on Venus may be strongly coupled. Large excursions in surface temperature predicted to follow a global or near-global volcanic event diffuse into the interior and introduce thermal stresses of a magnitude sufficient to influence widespread tectonic deformation. This sequence of events accounts for the timing and many of the characteristics of deformation in the ridged plains of Venus, the most widely preserved volcanic terrain on the planet.

Atmosphere↗
PubMed2004

Human skin permeation of Delta8-tetrahydrocannabinol, cannabidiol and cannabinol.

The purpose of this study was to quantify the in-vitro human skin transdermal flux of Delta8-tetrahydrocannabinol (Delta8-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 Delta8-THC and CBD. Tissue concentrations of Delta8-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 Delta8-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.

Abdomen↗
PubMed2000

Theory of solid-phase microextraction

The main objective of this contribution is to describe the fundamental concepts associated with solid-phase microextraction (SPME). Theory provides insight when developing SPME methods and identifies parameters for rigorous control and optimization. A mathematical model has been developed to understand the principal processes of SPME by applying basic fundamental principles of thermodynamics and diffusion theory. The model assumes idealized conditions and is limited to air, liquid, or headspace above liquid sampling. Theory for ideal cases can be quite accurate for trace concentrations in simple matrices such as air or drinking water at ambient conditions when secondary factors such as thermal expansion of polymers and changes in diffusion coefficients because of solutes in polymers can be neglected. When conditions are more complex, theory for ideal cases still efficiently estimates general relationships between parameters.

Journal Article↗
PubMed1983

Prospects for using implanted systems of assisted circulation and artificial heart with a radioisotope power source (biomedical, thermal, and radiation aspects).

The capacity of dogs to diffuse heat (up to 50 W) from an artificial heart and to tolerate prolonged intracorporeal ionizing radiation from a radioisotope power source (238Pu) was investigated, using electrical models of vascular blood heat exchangers that permit reproduction of elimination and heat transmission in autonomous systems. It was shown that up to 50 W can be discharged at temperatures of the wall-blood interface that do not exceed 43 degrees C. Clotting indexes, concentration of total protein, hemolysis, and serum enzyme activity during 1-1.5 months of heating remained within physiologically normal limits. A specific power load of up to 1.5 W/kg at ambient temperatures of 18-20 degrees C revealed no evidence of changes in heat production. By measuring the distribution of power of the dose absorbed around a 45-W plutonium source it was possible to estimate dose loads on critical organs and to assess overall risk of death from malignant tumors induced by radiation over a 10-year period: 6-12% for males and 8-14% for females. It is not very probable that use of the artificial heart with a radioisotope power source will be limited by thermal and radiational effects.

Animals↗
PubMed1998

Pulsed-field-gradient measurements of time-dependent gas diffusion.

Pulsed-field-gradient NMR techniques are demonstrated for measurements of time-dependent gas diffusion. The standard PGSE technique and variants, applied to a free gas mixture of thermally polarized xenon and O2, are found to provide a reproducible measure of the xenon diffusion coefficient (5.71 x 10(-6) m2 s-1 for 1 atm of pure xenon), in excellent agreement with previous, non-NMR measurements. The utility of pulsed-field-gradient NMR techniques is demonstrated by the first measurement of time-dependent (i.e., restricted) gas diffusion inside a porous medium (a random pack of glass beads), with results that agree well with theory. Two modified NMR pulse sequences derived from the PGSE technique (named the Pulsed Gradient Echo, or PGE, and the Pulsed Gradient Multiple Spin Echo, or PGMSE) are also applied to measurements of time dependent diffusion of laser polarized xenon gas, with results in good agreement with previous measurements on thermally polarized gas. The PGMSE technique is found to be superior to the PGE method, and to standard PGSE techniques and variants, for efficiently measuring laser polarized noble gas diffusion over a wide range of diffusion times.

Magnetic Resonance Spectroscopy↗