Search results for "Diffusion"
showing 10 items of 1615 documents
Forced Rayleigh scattering from non-harmonic gratings applied to complex diffusion processes in glass-forming liquids
1999
Abstract Tracer diffusion of 9,10-phenanthrenequinone (PQ) and its photoproduct in super-cooled phenolphthalein-dimethyl-ether (PDE) was studied by forced Rayleigh scattering. In order to investigate the spatial frequency dependence of the grating dynamics, several spatial harmonics of the grating with non-sinusoidal phase profile produced by non-linear recording were monitored. An optical scheme with a diverging reading beam is proposed for simultaneous reconstruction of the harmonic components.
Formation, diffusion, and aggregation of radiation-induced defects in MgO and α-Al 2 O 3
1997
MgO and M2O3 (corundum) are important ceramic materials with several technological applications including theirpossible use in fusion reactors' . Weare therefore studying the properties ofpoint defects (vacancies, interstitials, andtheir aggregates) and electronic defects (trapped holes and electrons) in these materials. Pair-potential (PP) calculationswere carried out using the Mott-Littleton (ML) two-region strategy, as implemented in the computer code CASCADEdue to Leslie. Details ofthe method, including the construction ofPPs, are in the literature3. For semi-empirical INDO(intermediate neglect ofdifferential overlap) calculations computer codes developed at the University of Riga were …
CHLOROPHYLL a AGGREGATES IN HYDROCARBON SOLUTION, A PICOSECOND SPECTROSCOPY AND MOLECULAR MODELING STUDY
1993
Chlorophyll a aggregates in 3-methylpentane solution have been studied by using picosecond absorption and fluorescence spectroscopy and molecular modeling. Chlorophyll a aggregates give rise to reversible temperature changes in the absorption and fluorescence spectra. Time-dependent anisotropies were used to estimate rotational correlation times of the aggregates. These were indicative of the sizes of the aggregates. The rotational diffusion of the monomer and the two identified aggregates was hydrodynamic over the viscosity range studied (0.29–1.8 cP). Molecular mechanics calculations were used to predict the minimum energy structures of several chlorophyll a dimers suggested earlier in th…
Gel chromatography
1968
Abstract The separation efficiency of gel chromatographic columns was investigated under a wide variety of experimental conditions. By use of reduced quantities it is possible to describe the separation efficiency for different gels and experimental conditions by a single equation. Deviations from this equation were found to be caused by an alteration of the diffusion coefficient within the gel with respect to the mobile phase. Factors governing the gel chromatography of p -oligophenylenes on poly(vinyl acetate) and poly(styrene) gels are generalized and discussed.
Evaluation of advanced silica packings for the separation of biopolymers by high-performance liquid chromatography
1987
Following previous studies of the use of non-porous monodisperse 1.5-microns n-octyl- and n-octadecyl-bonded silicas in gradient elution of proteins, this work was aimed at elucidating further the properties of this novel column material for peptide and protein separations in comparison with wide-pore silicas. First, it is demonstrated that with short columns (e.g., 35 X 8 mm I.D.) packed with these non-porous reversed-phase materials, mixtures of small peptides and mixtures of proteins can be very efficiently resolved. When the chain length of the bonded ligand was varied, the retention of a test set of proteins in gradient elution followed the ligand sequence C18 greater than C8 approxima…
Study of the effect of the membrane composition on ion transfer across a supported liquid membrane
2003
Abstract The rate of ion transfer across the supported liquid membrane (SLM) is studied in the rotating diffusion cell (RDC), varying the chemical composition of the SLM from net-cloth supported gel membranes to radiation-grafted polymer membranes. Steady-state current–voltage curves are measured as a function of the rotation rate, and values for the standard rate constant, k 0 , are determined for a series of tetraalkylammonium cations from the analysis of the initial slopes and the diffusion limiting currents. The analysis gives values for k 0 of the order of 10 −2 –10 −4 cm s −1 , which is in rather good agreement with the values found in the literature for this type of the system. As co…
Novel general expressions that describe the behavior of the height equivalent of a theoretical plate in chromatographic systems involving electricall…
2002
Novel general expressions are constructed and presented that describe the behavior of the height equivalent of a theoretical plate (plate height), H, as a function of the linear velocity, Vx, along the axis, x, of the column and the kinetic parameters that characterize the mass transfer and adsorption mechanisms in chromatographic columns. Open tube capillaries as well as columns packed with either non-porous or porous particles are studied. The porous particles could have unimodal or bimodal pore-size distributions and intraparticle convective fluid flow and pore diffusion are considered. The expressions for the plate height, H, presented in this work could be applicable to high-performanc…
1987
Effects of polysorbate 80 on amiodarone intestinal absorption in the rat
1995
Abstract Amiodarone is a widely used anti-arrhythmic agent which shows physico-chemical properties that are highly suitable for diffusion across lipophilic absorbing membranes, however,.its low aqueous solubility could represent the rate-limiting step for absorption, making it erratic and variable. In a previous paper, the influence of an anionic surfactant (sodium lauryl sulphate) at variable supramicellar concentrations was studied. The absorption rate constants of amiodarone decreased as the surfactant concentration increased, and absorption was unusually fast at lower surfactant concentrations. The previously proposed equations for interpreting the relationships between the amiodarone a…