Density gradients, effect

Philipsen P H T, te Velde G and Baerends E J 1994 The effect of density-gradient corrections for a molecule-surface potential energy surface. Slab calculations on Cu(100)c(2x2)-C0 Chem. Phys. Lett. 226 583... 

The density change on polymerization is typically about 20%, and this density gradient can cause significant secondary flows and natural convection effects. The experiments cited above for vinyl acetate polymerization were performed in a helical reactor. The centrifugal force in helical reactors induces secondary flows as well. The effects of helical flow have been analyzed, but were found to be less significant than the effects of natural convection [14]. 

Sphaeroplasts were prepared by slight modifications to published methods [12,13]. Lysis of sphaeroplasts was effected by a combination of osmotic lysis and gentle mechanical disruption [14]. Discontinuous sucrose-density gradients were constructed and fractions were then assayed for protein, PG and marker enzymes for different organelles. 

This discrepancy arises primarily from the fact that spontaneous liquid flows will always develop in any hquid even without artificial stirring (e.g., under the action of density gradients caused by local temperature or concentration fluctuations). This phenomenon has been termed natural convection. Electrochemical reactions reinforce natural convection, since the concentrations of substances involved in the reaction will change near the electrode surfaces, and also since heat is evolved. Gas evolution attending the reactions has a particularly strong effect on naturaf convection. 

Most organelle membranes, such as the tonoplast (6) and the Golgi apparatus (7), can be separated by density gradient ultracentrifugation of plant cell homogenates. However, other effective methods for the isolation of the plasma membrane (8,9) have been described. Moreover, another method that uses an aqueous two-phase system for the isolation of ER is also described (10). Those interested in these details for these methods should consult the original articles. 

In this approach a mixed solvent is chosen so that the relative sedimentation of the two components may give rise to a density gradient. The solute from a band that centres at the point where its effective density is equal to that of the solvent mixture. The band has a Gaussian shape with respect to solute concentration, the half-width is inversely proportional to the molecule weight of solute. A major importance is its sensitivity to small differences in effective density among the solute species. 

This is a method for the separation of molecules with very similar densities and depends upon the formation of a density gradient by the effect of centrifugal force on the supporting solute molecules but because of their low mass the technique demands a long period of centrifugation, e.g. 2-3 days. The test compounds are... 

Convection (of the electrolyte liquid phase as a whole) can be natural (due to thermal effects or density gradients) or forced (principal mass transport mode in hydrodynamic techniques). Still, however, close to the electrode surface a diffusion layer develops. 

At or near sonic velocity, as at the C-J plane and beyond, a minute divergence had a marked effect. Similarly, at the throat of supersonic nozzle, an infinitesimal change in cross-sectional area produced a finite acceleration of the flow. Objection was raised, however, by Duff Knight (Ref 17a) that evidence of a large density gradient at the end of the reaction zone is lacking. Wood Kirkwood (Ref 17b) suggested that even in normal detonation, the reactions are incomplete at the C-J point in a tube of finite diameter (Ref 22, p lid)... 

The density gradient resulting from wall friction, in addition to its effects on expl per-... 

Photochemistry can be used to demonstrate solvent effects in supercritical fluids. The analysis revealed trimodal fluorescence lifetime distributions near the critical temperature, which can be explained by the presence of solvent-solute and solute-solute clustering. This local aggregation causes an increase in nonradiative relaxations and, therefore, a decrease in the observed fluorescence lifetimes. Concentration and density gradients are responsible for these three unique lifetimes (trimodal) in the supercritical fluid, as contrasted with the single lifetime observed in a typical organic solvent. The... 

The increments should be less than the diameter of the charge each time to maximize the effect of this method. Although pressure and density gradients are reduced, they are not completely eliminated in this method and are proportional to the number and size of increments used. However, interfaces between the increments have been found to cause initiation problems in some cases. 

Different grades of both of the ABS and HIPS plastics have specific gravities in the range of 1.055-1.125 g cm-1. As a result such mixtures can not be effectively separated by density gradient procedures. 

The rate coefficient, k(p)> increases with sink density because, on average, the microscopic density gradient around each sink is reduced less then the microscopic density due to the neighbouring sinks. From Fig. 47, the extent of the increase in k(p) is quite small and would be difficult to observe experimentally. A reaction between a stationary species, A, and a diffusing species, B, occurs at a rate k [ A) [B], where k is the second-order rate coefficient, ft(p), above. If the Smoluchowski theory had been used instead, it would have a rate coefficient fe(0) and the concentration of the diffusing species B remains at its initial volume [B]0. The rate of reaction is (Q) [A] [B]0. These reaction rates are approximately the same, because [B]0/[B] fe(p)/fe(0). Under the circumstances where [B] > A], the importance of these competitive effects is small. 

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