Local composition enhancements

It has been suggested that the experimental isothermal kinetic rate constants of some reactions at near and supercritical conditions could not be explained solely by the thermodynamic pressure effect, but from the combination of local composition enhancement and density augmentation around reactants. 

Local solvation in the form of local density augmentation and local composition enhancements can influence reactivity, but this depends on the relative time-scales of the reaction and solvation processes. 

In supercritical fluids, the possibility of local composition enhancements of cosolvent about a solute suggests that we should see enhancement of anion fluorescence if the water cosolvent clusters effectively about the 2-naphthol solute. Although in liquids the water concentration must be >30% to see anion emission, the higher diffusivity and density fluctuations in SCFs could allow stabilization of the anion at much lower water concentrations provided that the water molecules provide sufficient structure. Therefore the purpose of these experiments was to investigate 2-naphthol fluorescence in supercritical CO 2 with water cosolvent in the highly compressible region of the mixture to probe the local environment about the solute. 

Ellington, J. B., K. M. Park, and J. F. Brennecke. 1994. Effect of local composition enhancements on the esterification of phtalic-anhydride with methanol in supercritical carbon-dioxide. Industrial and Engineering Chemistry Research. 33,965. 

Shalaev, V.M. (2001). Fractal nano-composites giant local-field enhancement of optical responses, In Nanoscale linear and nonlinear optics, Bertolotti, M. and Sibilia, C. (Ed.) AIP. 

Crack-tip growth mechanisms have been proposed that do not involve dislocation movement explicitly, but rather, in response to the stress field at the crack tip, interstitial atoms diffuse to the region of the stress field to reduce the stress substitutional atoms also will diffuse to the tip if the local stress is thereby reduced. Crack-tip growth would be increased if this local change in alloy composition enhances dissolution during slip displacement or alters the passive film such that it is more easily ruptured by dislocations emerging to the surface. That is, there is continuously produced at the crack tip a film that is more easily ruptured than the more stable passive film on the sides of the crack (Ref 158). 

These values are determined by experiment. It is, however, by no means a trivial task to measure the lamina compressive and shear strengths (52,53). Also the failure of the first ply of a laminate does not necessarily coincide with the maximum load that the laminate can sustain. In many practical composite laminates first-ply failure may be accompanied by a very small reduction in the laminate stiffness. Local ply-level failures can reduce the stress-raising effects of notches and enhance fatigue performance (54). 

Measurements of overall reaction rates (of product formation or of reactant consumption) do not necessarily provide sufficient information to describe completely and unambiguously the kinetics of the constituent steps of a composite rate process. A nucleation and growth reaction, for example, is composed of the interlinked but distinct and different changes which lead to the initial generation and to the subsequent advance of the reaction interface. Quantitative kinetic analysis of yield—time data does not always lead to a unique reaction model but, in favourable systems, the rate parameters, considered with reference to quantitative microscopic measurements, can be identified with specific nucleation and growth steps. Microscopic examinations provide positive evidence for interpretation of shapes of fractional decomposition (a)—time curves. In reactions of solids, it is often convenient to consider separately the geometry of interface development and the chemical changes which occur within that zone of locally enhanced reactivity. 

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