Heterogeneity, absence

Diffusivity and tortuosity affect resistance to diffusion caused by collision with other molecules (bulk diffusion) or by collision with the walls of the pore (Knudsen diffusion). Actual diffusivity in common porous catalysts is intermediate between the two types. Measurements and correlations of diffusivities of both types are Known. Diffusion is expressed per unit cross section and unit thickness of the pellet. Diffusion rate through the pellet then depends on the porosity d and a tortuosity faclor 1 that accounts for increased resistance of crooked and varied-diameter pores. Effective diffusion coefficient is D ff = Empirical porosities range from 0.3 to 0.7, tortuosities from 2 to 7. In the absence of other information, Satterfield Heterogeneous Catalysis in Practice, McGraw-HiU, 1991) recommends taking d = 0.5 and T = 4. In this area, clearly, precision is not a feature. 

The immobilization of reagents onto sorbents often results in increase of their sensitivity and, in some cases, selectivity, allows to simplify the analysis and to avoid necessity of use of toxic organic solvents. At the same time silicas are characterized by absence of swelling, thenual and chemical stability, rapid achievement of heterogeneous equilibrium. 

The outstanding feature of the preparation of 2,2 -bipyridine from pyridine under the influence of metal catalysts is the absence of isomeric bipyridines among the products. In this respect reactions using metal catalysts in a heterogeneous system differ from methods which have been used to prepare bipyridines in homogeneous sys-tems. ... 

Where cracks follow intergranular paths due to electrochemical effects related to grain boundary heterogeneity it is likely that selective attack should be observed in such locations by exposure to potent environments even in the absence of applied stress. This has been observed with, low-... 

Primary generalized seizures are also heterogeneous with respect to their clinical features. Such seizures can impose as absence epilepsy, which is characterized by a brief interruption of consciousness due to highly synchronized neuronal activity involving thalamocortical networks without increases in neuronal firing rate. On the other hand, tonic-clonic convulsions with loss of consciousness are often also primarily generalized. 

In ethanol In the absence of tin exhibits an emission maximum at 555 nm upon 366 nm excitation. Figures 3 and 4 show emission spectra of the Sn /flavonol complex on Irregularly shaped glass beads of 10-100 tm diameter, and of BuSn /flavonol accvmulated by Pseudomonas 244 cells respectively. The glass bead study serves as a model of tin adhesion to a small heterogeneous surface from which spectra can be directly obtained only by mlcrospectrofluorometrlc techniques. 

The mechanism of the UDMH dark decay is unknown, but it is presumed to be heterogeneous in nature. It is probably not wall adsorption, since much slower decay rates were observed previously in the absence of O2 ( ) ... 

Evidence exists that the relative solubility of amines and inhibitors in heterogeneous oil-water systems could be decisive in formation of nitrosamines and blocking these reactions, Nitrosopyrrolidine formation in bacon predominates in the adipose tissue despite the fact that its precursor, proline, predominates in the lean tissue (5,6,7). Mottram and Patterson (8) partly attribute this phenomenon to the fact that the adipose tissue furnishes a medium in which nitrosation is favored, Massey, et al, (9) found that the presence of decane in a model heterogeneous system caused a 20-fold increase in rate of nitrosamine formation from lipophilic dihexylamine, but had no effect on nitrosation of hydrophilic pyrrolidine. Ascorbic acid in the presence of decane enhanced the synthesis of nitrosamines from lipophilic amines, but had no effect on nitrosation of pyrrolidine. The oil-soluble inhibitor ascorbyl palmitate had little influence on the formation of nitrosamines in the presence or absence of decane. 

Ultrafiltration of heterogenous colloidal suspensions such as citrus juice is complex and many factors other than molecular weight contribute to fouling and permeation. For example, low MW aroma compounds were unevenly distributed in the permeate and retentate in UF in 500 kd MWCO system (10). The authors observed that the 500 kd MWCO UF removed all suspended solids, including pectin and PE. If PE is complexed to pectate in an inactive complex, then it is conceivable that release of PE from pectin with cations will enhance permeation in UF. At optimum salt concentration, less PE activation was observed at lower pH values than at higher pH (15). In juice systems, it is difficult to separate the effect of juice particulates on PE activity. Model studies with PE extracts allows UF in the absence of large or insoluble particulates and control of composition of the ultrafilter. In... 

The main advantage of a homogeneous immunoassay, compared to a heterogeneous immunoassay, is the absence of a separation step. This translates into a simpler procedure and easier automation. However, homogeneous assays are typically less sensitive and more susceptible to sample interferences which are removed in a separation step. 

The model predicts that in the presence of soot, the removal of NO decreases due to both heterogeneous generation of NO from N02 adsorbed on the surface of the soot particles and decrease of reactivity of the hydroxyalkyl radicals. It was found that NO removal, both in the presence and absence of soot, is about the same, however, the proportion of NO and N02 depends on the soot. With lower densities (107 cm-3) of soot particles, the final NO was primarily N02, whereas, with higher densities (109cm-3) of soot particles, the NO was mainly NO. 

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