Competitive effects

A more subtle complication to the analysis of diffusion-limited reactions in solution than those discussed above is due to the competition between reactants (say of type A) for the other reactant (species B). Consider two reactants Ai and A2 and two reactants B, and B2. If At and Bi react, then A2 and B2 have to react with each other or not react at all. Other combinations are possible. The fate of the first reaction affects the subsequent reaction. Clearly, such an effect can only be expected to be important when A and B are present in comparable concentrations. Given that the previous discussion has considered one or other reactant to be present in vast excess over the other, such an analysis and that including competitive effects are at opposite extremes of the concentration ratio range. 

So far, most analyses of the competitive effect have assumed one species, say A, to be stationary, and the other, B, to diffuse in the surrounding space. The concentration of B around any one A reactant (A ) is approximately given by the Smoluchowski [3] (or Collins and Kimball 

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Cr, Br , I, which cause pitting attack, and anions which form soluble complexes with aluminium , e.g. citrate and tartrate, which cause general attack. Competitive effects , similar to those observed on iron, are observed in the action of mixtures of inhibitive anions and chloride ions on aluminium. The inhibition of aluminium corrosion by anions exhibits both an upper and a lower pH limit. The pH range for inhibition depends upon the nature of the anion . 

The competition model and solvent interaction model were at one time heatedly debated but current thinking maintains that under defined r iitions the two theories are equivalent, however, it is impossible to distinguish between then on the basis of experimental retention data alone [231,249]. Based on the measurement of solute and solvent activity coefficients it was concluded that both models operate alternately. At higher solvent B concentrations, the competition effect diminishes, since under these conditions the solute molecule can enter the Interfacial layer without displacing solvent molecules. The competition model, in its expanded form, is more general, and can be used to derive the principal results of the solvent interaction model as a special case. In essence, it seems that the end result is the same, only the tenet that surface adsorption or solvent association are the dominant retention interactions remain at variance. 

Competition Effects on Carotenoid Absorption by Caco-2 Cells... 

In conclusion, the Caco-2 cell monolayer model has given original data on the competition effect of several nutrients on carotenoid uptake. Most of these data have been confirmed in several in vivo studies, including clinical studies, confirming that this model is a valuable tool to study competition effects on carotenoid absorption. 

Goldberg (2002) found no evidence of any competition in sorption of arsenate and arsenite on Al or Fe-oxides and montmorillonite, but only a small and apparent competitive effect of equimolar arsenate on arsenite sorption on kaolinite and illite. The minor competitive effect in this study was due to the small concentrations of arsenic which is very low for saturation site. Competition for sorption sites is evident by increasing the surface coverage of the sorbents. Arsenate prevents arsenite sorption on metal oxides when the surfaces of the sorbents are saturated by the anions (Jain and Loeppert 2000 Violante and Pigna 2002). 

Sleugh B, Moore KJ, George JR, Brummer EC (2000) Binary legume-grass mixtures improve forage yield, quality, and seasonal distribution. Agron J 92 24-29 Smolinska U, Morra MJ, Knudsen GR, James RL (2003) Isothiocyanates produced by Brassicaceae species as inhibitors of Fusarium oxysporum. Plant Dis 87 407-412 Soon YK, Harker KN, Clayton GW (2004) Plant competition effects on the nitrogen economy of field pea and the subsequent crop. Soil Sci Soc Am J 68 552-557... 

Traytak, S. D. (1996). Competition effects in steady-state diffusion-limited reactions renormalization group approach, J. Chem. Phys., 105, 10 860-10 867. 

Difficulties of incompatibility can arise with mixtures of basic dyes on acrylic fibres because of competition for the limited number of dyeing sites available and the differences between dyes in terms of affinity and rate of diffusion. The rate of uptake of each dye when applied in admixture with another is invariably slower than when the dye is applied alone at the same concentration. Competition effects of this kind can lead to serious practical problems unless the dyes are carefully designed and selected to have similar dyeing characteristics [97,98,104,105]. Dyes with exceptionally low affinity and rapid rates of diffusion have been developed, offering improved migration on acrylic fibres [106]. These dyes have migration properties not unlike those of monosulphonated acid dyes on nylon. 

In other words, the response (which for fast kinetics is more anodic compared to E° ), due to the competitive effects of the potential scan rate, moves towards more cathodic values by 30/n (mV) for every ten-fold increase in the scan rate. However, as shown in Figure 14, it is noted that at the same time the reverse peak tends to disappear, in that on increasing the scan rate, the species Z does not have time to restore Red. This is demonstrated by the current ratio zpr/zpf which is about 1 at low scan rates, but it tends to zero at high scan rates. 

The observed linearity of adsorption isotherms in various data sets in the literature and the absence of competitive effects are not evidence for partitioning alone, because such behavior can also be consistent with a physical adsorption model. 

What little is known about the microbiology of CH4 oxidation in the rice rhizosphere indicates complicated kinetics and competition effects. 

Kochhar, M. Phytotoxic and Competitive Effects of Tall Fescue on Ladino Cover as Modified by Ozone and/or Rhizoctonia sotani. Ph.D. Thesis. Raleigh North Carolina State University, 1974. 70 pp. 

Customer orientation and initial solution-free formulation of customers wishes, as an orientation for product development, appear to be promising approaches for innovations with regard to the application safety of chemicals-based products. However, the initiative for this is not mainly due to substance manufacturers, but rather to the chemicals users being close to the consumers. To what extent the commercial/industrial chemicals end-users (users of production auxiliary materials that are not included in the product) also transform the latent desire for application-safe products into effective demand behaviour, depends on other constellations of motives than those of private end-consumers. The employers liabihty insurance, chambers of commerce and industry, branch associations, trade unions and management boards of large-scale companies play a key role in making quality and competition effective as drivers for innovation here too. 

One of the recommendations in the FTC Staff Comment on Citizen Petitions was that the FDA consider requiring notification of whether the citizen petitioner has received, or will receive, consideration for filing the citizen petition and identification of the party furnishing the consideration. This information may be important in evaluating the competitive effect of the petition. 

The data on the reactivities of trichloroethylene and tetrachloroethylene further illustrate the competitive effects of substitutions on the 1- and 2-positions of ethylene. Trichloroethylene is more reactive than either of the 1,2-dichloroethylenes but less reactive than vinylidene chloride. Tetrachloroethylene is less reactive than trichloroethylene—analogous to the difference in reactivities between vinyl chloride and 1,2-dichloroethylene. The case of polyfluor-oethylenes is an exception to the generally observed large decrease in reactivity with polysubstitution. Tetrafluoroethylene and chlorotrifluoroethylene show enhanced reactivity due apparently to the small size of the fluorine atoms. 

Caffeine is a known inhibitor of phosphodiesterase. Caffeine has an effect on calcium-mediated signalling namely it causes an increase of cAMP activity. Caffeine also has a competitive effect on the central adenosine receptor and is thought to increase analgesic activity. It is also known to be somewhat effective... 

Kochhar, M. "Phytotoxic and competitive effects of tall fescue on ladino clover as modified by ozone and/or Phizoctonio solani." Ph.D. Thesis. North Carolina State University. Raleigh, NC. (1973). 

Activation of a-D-mannosidase from the limpet by Zn2+ and Cl-provides a particularly good example of the ways in which the kinetics of hydrolysis may be altered. Fig. 2 shows the effect of Zn2+, Cl-, or both, on the velocity of hydrolysis of substrate at varying concentration. Inspection of die curves reveals that Zn2+ increases the affinity of the enzyme for the substrate (competitive type of effect), whereas the main effect of Cl- is to increase the rate of hydrolysis (non-competitive effect). 

The effectiveness of a fixed-bed operation depends mainly on its hydraulic performance. Even if the physicochemical phenomena are well understood and their application in practice is simple, the operation will probably fail if the hydraulic behavior of the reactor is not adequate. One must be able to recognize the competitive effects of kinetics and fluid dynamics mixing, dead spaces, and bypasses that can completely alter the performance of the reactor when compared to the ideal presentation (Donati and Paludetto, 1997). The main factor of failure in liquid-phase operations is liquid maldistribution, which could be related to low liquid holdup in downflow operation, or other design problems. These effects could be critical not only in full-scale but also in pilot- or even in laboratory-scale reactors. 

Such NACs do not exhibit linear isotherms when they sorb under certain conditions with aluminosilicate clays (Fig. 11.66 Haderlein and Schwarzenbach, 1993 Hader-lein et al., 1996). Rather they show saturation behavior indicating an association with specific sites on the solid surfaces. This specific site interaction is also indicated by the observations of competitive effects among different NACs in sorption experiments. Further, the sorption enthalpies have been found to be much greater than excess enthalpies of aqueous solution of these sorbates (e.g., 4-methyl-2-nitro-phenol exhibits a sorption enthalpy of-41.7 kJ-mol"1). These data all indicate that there is a strong specific interaction of NACs with the aluminosilicate clay surfaces. 

What is especially noteworthy is that the pairwise approximation is so successful at modelling a spur containing 3 or 6 reactants and under conditions where some competitive effects might be anticipated. Clearly, such competitive effects are not very important. Further work on these lines is keenly awaited. Competitive effects are discussed more fully in Chap. 8, Sect. 2.3 and Chap. 9, Sects. 5 and 6. 

By contrast, when both the reactive solute molecules are of a size similar to or smaller than the solvent molecules, reaction cannot be described satisfactorily by Langevin, Fokker—Planck or diffusion equation analysis. Recently, theories of chemical reaction in solution have been developed by several groups. Those of Kapral and co-workers [37, 285, 286] use the kinetic theory of liquids to treat solute and solvent molecules as hard spheres, but on an equal basis (see Chap. 12). While this approach in its simplest approximation leads to an identical result to that of Smoluchowski, it is relatively straightforward to include more details of molecular motion. Furthermore, re-encounter events can be discussed very much more satisfactorily because the motion of both reactants and also the surrounding solvent is followed. An unreactive collision between reactant molecules necessarily leads to a correlation in the motion of both reactants. Even after collision with solvent molecules, some correlation of motion between reactants remains. Subsequent encounters between reactants are more or less probable than predicted by a random walk model (loss of correlation on each jump) and so reaction rates may be expected to depart from those predicted by the Smoluchowski analysis. Furthermore, such analysis based on the kinetic theory of liquids leads to both an easy incorporation of competitive effects (see Sect. 2.3 and Chap. 9, Sect. 5) and back reaction (see Sect. 3.3). Cukier et al. have found that to include hydrodynamic repulsion in a kinetic theory analysis is a much more difficult task [454]. 

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