Reactant mobility

Despite the current lack of clarity regarding the relationship between glass transition and chemical reaction kinetics, it is still quite feasible that chemical and biochemical reaction rates may be governed by mobility, i.e., the mobility that is most rate limiting to a particular reaction scheme (e.g., water mobility, reactant mobility, molecular-level matrix mobility, local or microregion mobility), but perhaps not simply by an average amorphous solid mobility as reflected by the Tg. Ludescher et al. (2001) recommend the use of luminescence spectroscopy to investigate how rates of specific chemical and physical processes important in amorphous solid foods are influenced by specific modes of molecular mobility, as well as by molecular structure. 

In the more difficult case of asymmetry in both mobilities (DA = 0, >b > 0) and initial concentrations, Sn > 0, a kind of master equation was derived and solved numerically, along with the Monte Carlo simulations [32], As it is seen in Fig. 6.26, for equal particle concentrations the decay asymptotics, shown by dashed lines, is reached very fast for both symmetrical and asymmetrical reactant mobilities (curves 1). It is no longer tme, however, for asymmetrical concentrations shown in curves 2. These latter demonstrate clearly an existence of the two different classes of universality for reactions with unequal concentrations one class corresponds to the case when both (A and B) reactants are mobile or those which are in majority whereas... 

There are numerous references that show a correlation between matrix properties and reaction rates (Labuza et al., 1977 Roos, 1993 Bell and Hageman, 1994 Buera and Karel, 1994 Bell, 1996), but arguably not to the exclusivity of solvent-based effects, and never with a direct link to the mobility of the reactant itself rather than the polymer. Yet there are a handful of studies that come very close to showing a direct link between reactant mobility and polymer mobility. 

Regardless of the mechanism, the results most clearly illustrate that a alone does not provide an adequate mechanism to explain the role of moisture in governing food stability. Model systems are perfect for studying the complexities of reactant mobility as a determinant for reaction rate. What is lacking is a study that examines multiple aspects of mobility in correlation to the rate of a chemical reaction. Changes in moisture content and affect matrix plasticity and thus mobility, solute solvency, rotational mobility, and translational mobility. Each of these aspects of mobility may or may not each have an effect on reaction rate, depending on the reaction and the chemical constituents within the system. 

In the second stage, diffusion process dominates the network formation as the reactants mobility is greatly reduced by polymerized networks. The incorporation of nano-alumina particles to the epoxy-amine system has an accelerating effect on the curing reaction. In this case, both physical interaction and chemical interaction at the nanoparticle surface are possible. 

The rates of biochemical reactions are dependent on the proximity and mobility of the reactants. Mobility is determined by the mutual interactions of the solvent with the solutes. The state of water (the solvent) determines the mobility of the solutes and in return, the solutes change the structural organization of nearby water molecules through hydrophihc and hydrophobic interactions. In the cytoplasm, the thermodynamic state of the medium (and therefore the molecular mobihty) determines the rate of metabolic activity. 

The sequence of events in a surface-catalyzed reaction comprises (1) diffusion of reactants to the surface (usually considered to be fast) (2) adsorption of the reactants on the surface (slow if activated) (3) surface diffusion of reactants to active sites (if the adsorption is mobile) (4) reaction of the adsorbed species (often rate-determining) (5) desorption of the reaction products (often slow) and (6) diffusion of the products away from the surface. Processes 1 and 6 may be rate-determining where one is dealing with a porous catalyst [197]. The situation is illustrated in Fig. XVIII-22 (see also Ref. 198 notice in the figure the variety of processes that may be present). 

Among the complications that can interfere with this conclusion is the possibility that the polymer becomes insoluble beyond a critical molecular weight or that the low molecular weight by-product molecules accumulate as the viscosity of the mixture increases and thereby shift some equilibrium to favor reactants. Note that we do not express reservations about the effect of increasing viscosity on the mobility of the polymer molecules themselves. Apparently it is not the migration of the center of mass of the molecule as a whole that determines the reactivity but, rather, the mobility of the chain ends which carry the reactive groups. 

The flux of flie adsorbed species to die catalyst from flie gaseous phase affects die catalytic activity because die fractional coverage by die reactants on die surface of die catalyst, which is determined by die heat of adsorption, also determines die amount of uncovered surface and hence die reactive area of die catalyst. Strong adsorption of a reactant usually leads to high coverage, accompanied by a low mobility of die adsorbed species on die surface, which... 

In solid state chemistry the limited atomic mobility in the solid state controls chemical changes and leads to explicit consideration of the relative location of potential reactants (the configuration) and solid state reactivity as controlled by solid state defects. The same factors dominate shock-induced solid state chemistry. 

M. Tammaro, M. Sabella, J. W. Evans. Hybrid treatment of spatio-temporal behavior in surface reactions with coexisting immobile and highly mobile reactants. J Chem Phys 705 10277-10285, 1995. 

A quite different approach was introduced in the early 1980s [44-46], in which a dense solid electrode is fabricated which has a composite microstructure in which particles of the reactant phase are finely dispersed within a solid, electronically conducting matrix in which the electroactive species is also mobile. There is thus a large internal reactant/mixed-conductor matrix interfacial area. The electroactive species is transported through the solid matrix to this interfacial region, where it undergoes the chemical part of the electrode reaction. Since the matrix material is also an electronic conductor, it can also act as the electrode s current collector. The electrochemical part of the reaction takes place on the outer surface of the composite electrode. 

Evidence concerning the identity of the mobile species can be obtained from observation [406,411—413] of the dispositions of product phases and phase boundaries relative to inert and immobile markers implanted at the plane of original contact between reactant surfaces. Movement of the markers themselves is known as the Kirkendall effect [414], Carter [415] has used pores in the material as markers. Product layer thickness has alternatively been determined by the decrease in intensity of the X-ray fluorescence from a suitable element which occurs in the underlying reactant but not in the intervening product layers [416]. 

Zero-order kinetic behaviour, in an unusual dehydration reaction [62], has been shown to be due to the constant area of reaction interface and this interface has been identified as original surfaces of the reactant crystallites which do not advance. Water molecules are mobile within the... 

The following assumptions are made (i) the activated complexes are in equilibrium with the reactants, (ii) the energy of a molecule is not altered when an activated complex is substituted for a nearest neighbour, and (iii) the products do not affect the course of reaction, except to define a boundary in surface processes. The various cases can be recognized from the magnitude of the pre-exponential term and calculated values [515] are summarized in Table 7. Low values of A indicate a tight surface complex whereas higher values are associated with a looser or mobile complex. 

Ni3C decomposition is included in this class on the basis of Doremieux s conclusion [669] that the slow step is the combination of carbon atoms on reactant surfaces. The reaction (543—613 K) obeyed first-order [eqn. (15)] kinetics. The rate was not significantly different in nitrogen and, unlike the hydrides and nitrides, the mobile lattice constituent was not volatilized but deposited as amorphous carbon. The mechanism suggested is that carbon diffuses from within the structure to a surface where combination occurs. When carbon concentration within the crystal has been decreased sufficiently, nuclei of nickel metal are formed and thereafter reaction proceeds through boundary displacement. 

Jacobs et al. [59,925,926] (Fig. 17). While this scheme conveniently summarizes many features of the observed behaviour, a number of variations or modifications of the mechanisms indicated have been proposed. Maycock and Pai Vemeker [924,933] emphasize the possible role of point defects and suggest, on the evidence of conductivity measurements, that the initial step may be the transfer of either a proton or an electron. Boldyrev et al. [46] suggest that proton conduction permits rapid migration of HC104 within the reactant and this undergoes preferential decomposition in distorted regions. More recently, the ease of proton transfer and the mobilities of other species in or on AP crystals have been investigated by a.c. [360] and d.c. [934] conductivity measurements. Owen et al. [934] could detect no surface proton conductivity and concluded that electron transfer was the initial step in decomposition. At the present time, these inconsistencies remain unresolved. 

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