The kinetics of mechanochemical reactions

Thermodynamic considerations may be insufficient to make a conclusion on essential occurrence of a reaction if the conditions determining its kinetics are not taken into account. 

The large value of activation energy in Arrhenius equation 

The main statements of the kinetics of solid-phase reactions which bind the interaction rate with the diffusion of one of components, at first over the surface of another component, and then in the bulk through product layer, are not suitable for mechanochemical reactions, since mechanical activation of a mixture involves continuous renewal of contact surfaces. On this grounds, it was assumed that the interaction rate is limited by the rate of chemical reaction at contact. 

Due to the pulsed action of balls on the reacting mixture, the process non-uniformity as in the time and in the space of a single mechanical action (the existence of gradient from the center of collision to its periphery), and changes of the conditions for chemical interaction in the course of activation, kinetic description of mechanochemical reactions is a complicated task. Therefore, one should not expect the creation of some universal reaction model only some particular models are possible. 

Mechanical activation of mixtures involves the changes in the conditions for chemical interaction, as it follows from Fig. 3.3 [4,5]. The first stage of interaction is connected with the progressive growth of surface area during activation. Real 

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As it was demonstrated in [7], a large number of papers on the kinetics of mechanochemical reactions report S-shaped experimental... 

According to X-ray study, the oxidation state of manganese ions greatly influenced on the kinetics of mechanochemical reaction of manganese... 

In chemical reactions initiated or accelerated by mechanical action, the estimation of the elastic energy involved is extremely difficult. This is mainly because of the lade of clear concepts concerning the mechanism of these reactions, and because of the methodological difficulties in measuring the amount of mechanical energy absorbed as well as the magnitude of the stresses that actually operate. Mechanochemical reactions are complex multistage processes, which include mechanic deformation of the molecule before the actual chemical reactions take place. In the kinetics of mechanochemical reactions we may consider reactions in constant mechanical fields and reactions in varid)le mechanical fields. 

Boldyrev V.V. About kinetic factors determining the specific of mechanochemical reactions in inorganic systems. Kinetika i Katahz 1972 13 1411-21. 

Water is one of the products formed via mechanochemical interaction between hydroxides. Its presence can substantially effect on the mechanism of transport processes and, thus, on the kinetics of mechanochemical processes, since some reaction products (initial or final) can be dissolved in it. Hence, the reaction could proceed with the participation of the dissolved forms of components. 

The possibility of mechanochemical reactions to occur via hydrothermal mechanism was demonstrated in [32] for the synthesis of calcinm hydrosilicates. Optimal water content values (humidity) of solid mixtures of correspondent hydrates were estimated for the achievement of local hydrothermal conditions (elevated temperature and pressure) in mechanochemical activators. It was shown experimentally that the main factor determining the kinetics and composition of the products was the molar fraction of the components, the same as for hydrothermal reactions, while the effect of temperatnre and pressnre was mnch weaker. 

The kinetics of tribochemical transformations, i.e. the ratio of reaction velocity to temperature, the intensity of mechanical actions and other factors, is defined largely by two sequential stages of the mechanochemical process, namely i) load application, deformations and distribution of elastic stresses in the system, and ii) the chemical reaction itself initiated by stresses. 

Successful applications of the macrocyclic force probes have been demonstrated in the kinetic study of mechanochemical reactions, especially multistep reactions. By incorporating an ester group in the stiff stilbene-based macrocycle, Boulatov and co-workers investigated its alkaline hydrolysis as a functimi of restoring force [110]. It was found that the hydrolysis of ester is insensitive to mechanical force, which is in agreement with MD simulations [112]. This is because the relative... 

In recent years, considerable efforts have been taken to follow reactions induced by mechanochemical treatment and to relate them with the milling parameters. Therefrom, attempts have been made toward unification of the influence of the milling parameters on the mechanochemical reactions [70,71,95-100]. Although the mechanisms of mechanochemical that is, mechanically induced reactions is not fuUy understood the overall kinetics may be derived from suitable measurements (e.g., structural, magnetic) of powder milled for various milling times. 

Because ATP and GTP are required in many biosynthetic and mechanochemical reactions, the ability to manipulate their concentrations is particularly important in many kinetic studies. The three enzymes most commonly employed for this purpose are hexokinase, phosphofruc-tokinase, and alkaline phosphatase. Each of these enzymes offers advantages and disadvantages, and one must consider the design requirements for any particular experiment before using one of these to deplete ATP or GTP. 

Gradual mechanochemical reaction systems have generally been found to exhibit sigmoidal reaction kinetics. The reaction rate initially increases with milling due to increasing activation and microstructural refinement of the reactants. The reaction rate then reaches a maximum at an intermediate milling time before decreasing as the reaction approaches completion due to dilution of the reactants by the product phases. ... 

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