Physical-Chemical Phenomena Influencing the Reaction

Summary. All of the phases and the physical and chemical mechanisms discussed in this section are important during the course of an emulsion polymerization reaction. They influence the reaction kinetics and the properties of the latex produced. Not all of the phenomena that can occur are understood in a quantitative manner. Nevertheless, considerable advances have been made in the fundamental understanding and the commercial exploitation of emulsion polymerization processes. The remainder of this chapter will focus on reactor types and reaction kinetics. 

The various solvent scales can be used to determine which property of a solvent has the greatest influence on reactivity or any other physical / chemical phenomena. An example of their use in a common reaction is given in the following Connections highlight, and we will also showcase their use in a Connections highlight concerned with the hydrophobic effect in the next chapter. 

In Fig. 1, various elements involved with the development of detailed chemical kinetic mechanisms are illustrated. Generally, the objective of this effort is to predict macroscopic phenomena, e.g., species concentration profiles and heat release in a chemical reactor, from the knowledge of fundamental chemical and physical parameters, together with a mathematical model of the process. Some of the fundamental chemical parameters of interest are the thermochemistry of species, i.e., standard state heats of formation (A//f(To)), and absolute entropies (S(Tq)), and temperature-dependent specific heats (Cp(7)), and the rate parameter constants A, n, and E, for the associated elementary reactions (see Eq. (1)). As noted above, evaluated compilations exist for the determination of these parameters. Fundamental physical parameters of interest may be the Lennard-Jones parameters (e/ic, c), dipole moments (fi), polarizabilities (a), and rotational relaxation numbers (z ,) that are necessary for the calculation of transport parameters such as the viscosity (fx) and the thermal conductivity (k) of the mixture and species diffusion coefficients (Dij). These data, together with their associated uncertainties, are then used in modeling the macroscopic behavior of the chemically reacting system. The model is then subjected to sensitivity analysis to identify its elements that are most important in influencing predictions. 

Historically, it dates from the early 1920 s. Indeed, in 1924 U.R. Evans proposed an equation showing the comparative influence of chemical and physical phenomena on the growth rate of a chemical compound layer. Unfortunately, its importance for understanding the essence of the process of reaction diffusion was not estimated properly at that time. Moreover, even now many researchers, especially physicists and metallurgists, tend to underestimate its significance. 

Polymerization is influenced by the physical structure and phase of the monomer and polymer. It proceeds in the monomer, and the chemical configuration of the macromolecules formed depends on whether the monomer is a liquid, vapor, or solid at the moment of polymerization. The influence of structural phenomena is evident in the polymerization of acrylic monomer either as liquids or liquid crystals. Supermolecular structures are formed in solid- and liquid-state reactions during and simultaneously with polymerization. Structural effects can be studied by investigating the nucleation effect of the solid phase of the newly formed polymer as a nucleation reaction by itself and as nuclei for a specific supermolecular structure of a polymer. Structural effects are demonstrated also using macromo-lecular initiators which influence the polymerization kinetics and mechanism. 

In this text, the conversion rate is used in relevant equations to avoid difficulties in applying the correct sign to the reaction rate in material balances. Note that the chemical conversion rate is not identical to the chemical reaction rate. The chemical reaction rate only reflects the chemical kinetics of the system, that is, the conversion rate measured under such conditions that it is not influenced by physical transport (diffusion and convective mass transfer) of reactants toward the reaction site or of product away from it. The reaction rate generally depends only on the composition of the reaction mixture, its temperature and pressure, and the properties of the catalyst. The conversion rate, in addition, can be influenced by the conditions of flow, mixing, and mass and heat transfer in the reaction system. For homogeneous reactions that proceed slowly with respect to potential physical transport, the conversion rate approximates the reaction rate. In contrast, for homogeneous reactions in poorly mixed fluids and for relatively rapid heterogeneous reactions, physical transport phenomena may reduce the conversion rate. In this case, the conversion rate is lower than the reaction rate. 

The colloidal nature of the reaction media has a significant influence on the course of an emulsion polymerization reaction. A number of distinct phases exist during different intervals of a batch reaction. Chemical and physical phenomena within these phases and at the interfaces can be important in determining reaction kinetics and the properties of the latex product. 

The views so far presented in this chapter may be summarized as being based upon the primary formation of addition compounds when two or more molecules react, these addition compounds then breaking down to form new molecules. In catalytic reactions, the first stage of the reaction is the same, but. in the second stage, one of the substances formed in the breaking down of the intermediate compound is identical in composition with one of the substances which took part initially in the reaction in the formation of the addition compound. While the experimental evidence is favorable to this view of catalytic reactions in many cases, it may be objected that physical influences may often modify the velocity of the reaction between gases. At present there is no experimental evidence of any kind available to prove or disprove the formation of definite chemical compounds in such cases, but on the other hand, evidence is accumulating that adsorption (or perhaps the solution of a gas or a liquid in a solid) is the important factor here. Just how far phenomena of this nature may be identical with the formation of definite chemical compounds (possibly so-called loose combinations) on a surface is not at present certain, but until direct evidence is obtained that such reactions must be included in a... 

The application of alkali metals at high temperatures can utilize chemical properties, or may be influenced by chemical reactions. The solutions of non-metallic elements in the alkali metals change physical properties of the pure metals. Dissolved elements are able to cause or to influence the corrosion phenomena. Some compounds are only formed or made stable in the presence of excess liquid alkali metal. Due to such an influence of chemical reactions on the behavior or metals, the new applications have initiated many studies in alkali metal chemistry. Interest has been concentrated on the elements lithium and sodium. The heavier alkali metals have found further interest in more recent work. 

Fundamental work by Luche resulted in the hypothesis that ultrasound can influence and change reaction pathways in reaction types with single electron transfer [186, 187]. Ultrasound is also believed to influence reaction systems by mechanical effects [187]. An empirical classification of sonochemical reactions is divided into three types of effects purely chemical effects induced by sonochemical cavitation, hydrodynamic effects (mechanically induced cavitation), and by-passing mass-transport limitation. The latter effects are based on physical rather than chemical phenomena and judged to be false sonochemistry [188]. Nevertheless, these false effects (e.g. emulsification) are often important. The three types of effect are ... 

Molecular simulation techniques can obtain the microscopic information that cannot be detected by current experimental conditions, but the conventional simulation methods stiU have inherent limitations with special mesoscopic scales of various complex forces and complex structure. It is necessary to establish a new mesoscale method that considers the chemical reaction and transport to the larger system at the same time. The roughness and chemical properties of catalyst supporting interface have great influence on chemical and physical adsorption stability of clusters. The problem is that the system is too large for traditional simulation in nano-/micro-/mesoscale. We need a new mesoscale method to study the effect of interface roughness on physical/chemistry phenomena. 

The thermal decomposition of a solid, which necessarily (on the above definition) incorporates a chemical step, is sometimes associated with the physical transformations to which passing reference was made above melting, sublimation, and recrystallization. Aspects of the relationships between physical transitions and decomposition reactions of solids are discussed in a book by Budnikov and Ginstling [1]. Since, in general, phase changes exert significant influence upon concurrent or subsequent chemical processes, it is appropriate to preface the main survey of the latter phenomena with a brief account of those features of melting, sublimation, and recrystallization which are relevant to the consideration of thermal decomposition reactions. 

Several possibilities exist to determine the influence of transport phenomena. The measurement of gas consumption in dependence on the interfacial area, the physical absorption coefficient, the rate of a chemical reaction following the absorption, and the concentration gradient (as the driving force of the absorption) allows decisions to be made on which regime is, in fact, in existence [40]. 

The bulk properties of macroscopic crystals cannot be affected drastically by the difference which exists between the structure of the interior and that of a surface film which is approximately 10,000 atoms deep. However, even for macroscopic crystals, rate phenomena such as modification changes which are initiated within the surface are likely to be influenced by the environment, which would include molecules which are conventionally described as physically adsorbed. Apparently it is not generally understood that even the presence of a noble gas can affect the chemical reactivity of solids. Brunauer (3) explained that in principle physical adsorption of molecules should affect the solid in the same manner as chemisorption. As action and reaction are equal, chemisorption may have a stronger effect on both the solid and the adsorbed molecule. 

The equations (3.109), (3.117) or (3.118) and (3.120) for the velocity, thermal and concentration boundary layers show some noticeable similarities. On the left hand side they contain convective terms , which describe the momentum, heat or mass exchange by convection, whilst on the right hand side a diffusive term for the momentum, heat and mass exchange exists. In addition to this the energy equation for multicomponent mixtures (3.118) and the component continuity equation (3.25) also contain terms for the influence of chemical reactions. The remaining expressions for pressure drop in the momentum equation and mass transport in the energy equation for multicomponent mixtures cannot be compared with each other because they describe two completely different physical phenomena. 

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