Network thermodynamics chemical reaction processes

The reliable long-term safety assessment of a nuclear waste repository requires the quantification of all processes that may affect the isolation of the nuclear waste from the biosphere. The colloid-mediated radionuclide migration is discussed as a possible pathway for radionuclide release. As soon as groundwater has access to the nuclear waste, a complicated interactive network of physical and chemical reactions is initiated, and may lead to (1) radionuclide mobilization (2) radionuclide retardation by surface sorption and co-precipitation reactions and (3) radionuclide immobilization by mineralization reactions, that is, the inclusion of radionuclides into thermodynamically or kinetically stabilized solid host matrices. 

All life processes are the result of enzyme activity. In fact, life itself, whether plant or animal, involves a complex network of enzymatic reactions. An enzyme is a protein that is synthesized in a living cell. It catalyzes a thermodynamically possible reaction so that the rate of the reaction is compatible with the numerous biochemical processes essential for the growth and maintenance of a cell. The synthesis of an enzyme thus is under tight metabolic regulations and controls that can be genetically or environmentally manipulated sometimes to cause the overproduction of an enzyme by the cell. An enzyme, like chemical catalysts, in no way modifies the equilibrium constant or the free energy change of a reaction. 

Generally, a critical evaluation of chemical thermodynamic data involves a network approach. A small sample of such a network approach is illustrated in Figure 1, for a few barium compounds. Each line represents a reaction (process), each node, represents a compound. The network approach has been discussed in reference (1). Various techniques are used by the analyst in evaluation of the thermodynamic consistency and reliability of individual reaction measurements. One of these is to analyze a given thermochemical network of measurements into various combinations of reactions that result in identical initial and final states. For each such loop the algebraic sum of changes in a thermodynamic variable (AH, AG) should equal zero except for the combination of experimental uncertainties. Analysis of these residuals from the various loops may reveal certain measurements to be inconsistent with the remainder of the reactions. Similarly, solutions of the entire network using both least sums and least squares techniques are valuable (2). The least sums technique minimizes the sums of the residuals whereas the least squares technique minimizes the sum of the deviations squared. Large residuals found in the solution are indicative of thermodynamic inconsistency with respect to the total set of measurements. 

The first question was answered by Network Thermodynamics (e.g. Oster et al., 1973 Schnakenberg, 1977 Peusner, 1985) adopting the bond graph technique. This approach benefits from the formal correspondence between certain interpretations of nonequilibrium thermodynamics and of electrical network theory. Adopting the notions of chemical impedance , chemical capacity and chemical inductance , chemical reactions as well transport processes can be represented by networks obeying KirchhofFs current and voltage laws. 

This example is not unique. Chemical explanations of biological phenomena have always to be complemented by evolutionary explanations. The structure of the metabolic network can be explained by the nature of the chemical reactions which form it, and the laws of thermodynamics but also by the evolutionary process which has recombined different pathways to generate new pathways and cycles. Biological phenomena have emerged from this complex history, in the sense that they were the products of this history. The attention paid to these phenomena in biological explanations is simply the recognition of the historical dimension of organisms. 

Thus, a general conclusion can be drawn that the viscoelastic properties of the full and semi-IPNs depend not only on the miscibility and thermodynamic interaction between two constituent networks, but also on the reaction kinetics. Both thermodynamic and kinetic factors determine the viscoelasticity of these systems due to superposition of the chemical and physical processes occurring in the systems during curing. 

As distinct from almost all polymer materials, the kinetics of IPN formation is a governing factor in development of the system morphology. For traditional polymeric materials their structure and morphology depend on the ways of processing, heat treatment, and other physical, not chemical, factors, while for IPNs all depends on the kinetic conditions. One may say that thermodynamics gives the general rules of the equiUbrium state and determines the path to equilibrium, whereas the kinetics allows the reahzation of the path and predetermines the real structure far from equilibrium. This specific feature of the IPN formation is connected with the fact that in the reaction system two processes proceed simultaneously the chemical process of network formation and the physical process of phase separation. As will be shown below, these processes are interconnected. 

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