Solid-phase reactant interactions

The association rate constants (k2) for solution-phase antibody systems are on the order of 107-108 M 1 s 1 [23] but those for reactions on synthetic solid phases and cell surfaces are two to four orders of magnitude slower, mainly as a consequence of the sluggish diffusion and slower mass transfer of the reactants to the interaction sites. The dissociation rate constant for heterogeneous systems (k j is on the order of 10 -10 5 s up to two orders of magnitude slower than for solution-phase systems, and for solid phase immunoassays, it is attributed to multivalent interactions and to surface coagulation or aggregation via translational diffusion in the presence of extensive cross-linking at the interface [23]. Most of the MIA-based assays described in the literature require equilibration times on the order of hours, and more synthetic efforts are required to reduce this analysis time. 

The adsorption process, unlike antigen-antibody interactions, is nonspecific. Thus, during the incubation of the immobilized antigen or antibody with enzyme-labeled antigen or antibody, the latter binds specifically to the immobilized immune reactant, but may also be adsorbed directly onto the solid phase. This nonspecific adsorption of enzyme activity can be minimized by inclusion of a nonionic detergent such as Triton X-lOO or Tween 20. These do not interfere with the antigen-antibody reaction but prevent formation of new hydrophobic interactions between added proteins and the solid phase without disrupting to any appreciable extent the hydrophobic bonds already formed between the previously adsorbed protein and the plastic surface. 

A catalyst is a substance that increases the rate at which a chemical reaction approaches equilibrium without, itself, becoming permanently involved in the reaction. The key word in this definition is permanently since there is ample evidence showing that the catalyst and the reactants interact before a reaction can take place. The product of this interaction is a reactive intermediate from which the products are formed. This substratexatalyst interaction can take place homogeneously with both the reactants and the catalyst in the same phase, usually the liquid, or it can occur at the interface between two phases. These heterogeneously catalyzed reactions generally utilize a solid catalyst with the interaction taking place at either the gas/solid or liquid/solid interface. Additional phase transport problems can arise when a gaseous reactant is also present in the liquid/solid system. 

In all of these systems, chemical reactions have a hierarchical structure. What we normally think of as one reaction is actually composed from several elementary steps, which are often called, collectively, the mechanism of the reaction. For processes with a solid catalyst, these steps describe the interaction of the catalyst s active sites with the fluid-phase reactants. For biochemical reactions, there are association and dissociation steps that involve complexes of the enzyme and the substrates. In all cases, unstable, short-lived intermediates may be formed and destroyed in a rapid succession of steps the overall reaction we observe might not... 

Plastic is by far the most popular solid phase, since it makes the procedures extremely simple. However, plastics may also have some important limitatons (i) they are immunoreactant-consumptive, i.e. often require 10 times more reactants than particulate solid phases or membranes (ii) the avidity of immobilized antibodies for large antigens decreases by 1-2 orders of magnitude (Zwolinski, G. Jo-sephson, L. cited by Parsons, 1981), probably due to the wide spacing of epitopes or paratopes (iii) the rate of antibody-antigen interactions is slower than in solution or with particulate solid phases (hours instead of minutes), due to the necessity of the free immuno-reactant to diffuse to the solid phase (association kinetics is largely dictated by diffusion rate Section 8.4) and, (iv) few suitable antibod-... 

The question arises, why do bi- or multi-phasic catalysts generally show better activity and selectivity than the active phase alone The aim of this paper is to answer this question by exploring the role of interfacial effects. We shall examine first how the thermodynamic and structural properties of one phase influence its interactions, not only with the gaseous reactants, but also with coexisting solid phases as a result of its bulk, surface, and defect structure. We will also examine the conditions necessary for these interactions and set up a structural classification of the main components of mild oxidation catalysts. This will lead finally to a discussion of the role of interfacial effects in catalyst performance using some illustrative examples. Thermodynamic and Structural Properties of Single Phase Catalysts... 

The interaction of a liquid and a solid phase presents additional considerations that are not described by the preceding simple rate equations. Diffusion is important for the permeation of reagents and solvents into the reactive sites of the solid support matrix. The concentration of the reactants also has an effect on the diffiision of materials into the solid support. Diffusion is the random movement of molecules from an area of high concentration to one of lower concentration and can be described by Eq. (3). 

As dicussed in Sec.1.Ill, general formulations of the equations for the rate constant are applicable, in principle, to both gas phase and dense phase reactions. The main dificulty in treating reactions in solution occurs when taking into account the influence of the solvent. Including the interactions between the reactants and solvent molecules is necessary for an accurate treatment based on a manydimensional potential energy surface of the whole system (reactants + solvent). This is at present an extremely complicated task. The unique way out of this difficulty is to introduce simplified models for different types of reactions which take into account the role of the solvent. The situation is simpler for reactions in solid phases, where vibrations of atoms and molecules are usually the only modes of motion involved in the reaction. Neglecting translation and rotation motions in liquid media is also sometimes possible as an acceptable approximation. 

A general schematic of FC is shown in Figure 2.1. The initial reaction medium consists of a porous matrix of solid reactants and inert diluents, where the pores are filled with gas-phase reactants. The combustion front propagates through the sample with a velocity, U, due to the chemical interaction between the gas- and condensed-phase reactants. Behind the front, the final product is formed, which in some cases may approach pore-free structure, since the volume of the final product grains is typically greater than that of reactant particles. 

Thus, the process of solid-phase synthesis of fluoramphibole from either chemical reactants or pure natural silicates and hydrosilicates, i.e. compounds of stoichiometric compositions, was represented by multistep physicochemical interactions conventional for the formation of non-stoichiometric inorganic compounds (Rabenau, 1970). The mechanisms of this process were estimated by the properties of new phases emerged during the decomposition of hydrosilicates. 

Figure 1 shows a schematic of a hthium battery. Lithium battery electrodes are usually made by coating a slurry of the active material, conductive filler, and binder onto a foil current collector. This porous configuration provides a high surface area for reaction and reduces the distance between reactants and the surfaces where reactions occur. In these porous electrodes, the electrochemical reaction is distributed over the surface of the particles of active material, and will vary across the depth of the electrode due to the interaction of potential drop and concentration changes in both the solution and solid phases. Porous electrode theory is used to understand these interactions. 

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