Steps in interfacial reactions

The interfacial processes can be greatly affected by the size of the pores of the solid phase. In the macropores, the size of the pores is greater than the thickness of the adhesion layer, and the diffusion is similar to the diffusion through the adhesion layer, that is, film diffusion. In meso- and micropores, the diffusion is called pore diffusion, which is a type of interparticular diffusion. 

In a stationary state, the rate of the overall interfacial reaction is determined by the slowest step. In laboratory experiments, the transport in the bulk solution can 

Two examples of empirical equations are shown here. One of them is the Elovich equation  

The other example is the power function equation, or, by another name, fractional power or modified Freundlich equation (Wahba and Zaghloul 2007)  

When diffusion is considered as the rate-determining step, the parabolic diffusion equation can be applied (Zimens 1945 Boyd et al. 1947 Crank 1956 Chute and Quirk 1967 Wollast 1967 Jardine and Sparks 1984 Sparks 1999). 

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Kinetics of chemical reactions at liquid interfaces has often proven difficult to study because they include processes that occur on a variety of time scales [1]. The reactions depend on diffusion of reactants to the interface prior to reaction and diffusion of products away from the interface after the reaction. As a result, relatively little information about the interface dependent kinetic step can be gleaned because this step is usually faster than diffusion. This often leads to diffusion controlled interfacial rates. While often not the rate-determining step in interfacial chemical reactions, the dynamics at the interface still play an important and interesting role in interfacial chemical processes. Chemists interested in interfacial kinetics have devised a variety of complex reaction vessels to eliminate diffusion effects systematically and access the interfacial kinetics. However, deconvolution of two slow bulk diffusion processes to access the desired the fast interfacial kinetics, especially ultrafast processes, is generally not an effective way to measure the fast interfacial dynamics. Thus, methodology to probe the interface specifically has been developed. 

The actual reaction between the carboxyl group on the CTBN and the epoxy resin in an amine-cured epoxy system is complex due to the low concentmtion of carboxyl groups compared with that of amine groups (a mtio of about 1 10), so the first step in the reaction is the formation of a salt with the amine which then proceeds to react with the epoxy groups (Scheme 1.47). As noted earlier, there will be solubility of the rubber in the resin, so there will be a copolymerization reaction as the matrix cures and the conditions for phase sep-amtion are achieved (Bucknall, 1977). It was recognized that this interfacial-reaction chemistry did not control the process of phase separation since this was governed by thermodynamic considerations (Pascault et al, 2002). 

The following physical and chemical steps are involved in interfacial reactions ... 

This hypothesis was discussed by Bockris in 1969, in a comprehensive article in Nature entitled, Are Interfacial Electron Transfer Reactions an Important Step in Biological Reactions These ideas were treated in the textbook. Modern Electrochemistry (1977). ... 

In this model it is assumed that the rate-limiting (slow) step in the reaction is still the breakdown of substrate to product. We also treat enzyme interfacial binding as an equilibrium process that can be described by an equilibrium dissociation constant ( J). We also assume that once the... 

When the reactive groups X were carbamates, the interfacial reaction was controlled by the carbamate thermolysis into primary amine, which was the rate determining step. The interfacial reaction was then occurring essentially after the softening/melting of the blend components, i.e. in the melt state as shown in Fig. 4.3(b). Phase dispersion was then... 

FIG. 1 Simplified schematic illustrating the t5rpes of steps involved in an interfacial reaction at a liquid-liquid interface. 

When the reactants involved in a step growth polymerization process are mutually immiscible, we can employ an interfacial polymerization method. Two solutions, each containing one of the monomers, are layered one on top of the other. This creates a phase boundary that forms wth the least dense liquid on top. The different monomers can then meet and polymerize at the interface. A commonly demonstrated example of this is the manufacture of nylon 610 by the interfacial reaction between an aqueous solution of hexamethylenediamine with sebacoyl chloride dissolved in carbon tetrachloride. Because the reaction only occurs at the interface, it is possible to pull the products from this interface to isolate the final product. 

Hydrolysis. NMR results show that TBT carboxylates undergo fast chemical exchange. Even the interfacial reaction between TBT carboxylates and chloride is shown to be extremely fast. The hydrolysis is thus not likely to be a rate determining step. Since the diffusivity of water in the matrix is expected to be much greater than that of TBTO, a hydrolytic equilibrium between the tributyltin carboxylate polymer and TBTO will always exist. As the mobile species produced diffuses out, the hydrolysis proceeds at a concentration-dependent rate. Godbee and Joy have developed a model to describe a similar situation in predicting the leacha-bility of radionuclides from cementitious grouts (15). Based on their equation, the rate of release of tin from the surface is ... 

For most phase-transfer catalysed reactions, the rate-determining step is the interaction of the reactive substrate with the anionic species in the organic phase and, compared with the corresponding interfacial reaction in the absence of the catalyst, rate enhancements of 107 are not uncommon. The virtual absence of water from the organic phase under strongly basic liquiddiquid or soliddiquid two-phase conditions allows for the formation of water-sensitive anions, such as carbanions (Chapter 6), and obviates the need for strictly anhydrous conditions and the use of bases such as sodium hydride or sodamide, etc. The phase-transfer catalytic process consequently has lower safety risks and is environmentally more friendly. 

Not unexpectedly, alkylation of the double carbonylated complex proceeds via a base-catalysed interfacial enolization step, but it is significant that the initial double carbonylation step also involves an interfacial reaction, as it has been shown that no pyruvic acid derivatives are obtained at low stirring rates. Further evidence comes from observations of the cobalt-catalysed carbonylation of secondary benzyl halides [8], where the overall reaction is more complex than that indicated by Scheme 8.3. In addition to the expected formation of the phenylacetic and phenylpyruvic acids, the reaction with 1-bromo-l-phenylethane also produces 3-phenylpropionic acid, 2,3-diphenylbutane, ethylbenzene and styrene (Scheme 8.4). The absence of secondary carbonylation of the phenylpropionylcobalt tetracarbonyl complex is consistent with the less favourable enolization of the phenylpropionyl group, compared with the phenylacetyl group. 

The concentration of acid should affect interfacial reactions in various ways such as through the hydrogen ion concentration, water activity, and behavior of anions. Hydrogen ion concentration may play a minor role because the dehydrogenation can be considered as relatively fast steps in the methanol oxidation. Therefore, the other two elements will be considered here. 

This compound was prepared by the interfacial reaction of a chloroform solution of the step 2 product with p-nitrophenol in water containing Na2C03. The product was... 

In glycerol monooleate/decane bilayers we find the steady-state conductance at zero current to be proportional to the first power of the ion concentration and to the second power of the ionophore concentration, as illustrated in Fig. 1. (The current-voltage characteristic is hyperbolic for all ionic species indicating that this molecule is in the equilibrium domain for the interfacial reactions, with the rate-limiting step being the ion translocation across the membrane interior.) The conductance selectivity sequence is seen to be Na>K>Rb>Cs, Li. 

It is shown elsewhere (Section 7.9.2) that an approximate numerical formula for this limiting diffusion current iL is iL = 0.02 nc, where n is the number of electrons used in one step of the overall reaction in the electrode and c is the concentration of the reactant in moles liter-1. Hence, at 0.01 M, and n = 2, say, iL = 0.4 mA cm-2—a current density less than may be desirable for many purposes. The problem is how to increase this diffusion-controlled limiting current density and obtain data on the interfacial reaction free of interference by transport at increasingly high current densities. 

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