Interfacial kinetic model

Dunn, I. J. (1968). An interfacial kinetics model for hydrocarbons oxidation, Biotechnol. Bioeng., 11, 467—487. 

Tjandra et al. (1998) have proposed an interfacial reaction model for the kinetics of the reaction between 1-bromo octane and sodium phenoxide to give 1-phenoxyoctane in a nonionic microemulsion. In this model the microemulsion is assumed to consist of the aqueous phase and the interface is covered by a monolayer of surfactant molecules. It is thus possible to assess the interfacial area from the concentration of the surfactant in the microemulsion medium. 

Although the Lewis cell was introduced over 50 years ago, and has several drawbacks, it is still used widely to study liquid-liquid interfacial kinetics, due to its simplicity and the adaptable nature of the experimental setup. For example, it was used recently to study the hydrolysis kinetics of -butyl acetate in the presence of a phase transfer catalyst [21]. Modeling of the system involved solving mass balance equations for coupled mass transfer and reactions for all of the species involved. Further recent applications of modified Lewis cells have focused on stripping-extraction kinetics [22-24], uncatalyzed hydrolysis [25,26], and partitioning kinetics [27]. 

The majority of RDC studies have concentrated on the measurement of solute transfer resistances, in particular, focusing on their relevance as model systems for drug transfer across skin [14,39-41]. In these studies, isopropyl myristate is commonly used as a solvent, since it is considered to serve as a model compound for skin lipids. However, it has since been reported that the true interfacial kinetics cannot be resolved with the RDC due to the severe mass transport limitations inherent in the technique [15]. The RDC has also been used to study more complicated interfacial processes such as kinetics in a microemulsion system [42], where one of the compartments contains an emulsion. 

Fig. 16.3 Quantum yield (QY) for electron and hole transfer to solution redox acceptors/donors as a function of the reduced variables y (related to the surface properties of the catalyst, i.e., ratio between interfacial electron transfer rate and surface recombination rate) and w (related to the ratio between surface migration currents of hole and electrons to the rate of bulk recombination), according to the proposed kinetic model [23],...

Selected entries from Methods in Enzymology [vol, page(s)] Activation of lipolytic enzymes by interfaces, 64, 341 model for lipase action on insoluble lipids, 64, 345 interfacial enzyme inactivation, 64, 347 reversibility of the adsorption step, 64, 347 monolayer substrates, 64, 349 kinetic models applicable to partly soluble amphiphilic lipids, 64, 353 surface dilution model, 64, 355 and 364 practical aspects, 64, 357. 

In [119], the hydrogen adsorption and desorption reactions in thin palladium electrodes were studied using the potential step method in order to analyze the mechanism of phase transformation. Transient current responses were recorded at the onset of the potential step for 47 pm thick Pd electrodes in 1 mol dm H2SO4 at ambient temperature. A model based on a moving boundary mechanism was proposed to account for the experimental i-t curves. It was found that the hydrogen adsorption reaction shows interfacial kinetic limitations and only numerical solutions can be obtained. Such kinetic limitations were not found for the desorption reaction and a semianalytical solution that satisfactorily fits the experimental data was proposed. 

Accordingly, the potential dependence of the electrode kinetics is determined by the variation of the activation energy with E, which is established by the position of the transition state on the energy profile in Fig. 1.13. This key aspect has been addressed in different ways by the different kinetic models developed. In the following sections, the two main models employed in interfacial electrochemistry will be reviewed. 

In extraction columns, it is possible to find droplet swarms where the local velocities near the droplet surface are higher, this being due to the lower free area available for the countercurrent flowing continuous phase. Wake and Marangoni influences make the prediction of a physical mass transfer coefficients difficult. With reactive extraction the influence of interfacial kinetics on overall mass transfer is generally not negligible. In any case, a combination of reactive kinetics with any eddy mass transfer model is recommended, whereas the latter could rely on correlations derived for specific column geometries. 

It has been shown that the magnitude of the rate constant for crossing the octanol-water interface makes the energy barrier significantly larger than the diffusional barrier. It has also been shown that for compounds with log Pow less than =1.2, the overall rates are faster and the interfacial kinetics term more important. However, detailed development of a model would be needed to understand what the relative importance of diffusion and interfacial terms (such as cuticle or membrane permeation) are in vivo. No clear dependence of interfacial rate constants on log Pow was seen, but the initial emphasis of such a study should be on the intermediate... 

Recently, kinetic models have been combined with the equilibrium data of the interfacial processes, taking into account that soils and rocks are heterogeneous and consequently have different sites. These models are called nonequilibrium models (Wu and Gschwend 1986 Miller and Pedit 1992 Pedit and Miller 1993 Fuller et al. 1993 Sparks 2003 Table 7.2). These models describe processes when a fast reaction (physical or chemical) is followed by one or more slower reactions. In these cases, Fick s second law is expressed—that the diffusion coefficient is corrected by an equilibrium thermodynamic parameter of the fast reaction (e.g., by a distribution coefficient), that is, the fast reaction is always assumed to be in equilibrium. In this way, the net processes are characterized by apparent diffusion coefficients. However, such reactions can be equally well described using Equation 1.126. 

Figure 2. Kinetic model for second-step reactions variation of K (or interfacial surface area) with A/O ratio for isobutane-to-olefin feed ratio of 5 1...

Table 3 gives a summary of the interfacial charge injection and recombination rate constants determined by direct spectroscopic techniques. The data are not directly comparable as different Ti02 preparations, solvents, electrolytes, time-scales and kinetic models were used by different experimentalists. Nevertheless, the table demonstrates the wide range of sensitized materials reported in the literature and provides a basis for further discussion. 

The effect on PEFC performance of 10-100 ppm CO in the anode feed stream is very substantial when Pt catalyst is used in the anode. It can be seen from Fig. 32 that the effect of CO can be qualitatively described in terms of some critical current density of value which drops with the level of CO in the anode feed stream. When current demand is below the critical current level, the PEFC maintains practically CO-free cell performance, whereas above that critical current level, cell performance drops sharply as the cell voltage falls well under the corresponding CO-free level. The explanation for this behavior was provided by a model that considered the interfacial kinetics at the PEFC anode in the presence of low levels of CO [42,66, 67]. 

The overall voltage loss in the fuel-cell cathode is a complex combination of interfacial kinetics and mass and charge transport losses. Consequently, model interfacial systems are best suitable to resolve and directly study the ORR at the Pt/ionomer interface. Such model... 

Fig. 9.3 Kinetic model illustrating the covalent inhibition of a lipolytic enzyme at a lipid/ water interface. Symbols and abbreviations are as follows A, total interfacial area (surface) V total volume (volume) E, free enzyme concentration (molecule/volume) , interfacial enzyme concentration (molecule/ surface) S, interfacial concentration of substrate (molecule/surface) I, interfacial concentration of inhibitor (molecule/surface) P, product concentration (molecule/volume) E S, interfacial enzyme-substrate complex... 

Fig. 9.16 Kinetic model illustrating the inhibition of HPL by orlistat in the aqueous phase and its reactivation at a lipid-water interface. The following symbols and abbreviations are used here E, free enzyme (molecule/volume) E, interfacial enzyme (molecule/surface) FA, fatty acid at the interface (molecule/surface) E -FA, interfacial enzyme-fatty acid complex (molecule/surface) THLc, closed reactive orlistat in the bulk (molecule/volume) THLo, open non-reactive orlistat at the interface (molecule/surface) -THLO, form 1 of cova-...

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