Kinetics of the Interfacial Reaction

6 Effect of the Molecular Characteristic Features of the Reactive Polymers 

Furthermore, reaction of cyclic anhydride with aliphatic primary amine is more rapid than with aromatic amine [35, 72]. When the mutual reactivity of the Omctional groups is not high enough, their concentration can be increased or catalyst or small-size molecules can be added [124, 125]. 

Scott et al. compared the development of the phase morphology in non-reactive and reactive polymer blends based on the reaction of maleic anhydride (MA) with amine and oxazoline (OX), respectively. The increase in the mixing torque and decrease in the average particle size were less pronoimced and delayed in time in the case of the MA/OX reactive pair compared to the MA/NH2 one, consistently with the lower reactivity of the former pair [46]. 

Sundararaj compared the final size of dispersions of reactive PS (MA-grafted PS, or PSMA) and neat PS in PA 6,6 and similarly of reactive EP (EP-MA) and unreactive EP in PS-Ox, at constant mixing time [73]. The content of the dispersed phase was very small (less than lwt%), so that the rate of coalescence was negligible, even in non-compatibilized blends [56, 74]. The fast MA/NH2 reaction in the PA 6,6/PSMA system was responsible for smaller particle size compared to the non-reactive PA 6,6/PS system. Such a difference was not observed in the case of the slower MA/Ox reaction. Therefore, the MA/Ox reaction was assumed to occin essentially beyond the initial melting/softening 

Step of the blending, i.e., when the major morphological changes had occurred. The main role of the interfacial reaction was then to stabilize the dispersion of the minor phase, which was actually what one observed for the uncompatibilized blend. 

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SECM has been applied to the investigation of various technologically important materials and interfaces, for example, metallic corrosion [91-96], fuel cell electrocatalysts [97], semiconductor photocatalysts [12, 60-63, 98], conducting polymers [49, 50, 85, 86, 99-103], liquid-liquid and liquid-gas interfaces [29, 30, 68]. The SECM may be used to image the substrate topography and/or reactivity, or with the tip at a fixed location, to study the local kinetics of the interfacial reactions of interest. 

In the electrochemical literature it is useful to refer to a reversible interface or interfacial reaction as one whose potential is determined only by the thermodynamic potentials of the various electroactive species at the electrode surface. In other words, it is only necessary to take into account mass transport to and from the interface, and not the inherent heterogeneous kinetics of the interfacial reaction itself, when discussing the rate of the charge transfer reaction. This nomenclature has two principal disadvantages. First, it neglects the fact that mass transport to the interface, whether migration or diffusion, is inherently an irreversible or dissipative... 

With this reaction, hafnium in the molten metal is preferably transferred to the molten salt phase, and consequently CUCI2 is reduced to metallic copper dissolving into the Zr-containing Cu-based low melting alloy. The kinetics of the interfacial reactions between molten metal and molten salt is usually very fast. The equilibrium composition for both phases can be established in laboratory experiments. Thermodynamic prediction of the reaction system gives very encouraging results for this separation process. 

Although the addition of a reactive polymer represents a major route for the compatibilization of immiscible blends on an industrial level, the exact mechanism and kinetics of the interfacial reactions remain uncertain. The reasons for this are the different reaction rates for the same functional groups on different polymers, and the effects of flow in a mixing device which can significantly accelerate the interfacial reaction, most likely due to convection as well as to the creation of a fresh interface [154]. 

Copolymers with a blocky structure have been designed as compatibilizers for immiscible polymer blends. More recently, most research effort has been devoted to reactive compatibilization, with special attention paid to the molecular characteristics of the reactive precursors of the compatibilizer, such as molecular weight, content, and distribution of the reactive groups and kinetics of the interfacial reaction. The interplay between these factors and the various processing factors are important. 

Nakahama et al. studied the kinetics of the interfacial reaction between the carboxylic acid end-group of polystyrene (PS-COOH) and either the epoxy end-group(s) of... 

According to this method, it is not necessaiy to investigate the kinetics of the chemical reactions in detail, nor is it necessary to determine the solubihties or the diffusivities of the various reactants in their unreacted forms. To use the method for scaling up, it is necessaiy independently to obtain data on the values of the interfacial area per unit volume a and the physical mass-transfer coefficient /c for the commercial packed tower. Once these data have been measured and tabulated, they can be used directly for scahng up the experimental laboratory data for any new chemic ly reac ting system. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

It was shown later that a mass transfer rate sufficiently high to measure the rate constant of potassium transfer [reaction (10a)] under steady-state conditions can be obtained using nanometer-sized pipettes (r < 250 nm) [8a]. Assuming uniform accessibility of the ITIES, the standard rate constant (k°) and transfer coefficient (a) were found by fitting the experimental data to Eq. (7) (Fig. 8). (Alternatively, the kinetic parameters of the interfacial reaction can be evaluated by the three-point method, i.e., the half-wave potential, iii/2, and two quartile potentials, and ii3/4 [8a,27].) A number of voltam-mograms obtained at 5-250 nm pipettes yielded similar values of kinetic parameters, = 1.3 0.6 cm/s, and a = 0.4 0.1. Importantly, no apparent correlation was found between the measured rate constant and the pipette size. The mass transfer coefficient for a 10 nm-radius pipette is > 10 cm/s (assuming D = 10 cm /s). Thus the upper limit for the determinable heterogeneous rate constant is at least 50 cm/s. 

The combination of resonance Raman microscope spectrometry and the CLM method allowed us to directly observe the Raman spectra of the liquid-liquid interface and the bulk phases by shifting the focal point of an objective lens. A schematic diagram of the measurement system is shown in Fig. 6. CLM/ Raman microscope spectrometry was applied in order to measure the rate of complex formation between Pd(II) and 5-Br-PADAP (HL) at the heptane-water interface and it was demonstrated that this method was highly useful for the kinetic measurement of the interfacial reaction [37],... 

Kinetic measurement of the interfacial reaction is still very important in relation to the biological reactions taking place at the cell membrane and the surface of biological organism. The catalytic mechanism of enzyme at the liquid-liquid interface is not well understood yet. The interaction between the enzyme and the substrate at the oil-water interface has to be investigated urgently. 

When the kinetics of an interfacial reaction can be described by Equation 1.126, the processes can be differentiated and the numerical value of the rate... 

What is the specific feature in the reaction at the liquid/liquid interface The catalytic role of the interface is of primary importance in solvent extraction and other two-phase reaction kinetics. In solvent extraction kinetics, the adsorption of the extractant or an intermediate complex at the liquid/liquid interface significantly increased the extraction rate. Secondly, interfacial accumulation or concentration of adsorbed molecules, which very often results in interfacial aggregation, is an important role played by the interface. This is because the interface is available to be saturated by an extractant or mehd complex, even if the concentration of the extractant or metal complex in the bulk phase is very low. Molecular recognition or separation by the interfacial aggregation is the third specific feature of the interfacial reaction and is thought to be closely related to the biological functions of cell membranes. In addition, molecular diffusion of solute and solvent molecules at the liquid/liquid interface has to be elucidated in order to understand the molecular mobility at the interface. In this chapter, some examples of specific... 

Bode, G., Lade, M. and Schomacker, R. (2000) The kinetics of an interfacial reaction in microemulsion with excess phase. Chem. Eng. Technol., 23, 405M09. 

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