Interfacial reactivity

We consider in the following the metal diHusion into silicon, which may take place at room temperature and thus produce the formation of an interfacial silicide without 

An important general trend is that interfaces with a higher reactivity require a lower temperature for formation of the sihcide [12]. For instance, refractory metals require temperatures in the range of600 °C to form disiUcides after reacting with Si, while quasi-noble metals react near 200 °C to form metal rich silicides. A summary of formation temperatures is recorded in Table 14.1. 

Silicide Formation 0 T C) Activation energy (eV) Crovrth law Diffusing species Melting T C) Eutectic temperatures (°C) 

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As mentioned earlier, a great deal of literature has dealt with the properties of heterogeneous liquid systems such as microemulsions, micelles, vesicles, and lipid bilayers in photosynthetic processes [114,115,119]. At externally polarizable ITIES, the control on the Galvani potential difference offers an extra variable, which allows tuning reaction paths and rates. For instance, the rather high interfacial reactivity of photoexcited porphyrin species has proved to be able to promote processes such as the one shown in Fig. 3(b). The inhibition of back ET upon addition of hexacyanoferrate in the photoreaction of Fig. 17 is an example of a photosynthetic reaction at polarizable ITIES [87,166]. At Galvani potential differences close to 0 V, a direct redox reaction involving an equimolar ratio of the hexacyanoferrate couple and TCNQ features an uphill ET of approximately 0.10 eV (see Fig. 4). However, the excited state of the porphyrin heterodimer can readily inject an electron into TCNQ and subsequently receive an electron from ferrocyanide. For illumination at 543 nm (2.3 eV), the overall photoprocess corresponds to a 4% conversion efficiency. 

Since the dependence of the i/i o6) ratio on d and the tip geometry can be calculated theoretically [8], simple current measurements with mediators which do not interact at the interface can be used to determine d. When either the solution species of interest, or electrolysis product(s), interact with the target interface, the hindered mass transport picture of Fig. 1(b) is modified. The effect is manifested in a change in the tip current, which is the basis of using SECM to investigate interfacial reactivity. 

The central issue which has to be addressed in any comprehensive study of electrode-surface phenomena is the determination of an unambiguous correlation between interfacial composition, interfacial structure, and interfacial reactivity. This principal concern is of course identical to the goal of fundamental studies in heterogeneous catalysis at gas-solid interfaces. However, electrochemical systems are far more complicated since a full treatment of the electrode-solution interface must incorporate not only the compact (inner) layer but also the boundary (outer) layer of the electrical double-layer. The effect of the outer layer on electrode reactions has been neglected in most surface electrochemical studies but in certain situations, such as in conducting polymers and... 

Case 3.2 When the interfacially reactive species are the undissociated molecules of the extractant adsorbed at the interface [i.e., the first rate-determining step of the two-step mechanism is the reaction between the metal ion and HB(ad)], the following equations will hold ... 

Several important energy-related applications, including hydrogen production, fuel cells, and CO2 reduction, have thrust electrocatalysis into the forefront of catalysis research recently. Electrocatalysis involves several physiochemical environmental dfects, which poses substantial challenges for the theoreticians. First, there is the electric potential which can aifect the thermodynamics of the system and the kinetics of the electron transfer reactions. The electrolyte, which is usually aqueous, contains water and ions that can interact directly with a surface and charged/polar adsorbates, and indirectly with the charge in the electrode to form the electrochemical double layer, which sets up an electric field at the interface that further affects interfacial reactivity. 

The Bailey and Watkins study shows that continuous liquid film formation is favoured by interfacial reactivity due to mutual solubility or to formation of intermetallics. This correlation between film stability and reactivity is due to several reasons ... 

The competition between the interfacial reactivities, the residual stresses and the elasto-plastic behavior of the components will be strongly dependent on the mechanical stability of the coating-substrate combination. Mechanical stability control has been assessed when making ceramic/metal junctions at high temperature (700°C - 1000°C) during which thick reaction zones tend to form by reactive diffusion in volume intermediate layers. ... 

The interfacial reactivity of functional electrodes can mostly influence the amperometric detection signal in the bioelectrocatalytic process. Herein, the electrochemical impedance spectroscopy (EIS) has been applied to investigate the interfacial charge transfer or mass transfer process of bare Ti02/Ti substrates and G0D-Ti02/Ti composite electrodes. The EIS measurements over a frequency from 100000 to 0.01 Hz are carried out in a conventional three-electrode system under a sinusoidal perturbation of 5 mV and a constant potential of -0.4 V. 

The sales of plastics continue to increase in a large part due to technical and economic advancements of polymer blends. Reactive blending is a useful technique for elastomers but, it appears that chemistry could also play an important role in the correct microstructure adjustment of thermoplastic alloys. Interfacial reactivity should be the focal point in maintaining the expected structure during subsequent stages of manufacture. Besides industrial examples, various kinds of polymeric co-reacting systems are also presented in order to emphasise the key factors of reactive blending. 

Clearly, chemistry has been most helpful in the development of industrial blends. Nevertheless, in many cases, the basic reaction conditions are not completely understood. The following is a description of laboratory studies carried out to examine the interfacial reactivity and two kinds of condensation reactions in immiscible polymer blends. 

Ardizzone, S. et al.. Hydrothermal route to pure phase ZiDj. Interfacial reactivity by XPS and electrochemical determinations. Colloids Surf. A, 90,45, 1994. 

The view of solid/liquid interfacial reactivity in Figure 1 is further complicated when the nature of a typical solid surface is considered. For many materials, the surface morphology is highly complex with a variety of active... 

Metal/Polymer Adhesion Effect of Ion Bombardment on Polymer Interfacial Reactivity... 

Specific interactions in binary blends of ethylene-vinyl acetate copolymer with various low molecular weight terpene-phenol tackifying resins (TPR) were systematically investigated, as a function of the composition of the blend and of the electron acceptor ability of the resin, by using attenuated total reflection FTIR spectroscopy. Molecular acid-base were evidenced between TPR hydroxyl groups and EVA carbonyl groups. Quantitative information on the fraction of acid-base bonded entities, the enthalpy and equilibrium constant of pair formation were obtained. A crystalline transition of the EVA copolymer was observed and discussed in terms of enthalpy and entropy considerations based on FTIR and calorimetric DSC investigations. Fundamental results are then summarised to predict the interfacial reactivity of such polymer blends towards acid or basic substrates. 16 refs. 

The interfacial reactivity of metalloporphyrins and chlorins is critically determined by their specific adsorption at the liquid/liquid junctions. The affinity of the water-soluble porphyrins for these interfaces is associated with their complex solvation structure arising from the hydrophobic central ring and peripheral ionisable groups. This chapter will also feature a brief overview on recent studies of the molecular organisation of water-soluble porphyrins as probed by a variety... 

Marie et al. (2001) have studied PA blends with poly(dimethylsiloxane) (PDMS) either in binary blends of the functionalized polymers or in ternary blends with a functionalized styrene copolymer. The efficiency of copolymer formation concurrent with morphology development and stabilization was studied for reactions between PA-amine and PDMS-anhydride, between PA-amine and PDMS-epoxy, and between PA-carboxylic acid and PDMS-epoxy. The effects of relative melt viscosities on interfacial reactivity and resulting morphology were noted. 

The study of amphiphile ordering at interfaces is necessary to understand many phenomena, like microemulsions, foams or interfacial reactivity. It is expected that the preferential orientation taken by these compounds at interfaces is entirely determined by their interactions with the two solvants forming the interface and the intermolecular repulsion or attraction within the monolayer. As mentioned above, the SH response at liquid/liquid interfaces is dominated by electric dipole contributions and is therefore surface specific. Neglecting the contribution from the sol-vant molecules, which usually only have a weak nonlinear optical activity, the passage from the macroscopic susceptibility tensor xP to the microscopic molecular hyperpolarizability p of the adsorbate is obtained by merely taking the SHG response of the amphiphile monolayer as the superposition of the contribution from each single moiety. Hence, it yields... 

To predict the behavior of a molecule in an aqueous solution and its ability to penetrate a bubble, react at the interface, or remain in the bulk liquid, Henglein proposed its hydrophobicity ratio, i.e., the number of hydrophobic to hydrophilic groups.20 For example, ethanol with two saturated hydrophobic carbon atoms and one hydrophilic hydroxyl group, has a ratio of 2. Solvation is important and sonochemical reactions of this compound should occur mainly in the bulk liquid. On the other hand, hexanols have a hydrophobicity ratio of 6. In a dilute aqueous solution, they are poorly solvated. Their volatility is too low to permit a reaction in the bubble, and an interfacial reactivity will be preferred. Thus, for aqueous solutions, the higher the hydrophobicity, the easier the reaction in the bubble or at the interface will be. Similar approaches, mutatis mutandis, for the solute-solvent interaction in organic solutions are unfortunately missing. 

Interfacial reactivity seems to be involved in the oxidation of hindered secondary amines (Fig. 8). 2i sonication experiments run with oxygen bubbling, hydroxyl radicals abstract hydrogen from the N-H bond, then the nitrogen-centered radical reacts with oxygen or combines with the hydroxyl radical. Subsequent steps give the stable nitroxide. 

Carter, P.W., Johns, T.P., 2005. Interfacial reactivity between ceria and silicon dioxide and silicon nitride surfaces. Electrochem. Sohd State Lett. 8 (8), G218—G221. 

Plueddemann has recently published a book on the subject of silane coupling agents. According to Plueddemann and references cited in his book, the mechanism of action of silane coupling agents involves not just interfacial reactivity, but also the formation of an interpenetrating polymer network between a polysiloxane surface layer and the overlying polymer. 

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