Heterogeneous Interfaces

Homogeneous nudeation occurs in the absence of a solid interface heterogeneous nudeation occurs in the presence of a solid interface of a foreign seed and secondary nudeation occurs in the presence of a solute particle interface. The mechanisms governing the various types of primary and secondary nudeation are different and result in different rate expressions. The relative importance of each type of nudeation varies with the precipitation conditions. 

R. J. Forster, Hopping Across Interfaces Heterogeneous Electron Transfer Dynamics, Interface 9(4) 24, 2000. 

This research evaluates the measurement of the "master" Eh of solutions in terms of heterogeneous electron-transfer kinetics between aqueous species and the surface of a polished platinum electrode. A preliminary model is proposed in which the electrode/solution interface is assumed to behave as a fixed-value capacitor, and the rate of equilibration depends on the net current at the interface. Heterogeneous kinetics at bright platinum in 0.1 m KCl were measured for the redox couples Fe(III)/Fe(II), Fe(CN)53-/Fe(CN)6, Se(VI)/Se(IV), and As(V)/As(Iin. Of the couples considered, only Fe(III)/Fe(II) at pH 3 and Fe(CN)g37Fe(CN)g at pH 6.0 were capable of imposing a Nemstian potential on the platinum electrode. 

Figure 1-1. Four regions of vapor reactivity within a CVD reactor. Only location 4 contributes to productive film growth. (1) In the vapor phase, homogenous, in the region of the substrate (2) in the vapor phase, homogenous, not in the region of the substrate (3) at a vapor/solid interface, heterogenous, not on the substrate however, on the reactor wall or other comparable surface (4) at a vapor/solid interface, heterogenous, on the substrate.

Irregularities in heart-cladding interface heterogeneities in index of refraction (rayleigh)... 

These reactions can involve species in the same phase (homogeneous electron transfer reactions) or the electron can transfer through an interface (heterogeneous electron transfer reactions). In the second case, the transfer can occur between molecules (e.g., electron transfers at liquid-liquid interfaces or mediated by redox active monolayers) or between an electronic conductor (the electrode) and a molecule. 

According to Vetter [11, 72], an electrochemical process can also be controlled by a chemical reaction in the bulk (homogeneous reaction) or at the interface (heterogeneous reaction). The corresponding expressions for the real and imaginary components of the impedance are presented for both cases in the paper by Vetter [11],... 

When the dissolved gas reaches a saturation limit in the polymer, it becomes supersaturated and finally diffuses out of the polymer system to form voids or bubbles [17-19]. The formation of bubbles represents a nucleation process in which voids are formed either without nucleating agent (self-nucleation process) or with solid nucleating agents at the liquid-solid interface (heterogeneous process). 

The main objectives of this chapter are to clarify the roles of the hydrophobic emulsifier additives added in the oil phase of O/W emulsions how they modify fat crystallization and where they interact within the emulsion droplets. One may ask why the hydrophobic emulsifiers accelerate the nucleation process. The answer may not be straightforward, because their influences on fat crystallization are controlled by their physical and chemical properties and the nature of the interactions with the fat molecules occurring in the oil phase and at the oil/water interfaces. However, the results we have obtained so far indicate that the addition of hydrophobic emulsifiers in the oil phase has remarkable effects on crystallization. Fat crystals typically form a number of polymorphs, whose crystallization properties are influenced by many factors, such as temperature, rate of crystallization, time evolution for transformation, and impurity effects, as is commonly revealed in various examples [27,28], It is reasonable to expect that these polymorphic properties of fats may interfere with the clarification of the essential properties of the interface heterogeneous nucleation that occurs in O/W emulsions. 

Figure 16 Schematic molecular model of interface heterogeneous nucleation.

The importance of the solid-liquid interface in a host of applications has led to extensive study over the past 50 years. Certainly, the study of the solid-liquid interface is no easier than that of the solid-gas interface, and all the complexities noted in Section VIM are present. The surface structural and spectroscopic techniques presented in Chapter VIII are not generally applicable to liquids (note, however. Ref. 1). There is, perforce, some retreat to phenomenology, empirical rules, and semiempirical models. The central importance of the Young equation is evident even in its modification to treat surface heterogeneity or roughness. ... 

There has been a general updating of the material in all the chapters the treatment of films at the liquid-air and liquid-solid interfaces has been expanded, particularly in the area of contemporary techniques and that of macromolecular films. The scanning microscopies (tunneling and atomic force) now contribute more prominently. The topic of heterogeneous catalysis has been expanded to include the well-studied case of oxidation of carbon monoxide on metals, and there is now more emphasis on the flexible surface, that is, the restructuring of surfaces when adsorption occurs. New calculational methods are discussed. 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

The equivalent equations for heterogeneous and quasi-heterogeneous systems (tire latter are small vesicles which can practically be handled as homogeneous systems, but which are nevertlieless large enough to possess a macroscopic solid-liquid interface) are dealt witli in section C2.14.7. 

Present research is devoted to investigation of application of luminol reactions in heterogeneous systems. Systems of rapid consecutive reactions usable for the determination of biologically active, toxic anions have been studied. Anions were quantitatively converted into chemiluminescing solid or gaseous products detectable on solid / liquid or gas / liquid interface. Methodology developed made it possible to combine concentration of microcomponents with chemiluminescence detection and to achieve high sensitivity of determination. 

For modern analysis, high sensitivity and selectivity of methods for the determination of these elements are important parameters. Catalymetric methods of analysis possess such characteristics. Using of heterogeneous catalytic reactions on the interface also penults to reduce determination time. 

When the nucleus is formed on a solid substrate by heterogeneous nucleation the above equations must be modified because of the nucleus-substrate interactions. These are reflected in the balance of the interfacial energies between the substrate and the environment, usually a vacuum, and the nucleus-vacuum and the nucleus-substrate interface energies. The effect of these terms is usually to reduce the critical size of the nucleus, to an extent dependent on... 

Fig. 7.5. Nucleation in solids. Heterogeneous nucleotion con take place at defects like dislocations, grain boundaries, interphase interfaces and free surfaces. Homogeneous nucleation, in defect-free regions, is rare.

When considering relea.se mechanisms, the physical and chemical heterogeneity of the adhesive/release interface cannot be ignored. At its most basic level, roughness of the release and PSA surface, the stiffness of the PSA and the method in which the PSA and release surface are brought together define the contact area of the interface. The area of contact between the PSA and release material defines not only the area over which chemical interactions are possible, but al.so potential mechanical obstacles to release. In practice, a differential liner for a transfer adhesive can be made to depend in part on the substrate roughness for the differences in release properties [21],... 

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