Interface control mechanisms

Peppas attribute the difference in disintegration rate between soluble and insoluble matrices to two proposed phenomena—an interface-controlled mechanism and a diffusion-controlled mechanism—as represented in the following equation ... 

Finally, these factors make it especially important to carefully choose appropriate control specimens (with no fiber coatings) and control tests to properly evaluate the effect of the coatings on the composite behavior. Unfortunately, this is not often done. All of these issues should be taken into consideration when comparing strengths of composites and the success of interface control mechanisms. 

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

In a kinetic regime system, the kinetics of solvent extraction can be described in terms of chemical reactions occurring in the bulk phases or at the interface. The number of possible mechanisms is, in principle, very large, and only the specific chemical composition of the system determines the controlling mechanism. Nevertheless, some generalizations are possible on considerations based... 

Figure 1-12 Control mechanisms of mineral dissolution in aqueous solutions. Data are from Berner (1978). A straight line is drawn to separate transport control and interface reaction control although there is no theoretical basis for whether the boundary should be linear. Almost without exception, those with transport control lie above a straight line, and those with interface reaction control lie below the line. The only significant departure from the rule is the dissolution of PbS04 (cross in the figure) that lies inside the region for the interface reaction control, but is actually controlled by both interface reaction and mass transport.

In heterogeneous solid state reactions, the phase boundaries move under the action of chemical (electrochemical) potential gradients. If the Gibbs energy of reaction is dissipated mainly at the interface, the reaction is named an interface controlled chemical reaction. Sometimes a thermodynamic pressure (AG/AK) is invoked to formalize the movement of the phase boundaries during heterogeneous reactions. This force, however, is a virtual thermodynamic force and must not be confused with mechanical (electrical) forces. 

To summarize the structure of a moving interface on the atomic scale depends on the atomic mechanism which operates in the structure transformation. The mode selection depends on the driving force and thus on the interface velocity. The interface mobility itself is determined by its structure and depends therefore on the driving force. This means that interface controlled reactions are normally nonlinear functions of the driving force. 

The objective is to reduce volatiles to below 50-100-ppm levels. In most devolatilization equipment, the solution is exposed to a vacuum, the level of which sets the thermodynamic upper limit of separation. The vacuum is generally high enough to superheat the solution and foam it. Foaming is essentially a boiling mechanism. In this case, the mechanism involves a series of steps creation of a vapor phase by nucleation, bubble growth, bubble coalescence and breakup, and bubble rupture. At a very low concentration of volatiles, foaming may not take place, and removal of volatiles would proceed via a diffusion-controlled mechanism to a liquid-vapor macroscopic interface enhanced by laminar flow-induced repeated surface renewals, which can also cause entrapment of vapor bubbles. 

Other reactor design considerations may be necessary in special cases. Monomer mass transfer, not normally a problem, can he important if the monomer- aqueous phase interface is small. This is more likely in systems involving gaseous monomers in which the large surface area of the monomer emulsion is not present. In such cases special attention must he paid to gas dispersion and transport. Giher factors that can have a significant effect on reactor design include latex viscosity, heat transfer rates, reaction pressure, and control mechanisms. 

Interfacial polycondensation has been studied in considerable detail in recent years, since this technique is quite useful for preparing high-melting polymers for fibre and other applications. This polymerization takes place in a two-phase system, with the propagation reaction occurring at or very near the interface. The mechanism is essentially diffusion controlled. 

The second method for catalyzing the membranes is to cast the same type of ink (TBA" " form of the ionomer) directly onto the membrane [44]. This process may have an advantage over the decal process in the formation of a more intimate membrane/ electrode interface. It may also be more amenable to scale-up. Indeed, initial attempts at laboratory-scale automated application of thin-film Pt/C//ionomer catalyst layers to ionomeric membranes have been quite successful. In this work, a computer-controlled mechanism of an X-Y recorder was applied to paint catalyst ink by the controlled repetitive motion of the pen of the recorder onto each of the membrane major surfaces. In this way, 100 cm areas of catalyzed membranes were re-producibly generated, yielding performances per cm of a similar level to that achieved previously with catalyzed membrane of 5 cm active area [44]. The laboratory-scale automation equipment is shown in Fig. 22. 

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