Mass transport, interfaces

All fluid interfaces contain an undisturbed layer of solution adjacent to the interface. Mass transport in this boundary layer occurs only by diffusion. The thickness of the boundary layer depends on temperature, stirring, and the interface itself. It is up to 0.1 cm thick in unstirred systems and approaches 10"3 cm in vigorously stirred systems3,31,32). 

To determine the elementary processes involved in a reaction mechanism occurring at an electrode/electrolyte interface (mass transport, chemical, and/or electrochemical reactions) requires the use of techniques to control the state of the electrode and to analyze the behavior of the interface. One begins by studying the steady-state regime. Although this study sometimes suffices for simple processes, it proves inadequate as the degree of complexity of the processes and their coupling increases. Nonsteady-state techniques must then be used [148,151,153]. 

Atkinson, A. Surface and Interface Mass Transport in Ionic Materials. Solid State Ionics 28/30 (1988)... 

Although a number of simplifying assumptions were introduced to facilitate the derivation of (8.14), it is important to note that the result for chemical potential (8.14) is independent of these assumptions. In other words, (8.14) provides a chemical potential for interface mass transport for any such system for which behavior is described by the same constitutive assumptions as were invoked above. This is so in cases where the system does exchange energy with its environment or in which there is dissipation due to interfacial shearing, for example. 

If the delta-r due to the exothennaUty of the reaction is not too high, such as in an EBR, the pseudohomogeneons model can properly represent the behavior of the reactor. In the pseudohomogeneons models, it is considered that the interface mass transport is faster than the reaction, that is, the mass and heat transfer in the interface are assumed to be insignificant, which implies that tanperature and concentration in the bulk and catalyst surface are both the same. 

When two reactants in a catalytic process have such different solubiUty properties that they can hardly both be present in a single Hquid phase, the reaction is confined to a Hquid—Hquid interface and is usually slow. However, the rate can be increased by orders of magnitude by appHcation of a phase-transfer catalyst (40,41), and these are used on a large scale in industrial processing (see Catalysts, phase-TRANSFEr). Phase-transfer catalysts function by faciHtating mass transport of reactants between the Hquid phases. Often most of the reaction takes place close to the interface. 

The part that marries the plasma to the mass spectrometer in ICPMS is the interfacial region. This is where the 6000° C argon plasma couples to the mass spectrometer. The interface must transport ions from the atmospheric pressure of the plasma to the 10 bar pressures within the mass spectrometer. This is accomplished using an expansion chamber with an intermediate pressure. The expansion chamber consists of two cones, a sample cone upon which the plasma flame impinges and a skimmer cone. The region between these is continuously pumped. 

This chapter attempts to give an overview of electrode processes, together with discussion of electron transfer kinetics, mass transport, and the electrode-solution interface. 

The devolatilization of a component in an internal mixer can be described by a model based on the penetration theory [27,28]. The main characteristic of this model is the separation of the bulk of material into two parts A layer periodically wiped onto the wall of the mixing chamber, and a pool of material rotating in front of the rotor flights, as shown in Figure 29.15. This flow pattern results in a constant exposure time of the interface between the material and the vapor phase in the void space of the internal mixer. Devolatilization occurs according to two different mechanisms Molecular diffusion between the fluid elements in the surface layer of the wall film and the pool, and mass transport between the rubber phase and the vapor phase due to evaporation of the volatile component. As the diffusion rate of a liquid or a gas in a polymeric matrix is rather low, the main contribution to devolatilization is based on the mass transport between the surface layer of the polymeric material and the vapor phase. 

OS 43] [R 14] [P 32] Using a three-liquid layer (water/oil/water) flow instead of a two-liquid layer flow at constant channel dimensions decreases the liquid lamellae width and doubles the absolute value of the organic/aqueous interface. As a consequence, mass transport is facilitated compared with the two-flow configuration. Hence it was found that a much higher yield was obtained for the three-liquid layer flow when performing experiments of both flow configurations imder the same experimental conditions (210 s, 0.2 pi min room temperature, 300 W, > 300 nm... 

Extraction Almost always Large uncertainty with respect to the operation time will remain unless pilot studies are performed. Impurities in raw materials of technical grade and in recycle streams can heavily influence mass transport through interfaces and separability of phases. 

Parthasarathy A, Srinivasan S, Appleby AJ, et al. 1992b. Pressure dependence of the oxygen reduction reaction at the platinum microelectrode/Nafion interface Electrode kinetics and mass transport. J Electrochem Soc 139 2856-2862. 

Interphase mass transport also represents a possible input to or output from the system. In Fig. 1.13., transfer of a soluble component takes place across the interface which separates the two phases. Shown here is the transfer from phase G to phase L, where the separate phases may be gas, liquid or solid. 

We begin by reviewing the principles of SECM methods, and present an overview of the instrumentation needed for experimental studies. A major factor in the success of SECM, in quantitative applications, has been the parallel development of theoretical models for mass transport. A detailed treatment of the theory for the most common SECM modes that have been used to study liquid interfaces is therefore given, along with key results from these models. A comprehensive assessment of the applications of SECM is provided and the prospects for the future developments of the methodology are highlighted. 

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. 

For small K, i.e., K = 0.5 in Fig. 17, the response of the equilibrium to the depletion of species Red] in phase 1 is slow compared to diffusional mass transport, and consequently the current-time response and mass transport characteristics are close to those predicted for hindered diffusion with an inert interface. As K is increased, the interfacial process responds more rapidly to the electrochemical perturbation in phase 1. The transfer of the target species across the interface generates an enhanced flux to the UME, causing... 

For the study of reactions at immiscible liquid-liquid interfaces, achieving well-defined contact between the two phases, with known interfacial area, under conditions where variable and high rates of mass transport are attainable, is nontrivial. 

The general criteria for an experimental investigation of the kinetics of reactions at liquid-liquid interfaces may be summarized as follows known interfacial area and well-defined interfacial contact are essential controlled, variable, and calculable mass transport rates are required to allow the transport and interfacial kinetic contributions to the overall rate to be quantified direct interfacial contact is preferred, since the use of a membrane to support the interface adds further resistances to the overall rate of the reaction [14,15] a renewable interface is useful, as the accumulation of products at the interface is possible. Finally, direct measurements of reactive fluxes at the interface of interest are desirable. 

In this chapter, we describe some of the more widely used and successful kinetic techniques involving controlled hydrodynamics. We briefly discuss the nature of mass transport associated with each method, and assess the attributes and drawbacks. While the application of hydrodynamic methods to liquid liquid interfaces has largely involved the study of spontaneous processes, several of these methods can be used to investigate electrochemical processes at polarized ITIES we consider these applications when appropriate. We aim to provide an historical overview of the field, but since some of the older techniques have been reviewed extensively [2,3,13], we emphasize the most recent developments and applications. 

A significant advance in this area was recently made by Li and coworkers [30,31], who developed a laminar flow technique, that allowed the direct contact of two liquids with better-defined mass transport compared to the Lewis cell. Laminar flow of the two phases parallel to the interface was produced through the use of flow deflectors. By forcing flow parallel to, rather than towards, the interface, it was proposed that the interface was less likely to be disrupted. Reactions were followed by sampling changes in bulk solution concentrations. 

To ensure that the detector electrode used in MEMED is a noninvasive probe of the concentration boundary layer that develops adjacent to the droplet, it is usually necessary to employ a small-sized UME (less than 2 /rm diameter). This is essential for amperometric detection protocols, although larger electrodes, up to 50/rm across, can be employed in potentiometric detection mode [73]. A key strength of the technique is that the electrode measures directly the concentration profile of a target species involved in the reaction at the interface, i.e., the spatial distribution of a product or reactant, on the receptor phase side. The shape of this concentration profile is sensitive to the mass transport characteristics for the growing drop, and to the interfacial reaction kinetics. A schematic of the apparatus for MEMED is shown in Fig. 14. 

As reversible ion transfer reactions are diffusion controlled, the mass transport to the interface is given by Fick s second law, which may be directly integrated with the Nernst equation as a boundary condition (see, for instance. Ref. 230 232). A solution for the interfacial concentrations may be obtained, and the maximum forward peak may then be expressed as a function of the interfacial area A, of the potential scan rate v, of the bulk concentration of the ion under study Cj and of its diffusion coefficient D". This leads to the Randles Sevcik equation [233] ... 

Biofilms adhere to surfaces, hence in nearly all systems of interest, whether a medical device or geological media, transport of mass from bulk fluid to the biofilm-fluid interface is impacted by the velocity field [24, 25]. Coupling of the velocity field to mass transport is a fundamental aspect of mass conservation [2]. The concentration of a species c(r,t) satisfies the advection diffusion equation... 

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