Complex diffusion

X10 cm /s. This is over 1,000 X smaller than the Hiffusion coefficient for this osmium complex diffusing freely in the acetonitrile solvent (obtained from the limiting current at the naked Pt electrode), and the observed PD corresponds to a very low permeability of the polymer film to luch bulky permeants. 

With the exception of Elder et. al., who used EXAFS to look at redox properties of copper complexes diffused in Nafion film modified electrodes (21), no in-situ studies have been reported on modified electrodes using EXAFS. 

Since many environmentally relevant complexes diffuse much slower than M (see Chapter 3), the distinction between Dm and Dml is important. Their ratio is usually labelled ... 

When all complexes diffuse as quickly as free M, DMLi = DM and the maximum obtainable limit for the permeability is ... 

Field measurements in sedimentary fluvial-type calcrete deposits also suggest that present-day groundwater in these areas may also display potential to both dissolve and precipitate uranium in the near surface. Chemical dilatancy and evaporation-driven diffusion that promote de-complexing, diffusion, and reprecipitation mechanisms are seen to play integral parts in the continued chemical reworking and modification of these calcrete-hosted carnotite deposits. 

The complex diffusion of the rodlike polymers during phase separation is also discussed. 

Engstrom et al. (1997) looked at diffusion couples in Ni-Cr-Al alloys and found that extremely complex diffusion paths were possible, see for example (Figs. 11.23(a) and (b)). These seem strange, but they are all consistent with the observed... 

In the preceding section it is seen how the theory of multicomponent systems gives the correct starting formulas for the analysis of ordinary and thermal diffusion in binary systems. Whereas these latter topics have been the subject of considerable investigation, there are a number of types of more complex diffusion problems of engineering interest for which little has been done. Several of these topics are discussed here, and an attempt is made to indicate to what extent they can be interpreted in terms of the theoretical development in the preceding sections. 

This problem is a good example of the importance of formulating a complex diffusion problem in terms of the equations of change. Hence the simplified treatment given here is discussed in terms of the simplified solutions to the three basic equations. 

The vacancy diffusion field around the toroidal loop will be quite complex, but at distances from it greater than about 2RL, it will appear approximately as shown in Fig. 11.12a. A reasonably accurate solution to this complex diffusion problem may be obtained by noting that the total flux to the two flat surfaces in Fig. 11.12a will not differ greatly from the total flux that would diffuse to a spherical surface of radius d centered on the loop as illustrated in Fig. 11.126. Furthermore, when d > Rl, the diffusion field around such a source will quickly reach a quasi-steady state [20, 26], and therefore... 

Fig.. 1 The electroactive complex diffuses from ihc bulk electrolyte solution tA) through the diffusion layer (B) to the Helmholtz double layer (C) to be discharged as metallic chromium D on Hie cathode surface (Fj. After General Motors color sketch I...

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

In facilitated transport (also known as carrier-mediated membrane transport), a substance combines with a specific carrier protein on the membrane, and the resultant protein-sub-stance complex diffuses to the other side of the membrane, where it dissociates to release the substance. The absorption of glucose from the intestines into the blood, for example, requires facilitated transport of glucose across the cellular membranes of the epitheleal lining of the intestines. Many amino adds cross cellular membranes by fadlitated transport. 

Another limitation of solid electrodes has been their complex diffusion-current response relative to time with slow-sweep voltammetry. The development of a capillary hanging-mercury-drop electrode (HMDE) by Kemula and Kublik,4,5 together with modem electronic instrumentation, allowed the principles of voltage-sweep voltammetry and cyclic voltammetry to be established. The success has been such that this has become one of the most important research tools for electrochemists concerned with the kinetics and mechanisms of electrochemical processes. These important contributions by Nicholson and Shain6 7 rely, as have all electrochemical kinetic developments, on the pioneering work by Eyring et al.8... 

Answer When electron-carrying complexes are bound together in a supercomplex, electron flow between complexes occurs in a solid state this electron movement is kinetically favored compared with the situation in which electron flow depends on each complex diffusing to and colliding with the next complex in the chain. 

For the case of a sphere, the control volume is given by a thin spherical shell of thickness dr and radius r. If we assume that the complex diffusion process inside the porous structure can be represented by Fick s first law, and we additionally suppose that the volume change due to reaction is negligible (i.e. the total number of moles is constant), we arrive at the following form of the mass conservation law for the reacting species i ... 

Although diffusion is a slow process compared to energy transfer and electron transfer at the shortest distances, it can be an exceptionally effective way to move electrons and protons over long distances. However, unlike the hard-wired cofactor chains that guide electron transfer in protein complexes, diffusion faces the problem of directing where... 

A complex, diffuse symptoms affecting all bodily systems. This is a complex, widespread neurotransmitter, the receptor sites of which are affected (usually adversely) by some psychotropic medications (anticholinergic side effects). 

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