Hindered diffusion

Passive transcellular diffusion hindered by efflux pumps... 

It is quite possible that the decrease of activation energy observed by Winter is coimected in all the cases considered through the influence of diffusion hindering at higher rates of exchange which was absent in the work of Dzisjak et al. (5) due to the forced circulation of the reaction mixture. 

Flgure 6.3 A Randles circuit where is the solution resistance, Qi is the double-layer capacitance, R t is the charge transfer, and Z is the diffusion-hindered impedance. 

Degradation phenomena around 250 °C are explained by cleavage of weak spots that represent unstable defects in the polymer chain. These weak spots can be structural defects (head-to-head links), contaminants incorporated by polymerization, and thermally unstable end groups. In addition, a small number of statistic chain cleavages may be able to cure in a diffusion-hindered environment ( cage effect ) by disproportionation. Figure 5.171. 

A priori, the best method to obtain a reactive mixture is the homogenity in the smallest scale possible when precursors have formed stable bonds by means of oxygen bonds. But it is not obvious that such bonds can resist the heating necessary for the establishment of a long-distance bond and for the crystalUzation of the desired phase. After drying, the mixture of precursors is homogenous but thermodynamically unstable. At ambient temperature, the low coefficients of diffusion hinder a phase separation. During heat treatment, there is therefore a competition between a separation of phase controlled by the speed of diffusion of elements and the crystallization of the desired phase. 

Ha T, Glass J, Enderle T, Chemla D S and Weiss S 1998 Hindered rotational diffusion and rotational ]umps of single molecules Phys. Rev. Lett. 80 2093-7... 

Diffusion within the largest cavities of a porous medium is assumed to be similar to ordinary or bulk diffusion except that it is hindered by the pore walls (see Eq. 5-236). The tortuosity T that expresses this hindrance has been estimated from geometric arguments. Unfortunately, measured values are often an order of magnitude greater than those estimates. Thus, the effective diffusivity D f (and hence t) is normally determined by comparing a diffusion model to experimental measurements. The normal range of tortuosities for sihca gel, alumina, and other porous solids is 2 < T < 6, but for activated carbon, 5 < T < 65. 

M. P. Bohrer, G. D. Paterson, P. J. Carrol. Hindered diffusion of dextran and ficoll in microporous membranes. Macromolecules 77 1170-1173, 1984. 

Olefins are hydrogenated very easily, unless highly hindered, over a variety of catalysts. With active catalysts, the reaction is apt to be diffusion limited, since hydrogen can be consumed faster than it can be supplied to the catalyst surface. Most problems connected with olefin hydrogenation involve some aspect of regio- or stereoselectivity. Often the course of reduction is influenced greatly by the catalyst, by reaction variables, and by hydrogen availability at the catalyst surface. 

In 1929 Pfeil" published a most interesting account of the way layered structures form and the manner in which they influence oxidation rates. From detailed studies of the growth and composition of scales he was able to show clearly how the formation of barrier layers reduced scale formation by hindering outward diffusion of iron through the scale. Naturally, this work had to be largely based on the study of scales of sufficient thickness so that the mechanism of the early stages of oxidation could not be studied in this way. Pfeil analysed the outer, middle and inner layers of scales formed... 

In systems comprised of cells in culture, there is no formal architecture (such as might be encountered in a whole tissue) that would hinder free diffusion. Such... 

FIGURE 2-17 Principles of SECM. (a) Tip far from the substrate surface diffusion of O leads to steady-state current. (b) Tip near a conductive substrate positive feedback of O. (c) Tip near the insulating substrate hindered diffusion of O. c = concentration a = radius of tip. (Reproduced with permission from reference 55.)... 

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