Mechanisms other than diffusion

However, pH does not always influence the rate or extent of absorption. For example, McElnay et al. found that captopril pharmacodynamic parameters (blood pressure, heart rate, and plasma renin activity) did not differ significantly between buffered and unbuffered sublingual administration, suggesting that manipulation of pH had little effect. It was, therefore, proposed that a mechanism other than passive diffusion was involved in the buccal absorption of this drug. 

To test for either adsorptive or electrostatic interactions, the SEC separation is performed at a variety of temperatures. If the separation occurs by size alone, the retention coefficient R(= V0/Ve) is independent of temperature very small variations may be observed as a result of gel swelling or microstructural changes to the gel. The presence of a significant dependence of R on T indicates the presence of a mechanism other than size exclusion. While R should not vary with 7 diffusion coefficients increase with T and so zone broadening occurs, leading to decreased resolution with increasing separation temperatures. 

The half-Ufe of PHY in rat brain was 11 min. PHY is rapidly concentrated in the rat s brain. The drag, or metabolite, appears to be concentrated in mitochondria in greater amounts by a mechanism other than simple diffusion. The effect of the drug on mitochondrial function are not known, nor is it known how these effects are related to the toxicity of PHY in humans." ... 

Where the summation extends over all the mechanisms other than impaction (i.e. interception, diffusion and thermophoresis). 

The excretion of the tocainide enantiomers into saliva was studied by Pillai et al. [140] after the IV injection of the racemic drug. It was reported that both enantiomers are excreted into saliva with saliva plasma ratios of 2.1 and 3.7 for the S and R enantiomers, respectively. The higher ratio for the R enantiomer was attributed to the involvement of transport mechanisms other than passive diffusion [140]. However, these mechanisms are not known at this time. 

In other words, for a Turing instability to occur, the activator must diffuse slower than the inhibitor. This is known as the principle of short-range activation and long-range inhibition. It is also known as local autocatalysis with lateral inhibition or local auto-activation-lateral inhibition (LALI), see for example [332, 319], local self-activation and lateral inhibition [280], or self-enhancement and lateral inhibition (SELI) [315] and has been applied to mechanisms other than reaction-diffusion. 

To surpass Robeson s upper bound, materials are emerging that rely on transport mechanisms other than solution-diffusion through glassy or rubbery polymeric materials. In particular, a number of materials have been developed that possess fixed microporosity (2 nm or less) in contrast to the activated, transient molecular gaps that give rise to diffusion in most polymers. These materials include amorphous and crystalline (zeolite) ceramics [68-69], molecular sieve carbons [70], polymers that possess intrinsic microporosity [71-72], and carbon nanotube membranes [73-76]. Transport in such materials is determined primarily by the average size and size distribution of the microporosity - the porosity can be tuned to allow discrimination between species that differ by less than one Angstrom in size. However, surface... 

Finally, returning briefly to foams, we note that the number of cells in a polyhedral foam can decrease by a mechanism other than him rupture by the mechanisms discussed above. As the pressure is largest for gas in the smallest cells, a driving force exists for transfer of gas from small to large cells by diffusion across the thin liquid Aims separating them. Ultimately this process leads to the disappearance of small cells and growth of large cells. Lemlich (1978) and Ranadive and Lemlich (1979) present a model of gas diffusion in foams. 

Scale Growth Controlled by Mechanisms Other than Solid State Diffusion 85... 

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