Diffusional barrier

Agmon N and Kosloff R 1987 Dynamics of two-dimensional diffusional barrier crossing J. Phys. Chem. 91 1988-96... 

Figure 2. Comparison of Calculated Diffusional barriers for 2,4,4-trimethyl-3-pentane (TMP) in Various Zeolites and Molecular Sieves... 

Isenberg I would like to address the question of the diffusional barrier once more. There is one argument that this barrier is flat sheet. On the other hand, Casey van Breemen has shown that in the region of the caveolae, there are interruptions of... 

Because most proteins are susceptible to protease degradation and denaturation in biologic fluids, most biopharmaceuticals must be administered by intravenous, intramuscular, or subcutaneous injection (see Table 5.5). High concentrations of proteases are found in the gastrointestinal tract, nasal mucosa, bronchioles, and alveoli, which severely limit the bioavailability of protein pharmaceuticals after oral, intranasal, and inhalation administration. Diffusional barriers to the passage of relatively large macromolecules preclude transdermal and mucosal administration of protein pharmaceuticals. Research is under way to develop methods that will protect protein drugs from proteolysis and improve transmembrane diffusion. 

In the situation where V2DA is of the same order or larger than the distance between any diffusional barriers in the system, so-called restricted diffusion is observed. In a W/O emulsion, for example, the water molecules are restricted in the extent of their diffusion by the presence of the boundaries of the water droplets. The extent of the restriction of the diffusion of the water molecules is reflected in the ratio R = E /E. An expression for the echo attenuation R-factor as a function of droplet diameter has been derived by Murday and Cotts for uniform spherical droplet sizes [7] ... 

While they are malleable, these hydrate shells form a diffusional barrier between the hydrocarbon and water phases. The shells do not usually become very thick, except at long times—periods of days. 

Operation of the sensor with porous diffusional barrier (Fig. 7.19b) is similar except that the constants have slightly different physical meanings. In that case, Aeff is the area of the cathode and d is the thickness of the porous layer. For a low concentration of oxygen... 

Apart from being a diffusional barrier, mucin can also interact with drugs to decrease their bioavailability, as has been shown with tetracycline [106], phenylbutazone, and warfarin [107]. On the other hand, studies in rats showed that binding of some water-soluble drugs to intestinal mucus was essential for their absorption and that damage to the mucus significantly reduced absorption [108], The acidic mucus is essential for lipid absorption and could be important for the diffusion of lipophilic drugs (see below). 

From these data one can deduce that there exists a proportionate relationship between the concentration of (ri6-C6H6)2V in the pentane phase and the amount of intrazeolitic (n6-CGH6)2V. Clearly under these impregnation conditions there appears to exist no appreciable diffusional barrier to the entry of (u6-C6H )2V into the supercages of NaY. 

Although we have assumed that there is no mucosal diffusional barrier for the CYP3A substrate, the unbound inhibitor concentration in the intestinal epithelia (7gm) may or may not be equivalent to that in plasma and the liver (7U). 7gm may exceed the unbound portal plasma concentration during the inhibitor absorption phase or be less than the unbound portal concentration postabsorption if there is not rapid equilibrium between the intracellular and portal plasma compartments (i.e., a basolateral membrane diffusional barrier exists). This obviously makes it challenging to anticipate the quantitative effect of an inhibitor on intestinal first-pass metabolism. 

Gwilt P, Comer S, Chaturvedi P, et al. The influence of diffusional barriers on presystemic gut elimination. Drug Metab Dispos 1988 16 521-526. 

Inspection of equation (4.6) shows how the effect of an enhancer on the diffusional barrier may be quantified. If an experiment is conducted such... 

It has been shown that the magnitude of the rate constant for crossing the octanol-water interface makes the energy barrier significantly larger than the diffusional barrier. It has also been shown that for compounds with log Pow less than =1.2, the overall rates are faster and the interfacial kinetics term more important. However, detailed development of a model would be needed to understand what the relative importance of diffusion and interfacial terms (such as cuticle or membrane permeation) are in vivo. No clear dependence of interfacial rate constants on log Pow was seen, but the initial emphasis of such a study should be on the intermediate... 

In order to cover glucuronidation reactions in incubations in microsomal fractions several modifications have been applied in order to optimize conditions. These comprise longer incubation times than necessary for oxidative reactions by cytochrome P450s, and use of modifiers, both to overcome the latency in activity due to the diffusional barriers of the endoplasmatic reticulum (Coughtrie and Fisher 2003 Csala et al. 2004). Modifiers used are detergents or the pore-forming peptide alamethicin (Fisher 2000). Also disruption of cells by sonication is applied (Ethell 1998). 

However, when the temperature increases further, the PNIPA network shrinks markedly, followed by a concomitant slowing down of the diffusion of reactants within the network. This in turn will lower the rate of reaction catalyzed by the metal nanoparticles. It is obvious that the increase of kapp by the raise of temperature is overcompensated by the diffusional barrier. Hence, the reaction rate must reach its minimum at the transition temperature. If the increase of temperature continues, the PNIPA network will not shrink any more and the density within the network stays constant. Now, the strong increase of kaW with T will be predominant and the... 

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