Permeability layers

Figure 7.8 Permeation of drugs through oil-soaked microfilters comparisons to Caco-2 permeabilities (dashed curves) [546] (a) oil-free (untreated hydrophilic) filters (b) unstirred water layer permeability versus log MW (c) octanol-soaked (impregnated) filters (d) isopropylmyristate-soaked filters.

The UWL permeability is nearly the same for drugs of comparable size, and is characterized by the water diffusivity (Daq) of the drug divided by twice the thickness of the layer (ftaq), Pu = Aiq / (2 h.Aq), in a symmetric permeation cell [40], The unstirred water layer permeability can be determined experimentally in a number of ways based on pH dependency of effective permeability [25,509,535-538], stirring rate dependence [511-514,552,578], and transport across lipid-free microfilters [25,546],... 

Figure 7.34 Permeability-pH profiles of ketoprofen (a) log-log plot solid curve represents effective permeability, and the dashed curve is the membrane permeability, calculated by Eq. (7.53). The latter curve levels off at the intrinsic permeability, Pq. The effective curve levels off to approximately the unstirred water layer permeability, Pu. (b) Direct plot the inset curve for the fraction neutral substance levels of at 100% (scale not shown). [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]...

TABLE 7.15 Intrinsic Permeabilities and Unstirred Water Layer Permeabilities Determined from Iso-pH Dependence of Effective Permeabilities 2% DOPC in Dodecane... 

Carbamazepine, antipyrine, terbutaline, and hydrochlorothiazide were treated as neutral molecules. Their effective permeabilities were corrected for the unstirred water layer using estimated unstirred water layer permeabilities, determined by the other molecules of similar lipophilicities and size. 

Figure 7 (Left panel) Relative absorption rate for a weak acid (pKa = 3) as a function of mucosal pH for increasing barrier (membrane) permeability (Pb) with fixed unstirred aqueous layer permeability (Pul). X = pHinflectionpoint in Eq. (4). (Right panel) Partition coefficient-dependent absorption rates for salicylic acid and the weak base ephedrine. (From Ref. 19.)...

The interest of using reciprocal values as in Eq. (5-12) is that an experimental plot of. 7 1 vs Q u2 must be a straight line parallel to the Levich linear variation, which passes through the origin. The ordinate value of the intercept of this straight line at 2 1/2 = 0 is /fjioo, and hence provides the layer permeability DF/8F. 

When the intrinsic PAMPA data (P0) are corrected for the appropriate unstirred water layer permeability (P "), the unified relationship between the biological barrier data (Papp) and the artificial membrane data is ... 

The resulting mesoporous layers don t usually exhibit extraporosity at a larger scale, but as smaller pores in the oxide walls (Figure 25.24). ° Due to limitations associated with intrinsic mesostructure characteristics, anisotropy resulting from preferential orientations, or boundaries between ordered domains, the tem-plated mesoporosity is usually not directly interconnected. " In such situations, it does directly define the selectivity of the membrane, which depends on the pore size of the oxide walls. However, the resulting hierarchical porosity (templated mesopores and smaller pores of the oxide walls) favors a decrease in layer permeability. The templated mesoporosity can also be used to functionalize the membrane. 

The layered model discussed here is a two-layer model top layer permeability is 5 md, and bottom layer permeabihty is 50 md. The ratio of vertical permeability to horizontal permeability is 0.1, and the total injection volume is 2 PV. The rest of the input data are the same as the homogeneous model. Figures 4.15 and 4.16 show the recovery factors versus Mjoc and for the initial water saturation of 0.5. 

This may be of importance in membrane filtration, where significant changes in concentration may take place in the boundary layer, where molecular conformation may influence gel layer permeability. However it may only be relevant for feeds of high NOM, high recovery and significant concentration polarisation. 

Such low-permeable layers can be formed not only on account of high-molecular surfactants. It has been shown in [29] that the adsorption layer permeability of proteins (bovine serum albumin, gamma-globulin) and lauiyl sulphate, determined by loss in weight of the evaporating film, is practically the same. Addition of higher alcohols to alkyl sulphates enhances the foam stability under dry air conditions. When lauryl alcohol was added to dodecyl sulphate, it turned out that the life time of the foam increased by 10 to 20 % at reduced air humidity. Note that this effect is more pronounced at maximum undersaturation of air by water vapours. 

Effective membrane permeability is related to membrane permeability and unstirred layer permeability as ... 

A solid layer permeable to the working irai that separates anode from cathode to prevent electron transfer between them inside a battery cell. 

D Quast, M Karel. Dry layer permeability and freeze-drying rates in concentrated fluid systems. Journal of Food Science 33 170, 1968. 

If the freeze-drying process is controlled by water vapor mass transfer through the freeze-dried layer - for example in the case of dried cake of very low permeability or for very favorable heat transfer conditions - the drying rate for one vial is proportional to the mean driving force P - Pc) - that is, to the water vapor pressure difference between the sublimation front interface and the drying chamber - and controlled by the freeze-dried cake mass transfer resistance, denoted J p, which is inversely proportional to the dried layer permeability defined by Darcy s law. Inthiscaseitholds ... 

Fig. 3.10 Simulation of the influence of cooling rate on dried layer permeability according to...

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