Permeability model for

While there are limitations associated with the use of an in vitro permeability model for assessing the transport of compounds across the buccal mucosa, it can still be useful in assessing and comparing the permeability of compounds under different conditions, such as pH, temperature, and osmolarity, which provide valuable information on the mechanisms involved in drug transport. Additionally, the preliminary effects of potential chemical penetration enhancers or formulation excipients may be assessed, and these may provide a substantial rationale for subsequently assessing the effect of these agents in man. 

The flow of liquid in ceramic green bodies of liquid volvune fraction, < i[=l - 4>], has been shown to follow permeability models for the flow of liquid relative to ceramic particles [8,9] given in spherical coordi-... 

Although the current permeability model properly reflects many of the important features of foam displacement, the authors acknowledge its limitations in several respects. First, the open pore, constricted tube, network model is an oversimplification of true 3-D porous structures. Even though communication was allowed between adjacent pore channels, the dissipation associated with transverse motions was not considered. Further, the actual local displacement events are highly transient with the bubble trains moving in channels considerably more complex than those used here. Also, the foam texture has been taken as fixed the important effects of gas and liquid rates, displacement history, pore structure, and foam stability on in situ foam texture were not considered. Finally, the use of the permeability model for quantitative predictions would require the apriori specification of fc, the fraction of Da channels containing flowing foam, which at present is not possible. Obviously, such limitations and factors must be addressed in future studies if a more complete description of foam flow and displacement is to be realized. 

Rawal A. A cross-plane permeability model for needlepunched nonwoven structures. J Textile Institute 2007 97 527-32. 

Woodfleld, D. and Bickert, G. (2001). An improved permeability model for fractal aggregates settling in creeping flow. Water Res., 35, 3807-3806. 

Barrier property to moisture and gas is another key demand for PU coatings. It is reported that impermeable nanoclays can effectively reduce the permeability of polymers by up to hundred times. The mechanism of permeabrhty has been widely discussed from experimental and theoretical aspects. Nielsen [44] proposed a permeability model for a regular arrangement of platelets based on the assumption, that is, rectangular platelets are uniformly dispersed and perpendicularly... 

Figure 17.13. Applicable permeability models for filled and two-phase polymer systems. The Geometric model with /= 0.125 most closely reflects observed MFC oxygen permeability...

Note that we use (10.42) for deciding on the local permeability, where we introduce a parameter a to represent the internal pore structure schematically. Further work is needed to establish the correct permeability model for bentonite with a multiscale pore structure. 

The permeability coefficient k for viscous flow through fibrous media is observed to vary in proportion to the bulk porosity and density of nanofiber mat and in inverse proportion to the pore size of the mat Tomadakis et al. (Robert and Lochner, 1990) conducted a comparative survey of various established permeability models for non woven mats, and the conduction-based model was found to yield results closest to experimental values. This model was modified to take into account bulk diffiision tortuosity, and the mathematical formula for evaluating permeability constant k for randomly overlapping fiber structure was derived as given by Eqn (8.8). 

The permeability models for polymer matrices with sheet-like fillers are well researched over years and found to yield reliable predictions for the permeability... 

Simacek, R Advani, S. Permeability Model for a Woven Fabric, Polym. Compos. 1996,77(19,887-899. 

Simacek, P. Advani, S.G. Permeability model for a woven fabric. Polym. 

Chen, S., Jacobi, D., Kwak, H., Altunbay, M., Kloos, J., 2008. Pore-connectivity based permeability model for complex carbonate formations. In Presented at the SPWLA 49th Annual Logging Symposium, Edinburgh, Scotland, 25-28 May. Paper E. 

The DDT mechanism for Case D is identical to that proposed for Case C. However, the model for DDT in Case D will differ from that of Case C because of compaction of the porous bed If from the above discussion the variation of is traced from case to case, it is seen that a minimum in the 8-compaction plane is a natural consequence of the effects of charge permeability and the proposed driving force for DDT The above model for DDT is based on exptl observation with granular and cast HE. However, as already mentioned, DDT in SP and HE should have many similarities. This contention is supported by the observations of Roth (Ref 17) and Wachtel (Ref 29a). Roth s smear camera... 

When a two- or higher-phase system is used with two or more phases permeable to the solute of interest and when interactions between the phases is possible, it would be necessary to apply the principle of local mass equilibrium [427] in order to derive a single effective diffusion coefficient that will be used in a one-equation model for the transport. Extensive justification of the principle of local thermdl equilibrium has been presented by Whitaker [425,432]. If the transport is in series rather than in parallel, assuming local equilibrium with equilibrium partition coefficients equal to unity, the effective diffusion coefficient is... 

The liquid-liquid patition systems discussed above are in fact very similar to various membrane-type interfaces and may serve as a model for them.A good example is, for instance, the distribution of a dissociated salt between an aqueous solution and a permeable organic polmer. ... 

Fig. 3.4 Permeability profiles for (a) warfarin (acid), (b) propranolol (base) and (c) morphine (ampholyte) based on a BBB PAMPA model (plON) composed of animal brain extract of lipids. The data (unpublished) were analyzed with the pCEL-X program (plON), with the refined parameters indicated in the three frames. In all three cases, there was evidence for the permeation of charged...

Since experimental determination of intestinal absorption is quite demanding, Caco-2 cell monolayers have been successfully used to model passive drug absorption. Several models for the prediction of Caco-2 permeability using PSA were developed, including those of van de Waterbeemd et al. [5] and Palm et al. [22] who found that relationships between Caco-2 permeability and PSA is stronger than with Clog D, Krarup et al. [23] who used dynamic PSA calculated for water accessible molecular surface and Bergstrom et al. [24]. 

FIG. 14 A model for the uptake of weakly basic compounds into lipid bilayer membrane (inside acidic) in response to the pH difference. For compounds with appropriate pki values, a neutral outside pH results in a mixture of both the protonated form AH (membrane impermeable) and unprotonated form A (membrane permeable) of the compound. The unprotonated form diffuse across the membrane until the inside and outside concentrations are equal. Inside the membrane an acidic interior results in protonation of the neutral unprotonated form, thereby driving continued uptake of the compound. Depending on the quantity of the outside weak base and the buffering capacity of the inside compartment, essentially complete uptake can usually be accomplished. The ratio between inside and outside concentrations of the weakly basic compound at equilibrum should equal the residual pH gradient. 

The pore geometry described in the above section plays a dominant role in the fluid transport through the media. For example, Katz and Thompson [64] reported a strong correlation between permeability and the size of the pore throat determined from Hg intrusion experiments. This is often understood in terms of a capillary model for porous media in which the main contribution to the single phase flow is the smallest restriction in the pore network, i.e., the pore throat. On the other hand, understanding multiphase flow in porous media requires a more complete picture of the pore network, including pore body and pore throat. For example, in a capillary model, complete displacement of both phases can be achieved. However, in real porous media, one finds that displacement of one or both phases can be hindered, giving rise to the concept of residue saturation. In the production of crude oil, this often dictates the fraction of oil that will not flow. 

We formulate an identification problem to determine the permeability distribution from the measured superficial velocity distribution. In this section, we first develop the model for our experiment, and present the estimation method. 

Artursson P, Neuhoff S, Matsson P, Tavelin S (2006) Passive permeability and active transport models for the prediction of oral absorption. In Taylor JB, Triggle DJ (eds) Comprehensive medicinal chemistry II. Elsevier, Oxford, Sect 5.11... 

One of the key parameters for correlating molecular structure and chemical properties with bioavailability has been transcorneal flux or, alternatively, the corneal permeability coefficient. The epithelium has been modeled as a lipid barrier (possibly with a limited number of aqueous pores that, for this physical model, serve as the equivalent of the extracellular space in a more physiological description) and the stroma as an aqueous barrier (Fig. 11). The endothelium is very thin and porous compared with the epithelium [189] and often has been ignored in the analysis, although mathematically it can be included as part of the lipid barrier. Diffusion through bilayer membranes of various structures has been modeled for some time [202] and adapted to ophthalmic applications more recently [203,204]. For a series of molecules of similar size, it was shown that the permeability increases with octa-nol/water distribution (or partition) coefficient until a plateau is reached. Modeling of this type of data has led to the earlier statement that drugs need to be both... 

The rough brush stroke agreement between model and experiment is illustrated by the results shown in Fig. 14, for which the correspondences of theoretical with experimental permeability coefficients for the compounds listed in Table 2, (3-adrenegic blockers studied by Lee et al. [207,208] and Schoenwald and Huang [191], are plotted. The calculated values utilized the physical model with pores [205]. Characteristic of... 

Ghosh [548] used cellulose nitrate microporous filters (500 pm thick) as scaffold material to deposit octanol into the pores and then under controlled pressure conditions, displace some of the oil in the pores with water, creating a membrane with parallel oil and water pathways. This was thought to serve as a possible model for some of the properties of the outermost layer of skin, the stratum comeum. The relative proportions of the two types of channel could be controlled, and the properties of 5-10% water pore content were studied. Ibuprofen (lipophilic) and antipyr-ine (hydrophilic) were model drugs used. When the filter was filled entirely with water, the measured permeability of antipyrine was 69 (in 10 6 cm/s) when 90% of the pores were filled with octanol, the permeability decreased to 33 95% octanol content further decreased permeability to 23, and fully octanol-filled filters indicated 0.9 as the permeability. 

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