Permeability, Permeation factors affecting

Barrier polymers, 3 375-405 applications, 3 405 barrier structures, 3 394-399 carbon dioxide transport, 3 403 flavor and aroma transport, 3 403-405 health and safety factors, 3 405 immiscible blends, 3 396-398 large molecule permeation, 3 388-390 layered structures, 3 394-396 miscible blends, 3 398-399 oxygen transport, 3 402 permanent gas permeation, 3 380-383 permeability prediction, 3 399-401 permeation process, 3 376-380 physical factors affecting permeability, 3 390-393... 

Since percutaneous permeation studies are frequently conducted using laboratory animal models such as rats, mice, and guinea pigs, it should be understood that wide dilferences exist between these models, including the thickness of the stratum corneum, the number of sweat glands and hair follicles, and the distribution of the papillary blood supply. These factors affect both the routes of transport and the resistance to penetration. In addition, the human skin differs from different animal species in biochemical composition and permeability. The subtle biochemical differences between human and animal skin may alter the reaction between permeant molecules and the skin [37]. 

Water vapor permeability (WVP) is a critical parameter in food packaging applications as contact widi water vapor may cause certain food items to lose texture. Polymer-water interaction, that is hydrophilicity or hydrophobicity of the polymer, is a crucial factor affecting the WVP. In general, water would permeate preferentially through... 

Another factor that affects membrane system design is the degree of separation required. The usual target of a gas separation system is to produce a residue stream essentially stripped of the permeable component and a small, highly concentrated permeate stream. These two requirements cannot be met simultaneously a tradeoff must be made between removal from the feed gas and enrichment in the permeate. The system attribute that characterizes this trade-off is called the stage-cut. The effect of stage-cut on system performance is illustrated in Figure 8.15. 

Three factors play key roles in determining the relative permeabilities of different polymers to a given penetrant molecule permeating by the solution-diffusion mechanism. The "free volume" available for molecule to traverse the polymer plays a major role, especially in the diffusivity. The cohesive forces between the polymer chains (i.e., how tightly the chains are held together) are also crucial. Finally, the solubility can be affected very significantly by the strength of the interactions between the penetrant molecule and the structural units in the polymer chains, i.e., by the "compatibility" of the polymer and the penetrant with each other. 

The rate and extent of intestinal permeation is dependent on the physicochemical properties of the compound (see Sections 16.1.2 and 16.4.3) and the physiological factors. Drugs are mainly absorbed in the small intestine due to its much larger surface area and less tight epithelium in comparison to the colon [17]. The permeation of the intestine may be affected by the presence of an aqueous boundary layer and mucus adjacent to cells, but for a majority of substances the epithelial barrier is the most important barrier to drug absorption. The lipoidal cell membrane restricts the permeability of hydrophilic and charged compounds, whereas large molecules are restricted by the ordered structure of the lipid bilayer. 

Eqn. 5 provides a very clear theoretical basis for the data of Fig. 1 (and similar data on other systems, as we shall see). The measured permeability coefficients for a set of solutes should parallel the measured partition coefficients, if the model solvent corresponds exactly in its solvent properties to the permeability barrier of the cell membrane. In addition, the molecular size of the solute is very likely to be an important factor as it will affect the diffusion coefficients within the membrane barrier phase. Data such as those of Fig. 1 will convince us that we have in our chosen solvent a good model for the solvent properties of the membrane s permeability barrier. We can now calculate values of PLx/K for the various solutes, and obtain estimated values of the intramembrane diffusion coefficient, and are in a position to study what variables influence this parameter. Fig. 3 is such a study in which data from Fig. 1 are plotted as the calculated values of f>n,c,n/A.t (calculated as P/K) against the molecular weight of the permeating solute. The log/log plot of the data has a slope of — 1.22, which means that one can express the dependence of diffusion coefficient on molecular weight (A/) in the form where... 

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