Permeation of molecules

SPAN module. It was mentioned at the beginning that the special polyacrylonitrile fibers of SPAN have a wall thickness of 30 gm, which is considerably thicker than the 8 gm wall thickness of the SMC modules [19]. As a consequence, the presence of stronger capillary effects from the special porous fiber material of the SPAN module would be a reasonable conclusion. Furthermore, the texture of the special polyacrylonitrile fibers is expected to have better surface properties, supporting the permeation of molecules as compared with synthetically modified cellulose. In conclusion, both convection and diffusion effectively contribute to the filtration efficiency in a SPAN module, whereas for the SMC membrane, diffusion is the driving force for molecular exchange, the efficiency of which is also considerable and benefits from the large surface-to-volume ratio. 

Finally, the skin itself affects the permeation of molecules. The barrier function depends on a variety of aspects such as the body region and the age of the patient. 

FIGURE 13.6 Proposed mechanism for permeation of molecules from ethosomal system through stratum corneum (SC) lipids. (Reproduced from Touitou, E. et al., J. Control. Release, 65, 403, 2000. With permission from Elsevier.)... 

The endothelium and basement membrane form a barrier that is difficult for the targeted drug Is cross (jee fSg, 5). IVko plasms membranes and the cell body must be passed before the complex reaches the subendrrtbelial space. Permeation of molecules across the rest of the vessel wall is largely influenced by the... 

All general porins contain pores with sizes allowing the permeation of molecules up to molecular masses of about 600 Da (Nikaido, 1994). The pores come with various selectivities. The porin from Rhodobacter capsulatus, for instance, contains a rather nonpolar binding site near the external end of the pore eyelet, indicating that it may pick up molecules... 

There are a number of useful in vitro measures of permeability that can be used to assess how well a given molecule will be absorbed. One of the most simple is the permeation of molecules through artificial (hexadecane) membranes (referred to as PAMPA studies). These measure transcellular permeation through bulk diffusion in systems that avoid the complexity of active transport. With these types of assays, compounds can be ranked on the basis of lipid permeation alone this can be a useful gauge of ability to penetrate the gut intestinal wall. 

Fa is the fraction of dose absorbed into enterocytes from the intestinal lumen after oral administration. The two major processes involved are (1) the dissolution of solid particles into gastrointestinal (GI) fluid and (2) the permeation of molecules across intestinal membranes. 

The principle of microscopic reversibility states that a given penetrant molecule is equally likely to make a diffusive jump towards either surface of a specimen at any given instant. A net diffusion and thus permeation of molecules from one surface to the other surface only occurs if... 

M. I. Kanehisa and T. Y. Tsong, Cluster Model of Lipid Phase Transitions with Application to Passive Permeation of Molecules and Structure Relaxations in Lipid Bilayers, /. Am. Chem. Soc. 100, 424 32 (1978). 

In conclusion, the results suggest that the size exclusion process is an equilibrium, entropy-controlled process and that the separation depends on the differential degree of permeation of molecules rather than the differential rate. This is equivalent to stating that the retention mechanism is a simple physical exclusion mechanism, and it is normally sufficient to consider the mechanism in these terms in order to understand what is happening in a size exclusion column. 

Terms other than permeability (Equation 12.6), such as permeant transmission rate, permeance, and thickness normalized flow, can be used to describe the steady-state permeation of molecules through the polymer films [2]. Permeant transmission rate is the amount of permeant passing through a plane of unit area normal to the direction of the flow during unit time (Equation 12.8). The term permeance is used when differences in partial pressure between both sides of the material are also taken into account (Equation 12.9), whereas thickness normalized flow considers material thickness but not difference in partial pressure (Equation 12.10). 

The permeation method is based on the slow penetration or permeation of molecules through a polymeric membrane. The permeation rate of the compound is determined by physical characteristics of the chemical and the membrane. Permeation is pressure and temperature dependent. Higher vapor-pressure compounds will have a higher permeation rate. Similarly, high temperature will create high internal vapor pressure that leads to higher permeation rate. 

General Principle Size-exclusion chromatography relies on the different rates of diffusion or permeation of molecules of different sizes through the pores of packing materials and not on the rates of adsorption and desorption. Size-exclusion chromatography functions as a molecular sieve. The distribution coefficient of molecules in different sizes of pores, K, is defined as... 

Figure 39.2 Schematic representation of H2- neutral H atoms (metals) and (b) mixed permeable membranes in the separation of H2/ ionic-electronic conductivity membranes N2 mixtures for two different separation mecha- (nonstoichiometric oxides). The chemical poten-nisms (a) permeation of molecules (PIMs) and tial gradient for H2 is illustrated qualitatively.

Danielli (15) shows that the permeation of molecules through a lipide layer requires a minimum kinetic energy for passage past energy barriers, of which those for transition between water and oil phases are prominent. 

To find experimentally the free volume distribution, a variant of gel chromatography was used that allowed estimation of the dimensions of molecules dissolved in eluent from the retention time tR in gel In [126,127], inverse gel-permeation chromatography was used and the dimensions of free space or vacancies in the network were determined from the retention time of test molecules in the gel. These molecvdes were of various sizes up to monomeric ones. In such a way, the part of space between the cross-links in the network was found, available for permeation of molecules (macromolecules) with known mean-square distance between the chain ends, . The larger the macromolecule dimension, the lower the total fraction of vacancies available for their penetration. Thus the distribution of vacancy size can be estimated. The main problem in the apphcation of this method consists in the transition from the distribution obtained for swollen gel (the necessary condition for gel chromatography) to distribution for pure network without solvent. 

The permeation of molecules through flawless polymers occurs by the steps of dissolution of a permeant in the polymer and diffusion of the dissolved permeant. The product of the diffusion coefficient D and the solubility coefiBcient 5 is referred to as the permeability coefficient. 

The different situations that can be encountered analysing the permeation of molecules through a porous membrane are reported below. When the pore diameter of a porous solid is in the macropore range, collisions between the molecules will occur much more frequently than collisions with the wall. In this case molecular diffusion is the dominant mechanism. As the size of the pores decreases (mesoporous solid), the number of collisions with the wall increases and can become more frequent and important than the molecule-molecule colUsions. At this point, Knudsen diffusion takes over. When the pore diameter becomes comparable to the size of the molecules (microporous solid), the molecules continuously collide with the walls. When this happens, diffusion behaves as an activated process and the term configurational regime is used to describe it. 

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