Mechanisms for permeation

Fig. 36. Mechanisms for permeation of gases through porous and dense gas-separation membranes.

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 basis of the flow models are ordinary differential mass balances for each component on either side of the membrane. The mechanism for permeation is substituted in the mass balances. When reaction occurs the kinetic expression is also added to the balances. The chemical reaction is assumed to take place in... 

McLeod L S, Degertekin F L and Fedorov A G (2009), Determination of the rate-limiting mechanism for permeation of hydrogen through microfabricated palladium-silver alloy membranes , J Membrane Sci, 341,225-232. 

In addition, any chemical aging (Section 23.5.3) could affect these mechanisms for instance, if an elastomer surface is degraded by contact with hostile chemicals, access to the interior may be facilitated and its permeation characteristics would change accordingly. 

The effect of pH and cation concentration on pectinesterase (PE) activation and permeation on 30 kD MWCO ultrafiltration (UF) membrane was evaluated. In order of increasing effectiveness, PE activity was stimulated by monovalent and divalent cations, poly amines and trivalent cations. A similar trend was observed for permeation on UF membranes. Cation addition and higher pH releases PE from an inactive complex, increases activity, and increases permeation. Higher cation concentration decreases activity and permeation. These results suggest a common mechanism is involved in PE activation and permeation. 

In addition, data obtained from infrared, thermal, and fluorescence spectroscopic studies of the outermost layer of skin, stratum corneum (SC), and its components imply enhancer-improved permeation of solutes through the SC is associated with alterations involving the hydrocarbon chains of the SC lipid components. Data obtained from electron microscopy and x-ray diffraction reveals that the disordering of the lamellar packing is also an important mechanism for increased permeation of drugs induced by penetration enhancers (for a recent review, see Ref. 206). 

Some of the polymeric membranes are suitable for bulk separation of hydrogen from impurities to enrich a dilute hydrogen stream. Dense polymers permeate gases by solution diffusion mechanism. The permeation rate of a gas species through a polymer membrane... 

All of the above considerations have sometimes led to a too rigid picture of the membrane structure. Of course, the mentioned types of fluctuations (protrusions, fluctuations in area per molecule, chain interdigitations) do exist and will turn out to be important. Without these, the membrane would lack any mechanism to, for example, adjust to the environmental conditions or to accommodate additives. Here we come to the central theme of this review. In order to come to predictive models for permeation in, and transport through bilayers, it is necessary to go beyond the surfactant parameter approach and the fluid mosaic model. 

The success of the Potts-Guy equation led many authors to advocate a single mechanism as the rate determining step for permeation through the skin barrier for all or at least a wide range of solutes diffusion was assumed to occur primarily via the interkeratinocyte lipids of the stratum corneum, a mixture of ceramides, fatty acids, and sterols. While from a macroscopic point of view these lipids may be modeled as a bulk solvent, on a microscopic scale they... 

The column packings used for SEC must be compatible with aqueous mobile phases and therefore must be hydrophilic in nature. The support surface must be inert to minimize interactions with protein analytes. The packing must be available in pore sizes suitable for permeation of a wide range of proteins, and the pores should be uniform in diameter. Because the separation only takes place within the pore system, the porosity of the packing should be as large as possible. The support material should be chemically compatible with SEC mobile phases and mechanically stable under high flow rates and pressures. 

Complementary use of PAMPA and Caco-2 cells for evaluation of absorption potential. PAMPA measurements are used to discard compounds with clear absorption problems whereas Caco-2 cells would be used to evaluate mechanisms of permeation or reasons for low permeation. It is highly unlikely that PAMPA measurements would be used to select compounds to be tested in vivo. 

The diffusion of a liquid or gas through a membrane is similar to the diffusion in liquid systems. The mechanism for each case involves a transfer of a small molecule to a hole in the liquid or membrane. For diffusion, the jumping frequency from hole to hole is dependent on the activation energy, k), for this jump, which is dependent on the size and shape of the diffusing molecule and the size of the holes in the membrane, i.e., the free volume. For permeation, the activation energy is on the order of 2(F40 kJ/mol. The values of Fp are somewhat higher. 

Physical or physico-chemical capability (Table 1), including mechanical strength, permeation, or sieving characteristics, is another important requirement of biomaterials. Cuprammonium rayon, for instance, maintains its dominant position as the most popular material for hemodialysis (artificial kidney). Thanks to its good mechanical strength, cuprarayon can be fabricated into much thinner membranes than synthetic polymer membranes as a consequence, much better clearance of low-molecular-weight solutes is achieved. 

The amount of small molecules in the permeate at low alkalinity increased with the relative concentration of small molecules (Figure 3). If hydrophobic bonding or simple solubility was the dominant mechanism for the association, the solubility of lignin molecules should be solely a function of pH regardless of the MWD of the parent solution. Consequently, the permeate MWD should be approximately the same and not a function of the MWD of the parent solution. Since the experimental permeate MWD varied with the parent MWD, the solubility or hydrophobic interaction mechanism was no longer accepted. 

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