Surface permeability

Vesicles are permeable to apolar non-ionic solutes, and, if ionic, can bind counterions at the inner and outer surfaces. Permeability to ions depends critically upon vesicle structure. 

From Equation (6.6), it is immediately clear, that by decreasing the pore size, only the surface dilTusion contribution is increased [third term on the right-hand side of Equation (6.6)], due to the increased surface area. It should be noted here that the additivity of surface permeability and gas phase permeability is still a matter of debate, since it is very difficult to determine the surface permeability alone. The experimentally measured surface permeability will always include a contribution due to gas phase permeability. 

Surface diffusion has been extensively studied in literature. An overview of experimental data is given in Table 6.1. Okazaki, Tamon and Toei (1981), for example, measured the transport of propane through Vycor glass with a pore radius of 3.5 nm at 303 K and variable pressure (see Table 6.1). The corrected gas phase permeability was 0.69 m -m/m -h-bar, while the surface permeability was 0.55 m -m/m -h-bar, and so almost as large as the gas phase permeability (Table 6.1). It is clear from Table 6.1, that the effects of surface diffusion, especially at moderate temperatures, can be pronounced. At higher temperatures, adsorption decreases and it can be expected that surface diffusion will become less pronounced. 

Oil reduction in deep-fat-fried products may be obtained through prefrying and/or postfrying treatments. Prefrying treatments are mainly based on the marked effect that the crust microstructure has in oil absorption, and mainly intend to reduce surface permeability. Postfrying treatments aim to remove surface oil before postcooling suction begins. 

For standards, see Table 1 ( Specific surface Permeability techniques ). 

Carman s Gas Permeability Method. A gas or a wetting liquid is made to flow through the porous material in a tube by applying vacuum or pressure. The pressure drop or flow rate is measured. For pigments, a modified procedure is used in which mainly nonlaminar flow takes place [1.16]. For standards, see Table 1.1 ( Specific surface Permeability techniques ). 

Figure 6 provides a comparison between measured spectra and theoretical spectra calculated under the assumption that the adsorption/desorption process is controlled by either intracrystalline diffusion (Fig. 6a) or external transport resistances such as surface barriers (Fig. 6b). For simplicity in the calculations, the crystallites have been assumed to be of nearly spherical shape with a concentration-independent transport diffusivity Dj or surface permeability a, respectively. Values of the intracrystalline mean lifetime are therefore given by...

Eq. (2)], (2) from the penetration through the crystallite surface [x harr R,J(3a), with a denoting the surface permeability with respect to the intracrystalline concentration, Eq. (12)], and (3) from long-range diffusion [Xp. iig, = / , /(15Du,ng range) Eq. (2)], where both the crystallites and pellets have been assumed to be of nearly spherical shape with radii R, and / , respectively. 

Nikaido, H. (2001) Preventing drug access to targets cell surface permeability barriers and active efflux in bacteria. Seminars in Cell ej Developmental Biology, 12, 215-223. 

If we abandon the very unlikely case of anisotropic diffusion with principal tensor axes which are not perpendicular to the crystal faces normal, molecular fluxes may quite generally be assumed to be directed perpendicular to the crystal surfaces. Hence, molecular uptake and release may be considered to proceed via one-dimensional diffusion quite generally, as long as the fluxes stemming from different crystal faces do not superimpose upon each other. This includes in particular the initial phases of uptake and release. We shall see that due to this reason, by measuring surface permeabilities, interference microscopy is in general able to quantify the intensity of surface resistances. 

The surface permeability a is defined as the factor of proportionahty between the particle flux through the surface, , and the difference Ceq - Csurf(f) between the intracrystalhne concentration estabhshed in equilibrium with the surrounding atmosphere and the actual concentration in the layer close to the crystal surface... 

Fig. 44 Surface permeability of the MOF-type crystal under study during methanol uptake as a function of the boundary concentration (mean value during the considered time step) via Eqs. 7 and 8, with the boundary concentration Csurf(t) taken from the margins of the measured concentrations. The polynomial fit to these data is given by the broken line. The full line shows the dependence of the permeability on concentration, which leads to the best fit of the recalculated concentration profiles to the experimental ones (Fig. 45)...

Fig. 45 Comparison of the transient concentration profiles during methanol uptake by the MOF-type crystal as recorded by interference microscopy (symbols) with the corresponding profiles recalculated from the measured diffusivities with surface permeabilities (full line in Fig. 44) which lead to the best fit to the experimental points...

Fig. 48 Comparison of simulated and experimental profiles for pressure steps 0 to 5 mbar (a), 5 to 0 mbar (b), 0 to 10 mbar (c), 10 to 0 mbar (d), 0 to 40 mbar (e), 40 to 0 mbar (f), 0 to 80 mbar (g), and 80 to 0 mbar (h). Tbe points refer to experimental measurements. Tbe lines are simulated from tbe 2-D finite difference solution with the same concentration dependence of transport diffusivities as determined from Fig. 47 (full line) and the surface permeabilities determined from the use of Eqs. 7 and 8. For the simulations it is implied that Dz > Dy...

Fig. 52 Surface permeabilities determined from the calculated concentration profiles. The solid line (uy) and the dotted line (Uz) represent the analytical dependence of the surface permeabilities which yields excellent agreement with the measmed concentration integrals (standard deviation a = 0.006)...

These data are displayed in Figs. 51 and 52. The transport diffusivity in the z direction is approximately twice as large as that in the x or y direction. This correlates nicely with the difference in the critical sizes of the windows on the diffusion paths in the different directions. It is interesting to note that also the surface permeability in the z direction slightly (by about 20%) exceeds that in the X andy directions. 

In fact, this behavior can be shown to hold quite generally. With the simplifying assumption of constant diffusivities and surface permeabilities, the normahzed concentration profiles during molecular uptake by a plate of thickness 2l are given by the relation... 

Body Surfaces Permeability Solubility Parathlon DEF Others Protective Clothing... 

Permeation of gas, vapor, or liquid through a polymer film consists of three steps (1) a solution of permeating molecules in the polymer, (2) diffusion through the polymer due to concentration gradient, and (3) emergence of permeating molecules at the outer surface. Permeability is therefore the product of solubility and diffusion so where the solubility obeys Henry s law one may write... 

Of the numerous macroscopic parameters used to quantify porous media, those gaiiung widest acceptance in the hterature for describing the flow of single phase fluids are voidage, speciflc surface, permeability and tortuosity. Their values can often be inferred from experiments on the streamline flow of single phase Newtonian fluids. 

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