Permeation depth

For small-molecule thermotropic smectic-A phases, typical values of two elastic constants are K 10 dyn and B 10 dyn/cm (Ostwald and Allain 1985). For lyotropic smectics, such as those made from surfactants in oil or water solvents, the layer compression modulus B can be much lower (see Chapter 12). From B and K, a length scale A. = ( 1 /B) 1 nm is defined it is called the permeation depth and its magnitude... 

Eor t7-limonene diffusion in a 50-pm thick vinyUdene chloride copolymer film, steady-state permeation is expected after 2000 days. Eor a 50- pm thick LDPE film, steady-state permeation is expected in less than one hour. If steady-state permeation is not achieved, the effective penetration depth E for simple diffusion, after time /has elapsed, can be estimated with equation 8. 

Some kinds of chromatography require relatively little optimization. In gel permeation chromatography, for example, once the pore size of the support and number of columns is selected, it is only rarely necessary to examine in depth factors such as solvent composition, temperature, and flow rate. Optimization of affinity chromatography is similarly straightforward. In RPLC or IEC, however, retention is a complex and sensitive function of mobile phase composition column type, efficiency, and length flow rate gradient rate and temperature. 

When the oxide is formed by anodizing in acid solutions and the sample is then left to rest at the OCP, some dissolution can occur. This process has been studied by a numbers of authors,70-75 especially in relation to porous oxides [cf. Section 111(4)]. It was found that pore walls are attacked, so that they are widened and tapered to a trumpet-like shape.70 71 Finally, the pore skeleton collapses and dissolves, at the outer oxide region. The outer regions of the oxide body dissolve at higher rates than the inner ones.9,19 The same is true for dissolution of other anodic oxides of valve metals.76 This thickness dependence is interpreted in terms of a depth-dependent vacancy concentration in the oxide75 or by acid permeation through cell walls by intercrystalline diffusion, disaggregating the microcrystallites of y-alumina.4... 

If the pore-mechanism applies, the rate of permeation should be related to the probability at which pores of sufficient size and depth appear in the bilayer. The correlation is given by the semi-empirical model of Hamilton and Kaler [150], which predicts a much stronger dependence on the thickness d of the membrane than the solubility-diffusion model (proportional to exp(-d) instead of the 1 Id dependence given in equation (14)). This has been confirmed for potassium by experiments with bilayers composed of lipids with different hydrocarbon chain lengths [148], The sensitivity to the solute size, however, is in the model of Hamilton and Kaler much less pronounced than in the solubility-diffusion model. 

Most models of the permeation of drugs through skin consider only steady-state conditions the drug amount in the donor is infinite and the concentration of accumulated drug in the acceptor is comparatively small and therefore negligible. Under these conditions the concentration-depth profile for a homogeneous membrane at time t is given by... 

Figure 19.4. The spiral wound membrane module for reverse osmosis, (a) Cutaway view of a spiral wound membrane permeator, consisting of two membranes sealed at the edges and enclosing a porous structure that serves as a passage for the permeate flow, and with mesh spacers outside each membrane for passage of feed solution, then wound into a spiral. A spiral 4 in. dia by 3 ft long has about 60 sqft of membrane surface, (b) Detail, showing particularly the sealing of the permeate flow channel, (c) Thickness of membranes and depths of channels for flows of permeate and feed solutions.

What is the size of the overlapping volume The complete independence of the constant in Equation 1 from the degree of polymerization shows that the overlapping volume always consists of the same portion of the volume of the polymer coil (12). This can be easily understood by assuming that two polymer coils are able to migrate nearly unhindered through each other. Then the mean depth of permeation and, therefore, the time of overlapping is determined only by the statistics of the free Brownian motion. Equation 1 is based on this assumption. 

Confocal Raman images were acquired from 50 pm (axial) x20 pm (lateral) areas with step sizes of 2 pm in the axial direction and 4 pm in the lateral direction. The map of the integrated area ratio 485cm /1004 cm 1 is plotted in Fig. 15.8. Most of the exogenous a-TAc permeated 15 pm into skin with a more-or-less monotonic diminishing gradient at depths between 15 and 30 pm. Small pockets of material are observed at depths of 30 35 pm. 

Adhesives are usually designed to bond dry surfaces as wood and metal, but very specially designed adhesives are formulated for bonding moist tissue surfaces. The human skin is moist and contains sweat glands that allow moisture to permeate the full thickness of the skin. If injured, the subcutaneous tissue hemorrhages because blood vessels are severed. Vessel diameter increases with tissue depth as can be observed in the Fig. 2.1, and any tissue adhesive or protective barrier must be capable of bonding to a moist surface and possibly applied over a bloody field. 

The second category of microporous membranes is the depth filter (b), which captures the particles to be removed in the interior of the membrane. The average pore diameter of a depth filter is often 10 times the diameter of the smallest particle able to permeate the membrane. Some particles are captured at small constrictions within the membrane, others by adsorption as they permeate the membrane by a tortuous path. Depth filters are usually isotropic, with a similar pore structure throughout the membrane. Most microfiltration membranes are depth filters. 

The photocurrent responses of nanocrystalline electrodes to stepped or pulsed illumination exhibit features on rather slow timescales. This is illustrated, for example, by Fig. 8.26, which is a set of photocurrent transients reported by Solbrand et al. [78] for band-band excitation at 308 nm of nanocrystalline Ti02 films of differing thicknesses permeated by 0.7 mol dm-3 LiC104 in ethanol. The 30 ns excimer laser pulse was incident from the solution side, and since the penetration depth of the light was much smaller than the film thickness, electron-hole pairs were effectively... 

The rational design of a reaction system to produce a polymer with desired molecular parameters is more feasible today by virtue of mathematical tools which permit prediction of product distribution. New analytical tools such as gel permeation chromatography are being used to check theoretical predictions and to help define molecular parameters as they affect product properties. There is a laudable trend away from arbitrary rate constants, but systems other than styrene need to be treated in depth. A critical review of available rate constants would be useful. Theory might be applied more broadly if it were more generally recognized that molecular weight distributions as well as rates can be calculated from combinations of constants based on the pseudo-steady-st te assumption. These are more easily determined than the individual constants in chain reactions. 

Therefore the knowledge of the lipophilicity expressed as logPow will still be of high interest in drug research. Pampa will not substitute CaCo2 or other cell types for the study of permeation but it is able to deliver predictive data about transcellular permeation of a high number of analytes. More time can then be used for the cell-based assays to investigate in depth the mechanism of permeation or efflux of compounds of interest. 

EDS analysis of yam cross sections could provide additional information, and document, for example, whether sulfur from the air permeates the fibrous core as well. Such is the case in the fume fading of textiles enclosed in environments with sulfur and nitrogen oxides. Elemental mapping of the fiber cross sections would show the depth of penetration of the absorbed or adsorbed species, and perhaps indicate the length of exposure, as well as of exposure to gaseous rather than aqueous environments. 

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