Diffusion thickness

The parameters that determine the release rate of a drug from a delivery device include polymer solubility, polymer diffusivity, thickness of the polymer diffiisional path, and the aqueous solubility, partition coefficient, and aqueous diffusivity of the drug. Finally, the thickness of the hydrodynamic diffusion layer, the amount of drug loaded into the matrix, and the smface area of the device all affect the release rate. 

Self-diffusivity tracer diffusivity intrinsic diffusivity Thickness... 

Figure 2. Calculated point-contact spectra for n- and a-bands (upper and lower panels, respectively) of MgE>2 [12] for ballistic (dashed curve), diffusive (thick solid curve) and tunnel (dotted curve) regimes of current flow. Real and imaginary parts of A (e) are taken from Fig. (1).

Now, the peak current becomes proportional to the potential scan rate. It should be noted that Equations (1.7) and (1.8) are formally analogous to those obtained for species in solution diffusing in a restricted space, under the so-called thin-layer conditions (by contraposition to unrestricted space diffusion, thick-layer conditions). 

Encapsulation of enzymes in LMs offers further improvements for immobilization of complex enzyme systems, as the enzymes / cofactors, etc. are situated in aqueous droplets surrounded by a stable liquid hydrocarbon film (Figure 1). Instead of the physical pores present in microcapsules, the HC barrier, which has a diffusion thickness of about 0.1-1.0 p, effectively blocks all molecules except those which are oil-soluble or transportable by the selected carriers. Encapsulation of enzymes in LMs is accomplished simply by emulsifying aqueous enzyme solutions. Hence, LMs offer many advantages over other systems used for separation and eirzyme immobilization they are inexpensive and easy to prepare they promote rapid mass transport they are selective for various chemical species they can be disrupted (demulsified) for recovery of internal aqueous solutions gradients of pH and concentration (even of small molecules) can be maintained across the HC barrier multiple enzyme / cofactor systems can be coencapsulated and enzymatic reaction and separation can be combined. Some of the potential disadvantages of LMs for enzyme encapsulation have been discussed earlier. 

Because pech (or Df) is proportional to rfpEcn (diffusion thickness) and, in addition, because 6 pvac (diffusion thickness) is proportional to ct>pvAC (in interface) and because A>vac (or i>e) is proportional to (ipvAC (diffusion thickness) the reptation behaviour for Z>a and I>q is confirmed experimentally. This is evidence in support of Eq. (15). 

Roof slope Cavity width, cm Ventilation area Diffusion thickness of the layers at inside of cavity, m... 

In the Belgian design guides, four vapour retarding classes have been developed, depending on the resistance to water vapour diffusion of a vapour retarder (expressed as the diffusion thickness or equivalent air layer thickness pd). Table 4.2 gives the boundaries of the vapour retarding classes, and some examples of materials. 

Application Air leakage, mVm /h (75 Pa) Air permeance, mVmVs/Pa Diffusion thickness wind barrier, m... 

Since wind barriers are located at the cold side of the thermal insulation (in cold climates) they should combine a sufficient air and water tightness with a high vapour permeance. Materials which combine these properties and are often recommended as wind barriers are the spunbonded plastic films (also called house-wraps). These films are composed of rolled synthetic filaments (typically polypropylene or polyethylene fibres) that are welded together to form a continuous porous fabric. Measuring procedures and results of vapour transfer properties of spunbonded plastic films are reported in detail by Janssens and Hens [36]. The diffusion thicknesses of the films are a few centimetres and of the same order of magnitude as an air boundary layer. 

Zw = Rdo (jS)for semi-infinite diffusion contribution and Zw = Rdo (jS) coth (jS) is related to diffusion through a film of thickness d, formed on the electrode, where Rdo is the diffusion resistance and the parameter S= d2(o/D, where d and D are the diffusion thickness and diffusion coefficient related to the transport process (Fedrizzi et al., 1992). 

In a real sampling situation, however, different diffuse thickness values must be used for each individual sample set, and can only help linearize the concentration function if the degree of scattering remains constant. 

As a general rule, overall aluminum diffusion is slowed as the nickel, chromium, and cobalt contents increase. Thus, higher temperatures and longer processing times are required to produce greater aluminum diffusion thicknesses as the base material increases in alloy content. 

Dermasanding is used with medium-depth chemical peeling to remove thick, keratotic lesions not amenable to peel alone. An example is the patient with diffuse, thick seborrheic keratoses with photoaging skin. Such lesions cannot be removed by peel as the only treatment. A medium depth peel is performed first then, after the skin has frosted appropriately, the keratotic lesions are removed by dermasanding (Fig. 14.12). 

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