Diffusion Principles

WELL-DEFINED DIFFUSION TUBE Fig. 13-4. Static sampler based on the diffusion principle. 

Tecator Ltd. [16] have described a flow injection analysis method for the determination of 0.2 -1.4 mg/1 (as NH3N) of ammonia nitrogen in soil samples extractable by 2 M potassium chloride. The soil suspension in 2 M potassium chloride is centrifuged and filtered and introduced into the flow injection system for the analysis of ammonia (and nitrate) one parameter at a time. Ammonia is determined by the gas diffusion principle, in which a PTFE membrane is mounted in the gas diffusion cell. 

Measurement of the transepidermal water loss (TEWL) is the standard method to determine SC barrier status. A disturbed skin barrier is characterized by high TEWL.7 The measurement of the TEWL is based on the diffusion principle in an open chamber. The density gradient is measured... 

Accordingly, the transport of salt requires larger elementary free volume than does the transport of water molecules. Hydrated ions are much larger than water, and hydrated cation and hydrated anion must move together because of coulumbic attractive force between them. Consequently, salt ions cannot permeate an amphoteric hydrophobic/hydrophilic polymer, of which the hydration value is low, i.e., less than few volume percent, by the solution-diffusion principle. Therefore, salt permeation through a hydrophobic polymer film such as low-density polyethylene (LDPE) and parylene C film should not occur. 

Mathematical Models Diffusion principles have been traditionally recognized as the most important determinants in skin absorption. Thus, Pick s law of diffusion provided the mathematical basis for early kinetic descriptions of percutaneous absorption. Pick s law simply states that... 

The above mass transfer equations, although based on sound molecular diffusion principles, are limited in their applicability in a number of ways. A basic condition for their validity is the assumption of equimolar or dilute unimolar mass transfer. This limits the NTU and HTU approach to processes that are essentially either binary (distillation) or ternary (absorption or stripping) with only one component crossing the phase boundary. Another shortcoming of the transfer units technique is its exclusion of energy balances or temperature calculations. 

The heater is equipped with an ignition device such as a glow plug. The combustion itself takes place in accordance with the diffusion principle with excess of air (a = 2.2-3). The heat output of this tool can be regulated within the range of 50-200 thousand kg cal/hr. 

The permeability of gases and vapors in a flawless polymer matrix is well established by the solution-diffusion principle in which permeability, P, is given by the product of solubility, s, and the diffusivity, D, i.e., P = sD. 

Because of significantly lower levels of solubility, permeation through a plasma polymer is often not governed by a truly solution-diffusion principle. Many plasma polymers show characteristics in between solution-diffusion type polymers and molecular sieves. This difference in permeation mechanisms might be utilized favorably in applications of plasma polymers. 

The pore diffusion coefficients in tentacle-based matrices are typically higher as the transport mechanism is not based on free diffusion but most probably on a faster film diffusion principle (Thomas et al, 2012, de Neuville et al, 2012). 

According to the diffusion principle, controlled-release drug delivery systems can be designed as a reservoir system or a matrix system. Drugs released from both reservoir and matrix type devices follow the principle of diffusion, but they show two different release patterns as shown in Fig. 22.3. 

One way of overcoming this potential compliance testing burden with the European Food Regulations is to make use of a validated migration model. These models have been discussed elsewhere [6-12] and are based upon Fickian diffusion principles. Recently a commercial software package has become available based upon the Piringer model [12]. 

In Figures 14.1-14.4 experimental migration results obtained in this study are compared with predicted migration values calculated from the measured additive levels in the polymer, Cp,o- Two similar empirical approaches were used to estimate the diffusion coefficient, D(cm /s), which in turn was used to calculate migration by Fickian diffusion principles. 

Fig. 21.3 Conceptual drawing of two filtering microdevices based on laminar flow and diffusion principles, (a) A junction where two liquids meet and flow downstream side by side, (b) H-filter. From (5) reproduced by permission of John Wiley and Sons...

FIG. 54 An illustration of the diffusion principle of the solute (impurity) into the solvent (molten tin). The concentrations of the impurities increase at the interface. (Courtesy of Panasonic.)... 

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