Diffusion cells

It must be emphasized that equation (10.14) is based on the assumption of strictly isobaric diffusion. In practice, we may ask, how closely must the pressures on the two sides of the diffusion cell be balanced if undue error is not to be associated with the use of (10.14) The answer, of course, is that pressure gradients must be small enough that the term in grad p on the right hand side of equation (10.4) is negligible coranared with the term in grad Since the viscous term dominates the coeffi-... 

For testing in a diffusion cell, on the other hand, equations (10.43) and (10.44) show that... 

The main uses of palladium [13] are in the electronics and electrical industries, in circuitry and in dental alloys. It finds many catalytic applications in industry, as well as in diffusion cells for the synthesis of hydrogen, and in automobile catalysts. Jewellery and three way auto-catalysts are the principal uses of platinum, which fulfils a wide range of roles in the chemical industry. 

The ratio of the overall rate of reaction to that which would be achieved in the absence of a mass transfer resistance is referred to as the effectiveness factor rj. SCOTT and Dullion(29) describe an apparatus incorporating a diffusion cell in which the effective diffusivity De of a gas in a porous medium may be measured. This approach allows for the combined effects of molecular and Knudsen diffusion, and takes into account the effect of the complex structure of the porous solid, and the influence of tortuosity which affects the path length to be traversed by the molecules. 

Axelsson, A Westrin, B Loyd, D, Application of the diffusion cell for the measurement of diffusion in gels. Chemical Engineering Science 46, 913,1991. 

FIG. 6 Schematic of the rotating diffusion cell. The reaction is usually followed by sampling the bulk solution of the outer phase using a suitable analytical technique. 

Figure 3.13 Cell designs for the thenael conductivity detector. A, flow-through cell B, semi-diffusion cell C, diffusion cell. (Reproduced with permission from ref. 163. Copyright Preston Publications, Inc.)...

Similar analysis can be applied to side-by-side diffusion cell systems, where stirring is effected by bubbling an 02/C02 gas mixture. For a bubbling rate of 40 mL gas/min, each UWL was estimated to be 282 pm [515]. 

Hidalgo, I. J. Hillgren, K. M. Grass, G. M. Borchardt, R. T., A new side-by-side diffusion cell for studying transport across epithelial cell monolayers, in Vitro Cell Dev. Biol. 28A, 578-580 (1992). 

Several investigators have used the two-chamber diffusion cell configuration. This experimental method has been found useful to determine diffusion coefficients [11] and to study drug transport from drug delivery devices [12],... 

Rigorous calibration is a requirement for the use of the side-by-side membrane diffusion cell for its intended purpose. The diffusion layer thickness, h, is dependent on hydrodynamic conditions, the system geometry, the spatial configuration of the stirrer apparatus relative to the plane of diffusion, the viscosity of the medium, and temperature. Failure to understand the effects of these factors on the mass transport rate confounds the interpretation of the data resulting from the mass transport experiments. 

The side-by-side diffusion cell has also been calibrated for drug delivery mass transport studies using polymeric membranes [12], The mass transport coefficient, D/h, was evaluated with diffusion data for benzoic acid in aqueous solutions of polyethylene glycol 400 at 37°C. By varying the polyethylene glycol 400 content incrementally from 0 to 40%, the kinematic viscosity of the diffusion medium, saturation solubility for benzoic acid, and diffusivity of benzoic acid could be varied. The resulting mass transport coefficients, D/h, were correlated with the Sherwood number (Sh), Reynolds number (Re), and Schmidt number (Sc) according to the relationships... 

Srinivasan et al. [17] have described a four-electrode potentiostat system which is suited to maintaining a constant voltage drop across a membrane in a two-chamber diffusion cell. This system was evaluated in connection with trans-dermal iontophoretic drug delivery of polypeptides. 

Various diffusion cell configurations have been used to investigate drug transport from ointments, creams, and gels through a membrane to an aqueous receptor phase. 

Side-by-side diffusion cell (membrane method) Diffusion coefficient determination mass transport studies 10-14... 

Diffusion cell for heterogeneous systems (membrane method) Mass transport studies from emulsions 18... 

Diffusion cell for gels (free boundary method) Mass transport studies from water-miscible gels 19... 

G Flynn, E Smith. Membrane diffusion I Design and testing of a new multifeatured diffusion cell. J Pharm Sci 11 1713, 1971. 

The sole purpose of the filter support and any applied extracellular matrix is simply to provide a surface for cell attachment and thus to provide mechanical support to the monolayer. However, the filter and matrix also can act as serial barriers to solute movement after diffusion through the cell monolayer. The important variables are the chemical composition of the filter, porosity, pore size, and overall thickness. In some cases, pore tortuosity also can be important. It is desired that the filter, with or without an added matrix, provide a favorable surface to which the cells can attach. However, in some cases these properties can also result in an attractive surface for nonspecific adsorption of the transported solute. In these instances, the appearance of the solute in the receiver compartment of the diffusion cell will not be a true reflection of its movement across the mono-layer. Such problems must be examined on a case-by-case basis. 

For the gas-lift, side-by-side diffusion cell system (Hidalgo et al., 1991) in which the effects of stirring on the ABLs in the donor and receiver chambers are equivalent,... 

It is assumed that the convective flow of water across the ABL, cell mono-layer, and filter owing to pressure gradients is negligible and that the cell mono-layer is uniformly confluent. When these conditions are not met, Katz and Schaeffer (1991) and Schaeffer et al. (1992) point out that mass transfer resistances of the ABL and filter [as described in Eq. (21)] cannot be used simply without exaggerating the permeability of the cell monolayer, particularly the paracellular route. An additional diffusion cell design was described by Imanidis et al. (1996). 

Imanidis G, C Waldner, C Mettler, H Leuenberger. (1996). An improved diffusion cell design for determining drug transport parameters across cultured cell monolayers. J Pharm Sci 85 1196-1203. 

Figure 14 The reversibilities of insulin permeation through polymer membranes in a two-compartment diffusion cell AH20 ( ), AS 15 (A), AS20 ( ), H ( ). Numbers indicate the content of styrene or HEMA in feed compositions in moles. H represents a cross-linked poly(2-hydroxyethyl methacrylate) (HEMA). (From Ref. 34.)... 

Flow in stirred vessels was also investigated by Holmes et al. (H5), who simulated mass transfer in a diaphragm diffusion cell stirred by magnetic stirrer bars. This is a good example of a simple model study with a direct practical purpose. A minimum stirring speed in such cells is necessary to avoid appreciable errors in the cell constant. The experiment permits this stirring speed to be related to the solution properties. 

CA in which many filled cells execute a random walk but never interact with one another, cannot give rise to stable pattern formation since the cells will move at random forever. However, if cells can interact when they meet, so that one diffusing cell is allowed to stick to another, stable structures can be created. These structures illustrate the modeling of diffusion-limited aggregation (DLA), which is of interest in studies of crystal formation, precipitation, and the electrochemical formation of solids. 

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