Diffusion-controlled system

The main conclusion to be drawn from these studies is that for most practical purposes the linear rate model provides an adequate approximation and the use of the more cumbersome and computationally time consuming diffusing models is generally not necessary. The Glueckauf approximation provides the required estimate of the effective mass transfer coefficient for a diffusion controlled system. More detailed analysis shows that when more than one mass transfer resistance is significant the overall rate coefficient may be estimated simply from the sum of the resistances (7) ... 

Rp data are meaningful for general or uniform corrosion but less so for localized corrosion, including MIC. In addition, the use of the Stem-Geary theory where the corrosion rate is inversely proportional to Rp at potentials close to is valid for conditions controlled by electron transfer, but not for the diffusion-controlled systems frequently found in MC. 

The facilitated transfers of Na+ and K+ into the NB phase were observed by the current-scan polarography at an electrolyte-dropping electrode [12]. In the case of ion transfers into the DCE phase, cyclic voltammetry was measured at an aqueous gel electrode [9]. Both measurements were carried out under two distinctive experimental conditions. One is a N15C5 diffusion-control system where the concentration of N15C5 in the organic phase is much smaller than that of a metal ion in the aqueous phase. The other is a metal ion diffusion-control system where, conversely, the concentration of metal ion is much smaller than that of N15C5. Typical polarograms measured in the both experimental systems are shown in Fig. 2. 

The facilitated ion transfers of some alkaline earth metals have been also studied in the DCE systems by the cyclic voltammetry. These systems perhaps have not been studied by any solvent extraction methods yet. Typical voltammograms in the N15C5 diffusion-control systems are shown in Fig. 6. The aqueous supporting electrolyte was MgCl2 instead of MgS04 in these measurements because BaS04 precipitated. 

In vitro release of methotrexate from the LDI based films is shown in Figure 7. The release was fast since the drug was imbedded in a thin film, in the order of 50 to 100 microns. The release profile was typical of one seen in diffusion controlled systems. The scattering of the release data was again reminiscent of the mass loss results, and the cause of that is not certain at this point... 

Because of their relative ease of production and cost compared with other methods of sustained or controlled delivery, dissolution and diffusion-controlled systems have classically been of primary importance in oral delivery of medication. Dissolution systems have been some of the oldest and most successful oral systems in early attempts to market sustaining products. 

Matrix diffusion-controlled system in which the drug is homogeneously dispersed in a hydrophilic polymer diffusion from the matrix controls release rate Nitrodur (nitroglycerin key). 

The first cathodic wave was studied by cycling the potential across it at various scan rates and the peak potentials were found to increase as indicative of a reversible, diffusion-controlled system, with ° = — 1.43 V vs. SCE. However, at sweep rates 20mV/s the peak anodic current is much smaller than expected which was interpreted by the authors as indicating that the reduced species undergoes a subsequent chemical reaction, i.e. an EC process. 

The shrinking core models described by Levenspiel cater for both reaction- and diffusion-controlled systems. Referring to the literature, how do these systems differ and which of these models do skeletal catalysts fit during their preparation by leaching ... 

Finally, the agitation rate does not affect the uptake rate if the particle diffusion controls the process. However, the latter criterion may be not safe the agitation in solution may have attained its limiting hydrodynamic efficiency, so that a change in the agitation rate has no effect on the uptake rate even in film diffusion-controlled systems. 

As with oral diffusion-controlled systems, there are two basic designs for transdermal diffusion-controlled systems matrix-type and reservoir-type systems. The matrix-type systems can be further classified as... 

Osmotic effects are often a problem in diffusion-controlled systems because imbibition of water swells the device or dilutes the drug. However, several devices... 

Korsmeyer, R.W. (1990) Diffusion controlled systems hydrogels in Polymers for Controlled Drug Delivery (ed. P.J. Tarcha), CRC Press, Boca Raton, p. 15. 

---