Diffusion layer Nernst-Brunner

Although it is possible to control the dissolution rate of a drug by controlling its particle size and solubility, the pharmaceutical manufacturer has very little, if any, control over the D/h term in the Nernst-Brunner equation, Eq. (1). In deriving the equation it was assumed that h, the thickness of the stationary diffusion layer, was independent of particle size. In fact, this is not necessarily true. The diffusion layer probably increases as particle size increases. Furthermore, h decreases as the stirring rate increases. In vivo, as GI motility increases or decreases, h would be expected to decrease or increase. In deriving the Nernst-Brunner equation, it was also assumed that all the particles were... 

The dependence of the limiting current density on the rate of stirring was first established in 1904 by Nernst (N2) and Brunner (Blla). They interpreted this dependence using the stagnant layer concept first proposed by Noyes and Whitney. The thickness of this layer ( Nernst diffusion layer thickness ) was correlated simply with the speed of the stirring impeller or rotated electrode tip. 

Dissolution rate, or simply dissolution, refers to the rate at which a compound dissolves in a medium. The dissolution rate may be expressed by the Nernst-Brunner diffusion layer form of the Noyes-Whitney equation ... 

Figure 5.2 Model of stationary diffusion in a stirred electrolyte. The Nernst-Brunner diffusion layer forms on an electrode surface of thickness because the viscosity of the electrolyte creates a gradient of convection between the bulk electrolyte and the electrode surface.

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