Layers, unstirred near membranes

Influence of unstirred layers near the membrane. Near the membrane there exist unstirred layers which under unfavourable conditions can exert a considerable influence on the fluxes and the membrane potential too. F. Helfferich (57) has drawn the attention to this effect. The thickness of these layers depends on the rate of stirring. Under good stirring conditions the film-thickness amounts to 20 to 1 10-3 cm. Under extreme conditions it can be reduced to 10-4 cm. It is not always possible to eliminate its influence (139). The transport in the films is diffusion-controlled. In some cases the effect of the films can be involved in the calculations. As an example the case of selfdiffusion is given here. A cation-exchange resin separates two solutions of identical chemical composition. The cations on either side are isotopes of the same element. 

If an electrical current is passed across a cation-exchange membrane, salt depletion takes place in the surface layer facing the anode. Supply of salt occurs by diffusion across the unstirred film. Maximum diffusion flow occurs if the salt concentration near the membrane equals zero. 

Figure 1-11. Hie effect of unstirred layers (see dashed lines) on the concentration of species j near a membrane across which the solute is diffusing.

FIG. 25 The reduction of acetaldehyde catalyzed by alcohol dehydrogenase (ADH) in the presence of NADH proceeds at the trans side. The enzyme/coenzyme and substrate are added to opposite sides of the membrane. After the addition of acetate the membrane is highly permeable not only to acetaldehyde and alcohol, but to protons as well. pH profiles within the unstirred layer near the planar bilayer lipid membrane are measured with the help of a microelectrode driven by a hydraulic microdrive. (Schematic redrawn from Ref. 33 by the chapter authors. Copyright Academic Press, 1996.)... 

SM Dzekunov, YN Antonenko. Dynamics of formation and dissipation of local pH gradients in the unstirred layers near bilayer lipid membranes. Bio-electrochem Bioenerg 41 187-190, 1996. 

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