THE EFFECT OF A TRANSMEMBRANE OSMOTIC FLUX ON THE ION CONCENTRATION DISTRIBUTION IN THE IMMEDIATE MEMBRANE VICINITY MEASURED BY MICROELECTRODES

The osmotically induced transmembrane water flow is accompanied by sol ute concentration changes within the unstirred layer adjacent to membr anes. Experimental concentration profiles, measured by means of microe lectrodes in the immediate vicinity of a planar lipid bilayer, are com pared with theoretical ones predicted from the standard physiological model in which the osmotic advection is countered by back-diffusion of the solute only. An increase of the apparent osmotic flow rate is ind uced by an increase of the osmotic gradient and by rigorous stirring. The polarization effect decreases in the latter case due to an increas e of the transfer rate of solutes between the bulk solutions and the m embrane surfaces, whereas it increases in the former case. The observa tions show that the concentration profile is not well described by the standard approximation. The discrepancy becomes increasingly large wi th increased volume flow. Based on a modified theoretical description of the interaction between water flux and diffusion, the hydraulic con ductivity of the bilayer is calculated from the measured uniexponentia l concentration profiles. The common approximation that there is a dis crete boundary between the stirred and unstirred regions adjacent to t he membrane is substituted by the model of a stagnant point flow that takes into account a gradual change of the stirring velocity in the im mediate membrane vicinity. Supported by experimental observations, thi s approach predicts a shortening of the unstirred layer if the transme mbrane osmotic gradient is increased under gentle stirring conditions.