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Diffusive transport of a substance through a membrane separating solutions of different concentrations leads to the formation of concentration boundary layers (CBLs) adjacent to its surfaces. This phenomenon, commonly referred as concentration polarisation, causes the permeability coefficient measured in a membrane system to be smaller than the permeability of the membrane. The diffusion processes in the membrane system consisting of two cells separated by a horizontally located membrane were studied by using aqueous binary solutions of ethanol. To observe the time evolution of the CBLs, the laser interferometric technique was used: the interference fringe pattern provided quantitative measurement of the substance concentration C(x,t) at position x and time t. The concentration profiles were constructed by recording the interferograms with a given time step Δt; and on this basis, the CBL thicknesses were obtained. On the basis of these investigations, a practical and valuable method is proposed which can determine the membrane permeability coefficient under concentration polarisation conditions without the need for stirring solutions.

We have demonstrated that the interferometric technique combined with digital image analysis is a good and useful method for visualisation and investigation of the concentration polarisation process. The interferometric system is a new instrument for testing precise measurements of the membrane permeability coefficient under con- centration polarisation conditions. Using this system, a concrete investigation was performed for four membranes with different structures. Solute permeability through the membranes was determined by measuring the solute diffusion for different ethanol concentrations. The membrane permeability coefficient values obtained for different membranes and different initial concentrations of solute indicate that the membrane permeability coefficient seems to be independent of initial concentration of solute.