Fig. 2. The reflection coefficient, , according to the Fritz-Marine Membrane Model (FMMM); top as a function of the porosity at an average concentration of 0.1 mol L^-1, and bottom as a function of the average concentration at a given porosity of 0.5. Curves were calculated using Eq. [4] for monomineral membranes made of kaolinite, illite or montmorillonite with values for the cation-exchange capacity (CEC) of respectively 15, 25, and 100 cmol_c kg^-1. For the frictional coefficient, R_wm, an initial value of 0.1 was taken which decreased linearly in the porosity range between 0 and 0.4. Above a porosity of 0.4 the value of R_wm was taken to be zero. Values of the other frictional coefficients are given in the text.