Ionic behavior across charged membranes in methanol-water solutions. 2. Ionic mobility

The permeability coefficients across cation-and anion-exchange membranes were measured for LiCl methanol-water solutions. The experimental data were analyzed on the basis of the Donnan equilibrium and the Nernst-Planck equation of ion flux considering the effect of the mean activity coefficient of an electrolyte in an external solution, and the theoretical predictions agreed well with the experimental data. The mobilities of ions in the membrane were determined by using the results of the effective membrane charge densities and the cation-to-anion mobility ratios obtained from the previous paper and a nonlinear regression method. The results showed that the mobility of ions in the membrane would decrease with an increase in the weight fraction of methanol. It is postulated that the counterions prefer to make ion pairs with the fixed charge groups in the mixed solvent system since the dielectric constant of methanol is smaller than water. Thus the ion pairs between the counterions and fixed charge groups will increase with the increase in methanol concentration, and the mobilities of the counterions will decrease.