IONIC BEHAVIOR ACROSS CHARGED MEMBRANES IN METHANOL-WATER SOLUTIONS -2 - IONIC MOBILITY

The permeability coefficients across cation-and anion-exchange membran es were measured for LiCl methanol-water solutions. The experimental d ata were analyzed on the basis of the Donnan equilibrium and the Nerns t-Planck equation of ion flux considering the effect of the mean activ ity coefficient of an electrolyte in an external solution, and the the oretical predictions agreed well with the experimental data. The mobil ities of ions in the membrane were determined by using the results of the effective membrane charge densities and the cation-to-anion mobili ty ratios obtained from the previous paper and a nonlinear regression method. The results showed that the mobility of ions in the membrane w ould decrease with an increase in the weight fraction of methanol. It is postulated that the counterions prefer to make ion pairs with the f ixed charge groups in the mixed solvent system since the dielectric co nstant of methanol is smaller than water. Thus the ion pairs between t he counterions and fixed charge groups will increase with the increase in methanol concentration, and the mobilities of the counterions will decrease.