Effects of monovalent ions on membrane potential and permselectivity: evaluation of fixed charge density of polymer based zirconium aluminophosphate composite membrane

The composite of poly(vinyl chloride) (PVC) with zirconium aluminophosphate (ZrAlP) employed as additive was prepared by sol-gel method. The physico-chemical properties were elucidated by FT-IR, XRD, TG-DTA and SEM. The influence of polymer ratios on stability of the membrane was investigated; with 25% polymer it shows greater stability. The porosity and water uptake properties were also observed. The membrane potential in different monovalent electrolyte solutions with varying concentrations (1-0.001 M) followed the trend as LiCl > NaCl > KCl > NaNO 3 > KNO 3 attributed to the different sizes of electrolytes. The membrane potential increased with the decrease in cation size. For anions, differences found are according to the size, mobility ratio and strong interaction with the membrane matrix. Small size and higher mobility of Cl − ion shows greater value of membrane potential whereas NO 3 − ion shows lower value because of its large size and strong interaction with the membrane matrix. The potential shows positive values with decrease in electrolyte concentrations confirming the membrane to be cation selective. Membrane potential also gave fixed charge density value of the membrane; evaluated and compared using various theoretical models: (a) Nagasawa, (b) Kobatake and (c) Teorell, Meyer and Sievers. Li + ion shows highest value of fixed charge density in all the methods as the Donnan exclusion is highest for the electrolyte of smaller cation size. The experimental results comparatively agreed with theoretical predictions. Extended form of TMS theory was used to calculate the distribution coefficient, charge effectiveness and transference number of ions. The prepared membrane also shows highest permselectivity for Li + counter-ion while lowest for K + , as potassium counter-ion shows strong interaction with the fixed charge groups on the polymer, thereby, decreasing the value. Co-ion (NO 3 − ) with higher hydrated radii and lower charge density also tended to result in low membrane permselectivity. Thus, the composite membrane can be efficiently used in various electro-membrane processes. The composite of poly(vinyl chloride) (PVC) with zirconium aluminophosphate (ZrAlP) employed as additive was prepared by sol-gel method.