A Comparison of Electrochemical and Electrokinetic Parameters Determined for Cellophane Membranes in Contact with NaCl and NaNO3 Solutions

Abstract

Electrochemical and electrokinetic characterizations of cellophane membrane samples have been carried out by measuring membrane potential, salt diffusion, and tangential streaming potential, which allow the determination of different characteristic membrane parameters. Experiments were made with the membrane samples in contact with NaCl and NaNO3 solutions at different concentrations and under different external conditions (concentration gradients), in order to obtain differences in transport and membrane characteristic parameters, depending on the electrolyte considered. Salt permeability across the membrane, which was obtained from diffusion measurements, is about twice as high for NaCl solutions as for NaNO3 solutions, which is attributed to the different sizes of the electrolytes. Membrane potential measurements keeping the concentration ratio constant (C1/C2=2) were used to determine both the effective fixed charge concentration in the membrane, Xf, and the average value of transport numbers, 〈ti〉; taking into account these values, concentration dependence of membrane potential under a different external condition (C1=cte=0.01 M, 5×10−3≤C(M)≤5×10−2) was predicted. Results show that cellophane membrane behaves as a weak cation-exchange membrane and its permselectivity to cations is practically independent of the electrolyte considered. From electrokinetic results, assuming a Langmuir-type adsorption of anions on the cellophane surface, the number of accessible sites per surface unit was obtained, which is higher for Cl− than for NO3−, in agreement with the small radii of chlorine ions; however, no significant differences in the specific adsorption free energy were found (ΔGNacl=−22.0×103 J/mol) and (ΔGNaNO3=−23.2×103 J/mol).