Membrane potentials across nanofiltration membranes: effect of nanoscaled cavity structure

The charge characteristics of two kinds of commercial nanofiltration (NF) membranes, DK and MPF-34, were investigated. In the present study, the membrane potentials across NF membranes were measured in various electrolyte solutions-HCl, KCl, NaCl, and MgSO4. The experimental results were analyzed based on the Donnan equilibrium and the Nernst-Planck equation of ion flux, and theoretical predictions agreed well with the experimental results. The effective charge densities and cation-to-anion mobility ratios in the membranes were determined by a nonlinear regression method. Our analysis showed that (i) NF membranes had a very small effective charge density and (ii) the contribution of diffusion potential was substantial for NF membranes, especially for DK membrane. The latter indicates that diffusion potential is dominant factor for the membrane potential phenomena across membranes with solvent permeability. The potentiometric titration measurements, on the other hand, also suggested that the fixed-charge density of NF membranes will be very low. Finally, it is revealed that the nanoscaled cavity structure of NF membranes has an important role in the generation of diffusion potential.