Electrokinetic and electrochemical characterizations of porous membranes

Surface and bulk membrane have been electrically and chemically characterized by different kinds of measurements. Tangential streaming potential, impedance spectroscopy, membrane potential and salt diffusion were measured with a porous polysulfone membrane for ultrafiltration when it was in contact with NaCl solutions at different concentrations (5×10−5<c<5×10−2 M). From these experiments some membrane and transport characteristic parameters such as the ζ potential, the surface charge density, ionic transport numbers, concentration of fixed charge in the bulk membrane and salt and ionic permeabilities across the porous membrane were determined. From electrokinetic results, assuming for the membrane a Freundlich heterogeneous adsorption of Cl− ions, the following parameters were determined: membrane proper charge density (σo=−1.2×10−3 C m−2), density of accessible sites (N=6×1017 m−2) and mean adsorption free energy (ΔGads=−3.9 kJ mol−1); membrane potential results also indicate a weak cation exchange character for this ultrafiltration membrane: fixed charge concentration Xf=−(5.0±0.2)×10−3 M, cation transport number t+=(0.41±0.4). A reduction in salt permeability due to concentration–polarization was determined by measuring salt diffusion at different stirring rates. Chemical composition of the membrane surface was determined by X-ray photoelectron spectroscopy (XPS); the homogeneity of the membrane surface in the range between 2 and 9 nm was obtained from XPS data by using five different take-off angles (chemical depth profile).