Effect of pH on electrokinetic and electrochemical parameters of both sub-layers of composite polyamide/polysulfone membranes

Electrokinetic and electrochemical characterisations of a composite polyamide/polysulfone membrane (PAC-1) were carried out by measuring the streaming potential along the membrane surface (polyamide active layer) and the membrane potential at different pHs (3<pH<9). These results allow the determination of some characteristic parameters such as the active layer charge density and the ionic transport number. Porous sublayer characterisation was obtained from measurements made with an ultrafiltration polysulfone membrane (PPS) similar to that used as support of the composite one. Measurements of tangential streaming potential at different pHs allow the determination of some PAC-1 and PPS surface material parameters (polyamide and polysulfone, respectively), such as the isoelectric point (ie.p.), or the pK(a) and the number of acidic sites on the membrane surface (N-a), if a dissociation site model is assumed. The following values are been obtained: polyamide surface, N-a = (1.4+/-0.2)x10(16) m(-2), pK(a) = (5.7+/-0.3) and ie.p. = (4.3+/-0.2); polysulfone surface, N-a = (1.4+/-0.1)x10(16) m(-2), pK(a) = (4.2+/-0.1) and ie.p. = (3.1+/-0.1). From both types of measurements, a change in the electrical character of the polyamide active layer was obtained, from a negative behaviour at acidic pH (7.6) to slightly positive when pH<4, while the porous sublayer behaves as a very weak electronegative material for the whole range of pH studied.