Electrokinetic phenomena in composite organics microporous membranes with a low charge density

This work builds on the knowledge of electrical properties of the nanosurfaces being in contact with electrolytes solutions. Streaming potential (SP) and membrane potential (MP) are considered. The membrane electrical properties were studied with SP on a function of the MWCO and ionic strength. The membranes studied are composite and organics, the first with a mesoporous skin layer in polyethersulfon (PES) and the second with a microporous skin layer in polyamid (PA), both with a sublayer in polysulfon (PS). As the main pressure fall in the skin layer of these membranes, the SP measurements characterize this single layer in term of isoelectric point (pi) or surface charge density. The ionic strength effect on SP measurements show that the electrical attraction between solutes and membrane increase with the MWCO decrease. The isoelectric point of a PES membrane is determined equal to 3.1 independantly to the ionic strength and the kind of electrolyte solutions. In this case, the ions determining the charge are the ions of water (H+, OH-). For the microporous PA membrane, the pi is dependant on the ionic strength and on the nature of the electrolyte solutions. For exemple the pi in the presence of KCI is 4.4 at a concentration of 0.0001 mol.L-1 and 5.8 at 0.001 mol.L-1. For an electrolyte concentration fixed (0.0001 mol.L-1), the pi of the PA are 4.4, 5.6 and 6.8 for the respective electrolyte solutions of KCI, NaCl et CaCl2. Then we observed that the surface activity of such counter-ions is different for the PA material. The transport numbers of cations Na+, K+, Ca2+ determined by MP measurements through the PA membrane : 0.44, 0.49 and 0.42 can be correlated with pi measurements. On the other hand we have quantified the charge density of the mesoporous membrane by the Aizawa method. The values obtained are in the range of 0.1 - 1 mu C.cm(-2) (60-550 mol.m(3)).