Determining the zeta potential of porous membranes using electrolyte conductivity inside pores

The zeta potential is an important and reliable indicator of the surface ch arge of membranes, and knowledge of it is essential for the design and oper ation of membrane processes. The zeta potential cannot be measured directly , but must be deduced from experiments by means of a model. The possibility of determining the zeta potential of porous membranes from measurements of the electrolyte conductivity inside pores (lambda (pore)) is investigated in the case of a ceramic microfiltration membrane. To this end, experimenta l measurements of the electrical resistance in pores are performed with the membrane filled with KCI solutions of various pHs and concentrations, hp,, , is deduced from these experiments. The farther the pH is from the isoelec tric point and/or the lower the salt concentration is, the higher the ratio of the electrolyte conductivity inside pores to the bulk conductivity is, due to a more important contribution of the surface conduction. Zeta potent ials are calculated from hp,,, values by means of a space charge model and compared to those calculated from streaming potential measurements. It is f ound that the isoelectric points are very close and that zeta potential val ues for both methods are in quite good agreement. The differences observed in zeta potentials could be due to the fact that the space charge model doe s not consider the surface conductivity in the inner part of the double lay er. Measurements of the electrolyte conductivity within the membrane pores are proved to be a well-adapted procedure for the determination of the zeta potential in situations where the contribution of the surface conduction i s significant, i.e., for small and charged pores. (C) 2001 Academic Press.