The effects of electrostatic interactions on the rejection of colloids by membrane pores-visualisation and quantification

The pressure driven membrane processes of microfiltration and ultrafiltration are usually classified in terms of the size of solutes (colloids) separated. However, theoretical calculations have shown that electrostatic double-layer interactions can have a strong influence on rejection at the pores of such membranes. The present paper provides experimental evidence to support these findings. Firstly, atomic force microscopy in conjunction with the colloid probe technique is used to measure directly the repulsive electrostatic force experienced when a single colloidal particle approaches a microfiltration membrane. Scanning of such a membrane with the colloid probe provides a direct visualisation of the membrane surface as would be experienced by a colloidal particle during filtration. Secondly, filtration flux/time data is presented for the case of filtration of particles of size very close to the pore size in an ultrafiltration membrane. For such a case, theoretical calculations allow definition of a critical pressure at which the hydrodynamic force transporting the colloid toward the membrane pore is exactly balanced by the opposing electrostatic force. The experiments show that operating above this pressure results in a rapid loss in filtration flux, but operation below this pressure allows continuing filtration with only a minor decrease in flux, in agreement with the calculations.