USING MICELLES TO DETERMINE THE PORE-SIZE OF UF MEMBRANES

To estimate the pore size of ultrafiltration (UF) membranes, a method using micellar filtration is proposed. Lithium dodecylsulfate (LiDS) forms micellar aggregates of spherical shape even at high ionic strength. The surfactant rejection rate of micellar solutions shows a remarkable dependence upon the ionic strength (LiCl: 0 to 1 mol/l). From the size dependence of the aggregates on the ionic strength (micelle radius plus ionic atmosphere thickness) and the rejection rate of the micellized surfactant, we determine the mean pore diameter of a 40 kDa UF polysulfone membrane. Obtained results are in accordance with the hindered transport theory through capillaries: small LiDS micelles behave like neutral hard spheres in the presence of excess salt. Microscopic examination of the membrane surface shows a quasi-uniform distribution of pores. Pore surface density can be easily determined, but, due to gold aggregates at the pores entrance, the average mean diameter obtained by this method appears to be too large.