Evidence of nanopores in sol-gel based TiO2 and TiN ultrafiltration membranes

An essential criterion for applying TiO2 and TiN ultrafiltration (UF) membr anes in the cross-flow filtration is the pore size distribution in the acti ve filter layer. This layer is very thin (0.1-0.5 mu m) compared to the ent ire cross-section of the membrane. Therefore the pores inside the layer can not be determined by means of the classical method of N-2-adsorption, but o nly by high resolution techniques which are demanding (as the pore sizes am ount to only a few nanometers), and require intricate preparation methods. However, adsorption measurements are easy to perform in so-called unsupport ed layers which are manufactured by the crystallization of the gel in a gla ss dish. Using high resolution electron microscopy and image analysis metho ds we have directly evidenced (i) the pore and particle sizes and (ii) the very good agreement of the results of the adsorption measurements in unsupp orted layers with the real pore size distributions in the UF layer of the m ultilayer membranes. Converting the volume distribution of the spatial pore diameters from the N-2-adsorption measurements into a number distribution of the chord lengths allows the comparison of both methods. In addition, sc anning electron microscopy combined with energy dispersive X-ray microanaly sis revealed the thicknesses of the UF layers of the multilayer membranes t o be 130 nm for TiO2, and 550 nm for TiN. No sol particles had penetrated i nto the multilayer supports. (C) 2000 Published by Elsevier Science S.A. Al l rights reserved.