PERMEABILITIES OF UNSUPPORTED CERAMIC MEMBRANES OF ALUMINA

Most models for hydrodynamic permeability predict that the flow of an aqueous solution through a membrane that bears an electrostatic charge on the pore walls will be retarded when compared to the flow through uncharged pores under otherwise identical conditions. The prediction of flow retardation is based on the concept of a spontaneously generated streaming potential and subsequent electroosmotic backflow. To test the models, two types of unsupported membranes of alumina (Al2O3) were prepared which differed in composition, microstructure, and pore size (3.5- and 14.1-nm radii). The data from electrophoretic mobility titrations performed on ground membranes established that the isoelectric point was pH 9.5-10.0 and that the electrostatic charge was fully developed at pH less-than-or-equal-to 7.0. However, the hydrodynamic permeabilities measured at pH values of 4, 7, and 10 were not significantly different from each other for either type of membrane. Electroosmotic permeabilities were also much lower than predicted from the classic Smoluchowski equations. The results are consistent with the permeability models that correct the Smoluchowski equations for overlap of the electrical double layers which occurs in small pores (i.e., Kr less-than-or-equal-to 5).