Sieving characteristics of multilayer ultrafiltration membranes

Almost all commercial ultrafiltration membranes have an asymmetric structure. However, the unusual transport characteristics of these asymmetric, or multilayer, structures has not been widely appreciated. The general equations describing solute transport through a multilayer membrane are developed and used to examine the physical basis for the unusual transport properties of these membranes. Experimental data are obtained for the transport of polydisperse dextrans through asymmetric and symmetric ultrafiltration membranes,both alone and in sandwich (series) combinations. The data clearly show that the sieving characteristics of these multilayer membranes are dependent upon both the detailed properties and the orientation of the different layers. The sieving coefficients for traditional asymmetric membranes are shown to increase by well over an order of magnitude when used in the reverse (skin-side down) direction. Experiments with appropriate sandwich arrangements demonstrate that these composite structures can have higher selectivities than conventional membranes. The implications of these transport phenomena are examined in several different ultrafiltration applications.