a porous layer is provided on one or both surfaces of a mass - transfer membrane to enhance or modify the passage of ions . the membrane may be a film of any of a variety of polymeric materials , and the porous layer may be a surface layer on the membrane or a separate porous structure placed in contact therewith . the porous layer or layers render certain normally ion - impermeable polymeric films capable of passing ions in an electrodialysis - type system .