Polysulfone-aerosil composite membranes Part 2. The influence of the addition of aerosil on the skin characteristics and membrane properties

Presented organo-mineral membranes are the result of the addition of minera l fillers (Aerosil 200) to a solution of polysulfone (PSf) in N-methylpyrro lidone (NMP). They are prepared using the wet phase inversion process by ca sting the suspension on a glass plate and immersing it in water. The additi on of aerosil to the casting solution, up to 2 vol.%, increases the resulti ng membrane permeability while maintaining their retentive properties. At a higher concentration of filler (3 vol.%), a decrease of the permeability a nd rejection properties as a function of operating pressure is observed. Th e effect of the filler concentration on the pore size distribution was exam ined using field emission scanning electron microscopy (FESEM) and the flow distribution through the Surface pores was calculated. Adding more filler to the solution changed the pore size distribution from a log-normal distri bution to a bimodal distribution. At a certain filler concentration, the sm all amount of large skin pores influences the flow distribution through the membrane in such a way that the retention properties of the membranes decr ease. The formation of the skin pores is explained by the presence of stres s due to the amount of filler. The apparent permeability of the membranes i s a result of the two opposite effects both initiated by the presence of th e filler: an increase of the filler concentration enhances the surface poro sity but slows down tl le membrane formation process that results in a thic ker skinlayer. (C) 2000 Elsevier Science B.V. All rights reserved.