CROSS-FLOW ULTRAFILTRATION OF MICELLAR SURFACTANT SOLUTIONS

A steady-state fouling-resistance and osmotic-pressure model is used t o predict flux in the laminar, cross-flow ultrafiltration of micellar cetyl (=hexadecyl)pyridinium chloride (CPC) solutions at 0.01-M NaCl b ackground electrolyte. The model assumes a constant overall hydraulic resistance including the effect of surfactant fouling and native membr ane resistance. Measurements of osmotic pressures of CPC solutions at 0.01-M NaCl as a function of surfactant concentration describe the eff ect of concentration polarization on permeate flux. Two types of asymm etric polyethersulfone membranes are used: 5,000 molecular weight cuto ff (MWCO) membranes that allow partial monomer permeation, but quantit atively reject all micelles; 50,000 MWCO membranes that allow some mic elle permeation. For the former the intrinsic rejection coefficient fo r monomer, measured separately, is sufficient to describe surfactant r ejection, without adjustable parameters. Predictions of the volumetric flux of the permeate, including the value of the limiting flux, agree well with the experimental results over the entire range of pressure drop, axial velocity, and bulk surfactant concentration. For tile 50,0 00 MWCO membranes the data are described using a best-fit value of til e over all surfactant rejection coefficient. For the first time, unusu al behavior is observed experimentally in which the flux levels off wi th increasing pressure drop across the membrane, only to increase shar ply again at higher applied pressure drop. Both effects are in accord with the proposed model. No gel layer need be postulated to explain th e flux behavior of either membrane type.