MICELLAR ULTRAFILTRATION IN AN UNSTIRRED BATCH CELL AT CONSTANT FLUX

An unsteady mass-transfer model is developed to describe the ultrafili tration of micellar surfactant solutions in an unstirred batch cell at constant flux. Excellent agreement between the model and new experime nts for asymmetric polyethersulfone membranes [5000 and 50,000 molecul ar weight cutoff (MWCO)] with aqueous hexadecyl(= cetyl)pyridinium chl oride (CPC) solutions in 0.01 M NaCl allows quantitative characterizat ion of the intrinsic membrane rejection properties for both surfactant monomer and micelles, and supports the physics at the membrane surfac e presumed in the model. The 5000 MWCO membrane rejects all of the mic elles (hydrodynamic radius approximately 2.5 nm) and most of the singl y dispersed surfactant molecules, or monomers (hydrodynamic radius app roximately 0.42 nm); the intrinsic rejection coefficient of this membr ane for micelles is 1.0 and for monomers 0.80. For the 50,000 MWCO mem brane, two pore types are necessary to explain the observations. The s mall-pore intrinsic rejection coefficients for monomers and micelles a re 0.75 and 1.0, respectively, while the large-pore rejection coeffici ents are 0.4 and 0.85, respectively. At least 88% of the permeate flow for the 50,000 MWCO membrane is through pores that completely reject micelles. More of the monomers and micelles are sieved by the membrane s than is expected from their advertised molecular weight cutoffs. Cal culations of intrinsic rejection using estimates of pore and solute si ze indicate qualitatively that repulsive electrostatic interactions an d surfactant adsorption significantly influence rejection.