Equilibrium Distribution Model of Betaine between Surfactant Micelles and Water: Application to a Micellar-Enhanced Ultrafiltration Process

Abstract

The aim of this work was to determine the optimal surfactant concentration and operating conditions for betaine recovery from aqueous solutions by micellar-enhanced ultrafiltration (MEUF). Initial centrifugal ultrafiltration experiments with commercial 5 kDa cutoff polyethersulfone membranes showed that optimal betaine extraction was achieved with 200 mol/m3 of sodium dodecyl sulfate (SDS) as surfactant, at pH 1, with phosphoric acid for pH adjustment. The degree of betaine extraction increased with SDS concentration but remained constant with increasing betaine concentration for a given initial SDS concentration. The equilibrium distribution constants, KS, of betaine between SDS micelles and water were determined assuming a theoretical pseudophase separation model. Continuous MEUF with a 5 kDa cutoff TiO2 tubular ceramic membrane was performed in order to concentrate the micellar solution. Complete rejection of the micelles by the membrane was observed, resulting in 54% of the betaine and 99% of the SDS in micellar form, in agreement with the values estimated using the proposed theoretical model.