Micellar enhanced ultrafiltration (MEUF) of methylene blue with carboxylate surfactants

Alkylethoxy carboxylates CiEjCH2COOH of variable alkyl chain length and a different number of ethylene oxide units (RLM45 and RO90) are investigated as alternative surfactants to the classical anionic sodium dodecyl sulfate (SDS) in micellar enhanced ultrafiltration (MEUF) of dissolved methylene blue (MB) model dye. After adjustment of pH to 7, where the alkylethoxy carboxylates are in their anionic form and bind MB via electrostatic interactions, a removal efficiency of about 95% was obtained for a 1 g L-1 RLM45 solution applying a 10 kDa cellulose membrane. The performance of RLM45 is comparable to SDS, but it is not influenced by the precipitation reaction that is the major reason for the good performance of SDS below its CMC. To investigate the size effect of the micelles, the molecular weight cut-off (MWCO) of the membranes was varied. For the 5 kDa and 10 kDa membranes, the MB removal for SDS is almost quantitative and slightly higher as for the alkylethoxy carboxylates. With increasing MWCO, the MB removal efficiency for SDS strongly decreases. For the 30 kDa membrane, the removal efficiency for the carboxylate surfactants is already better than for SDS. For the 100 kDa membrane, the removal efficiency for SDS drops significantly to values below 10% because of the small size of the micelles. In contrast, the micelles of the alkylethoxy carboxylates are much larger allowing for MB removal in the order of > 80%.