Ultrafiltration of sodium dodecylsulfate solutions

The present paper is devoted to ultrafiltration (UF) of aqueous solutions containing micellized sodium dodecylsulfate (NaDS) through zircon membranes, following permeation data of cetyltrimethylammonium bromide (CTABr), which we have reported recently.The experimental results are analyzed using the same model of diffusion in the membrane matrix based on the thermodynamics of irreversible processes (TIP). A comparison with the classical model of resistances in series leads to a new analysis of the measured resistances in terms of membrane–permeant and permeant–permeant interactions. NaDS micelles being able to cross the membrane as permeants, in contrast with CTABr micelles, the comparison of the behaviors of anionic and cationic surfactants is worthwhile. The resistance of the membrane strongly depends on the composition of the feed. The TIP approach allows us to relate the membrane resistance to the concentrations of all the species present in the feed, namely free ions, spherical micelles and rod-like aggregates. It is shown that the resistance is highly influenced by free ions (dodecylsulfate (DS⁻) and sodium (Na⁺)), very slightly by negatively charged spherical micelles, and weakly by rod-like aggregates, also negatively charged.