The Effects of Surfactant Additives on Properties and Morphologies of Cellulose Acetate Membranes Prepared by Phase Inversion

Cellulose acetate is frequently used in reverse osmosis (RO) and ultrafiltration (UF) applications. In the present study, effects of different surfactant additives including Span-20 (sorbitanmonolaurate), Tween-40 (polyoxyethylenesorbitanmonopalmitate) and Tween-60 (polyoxyethylenesorbitanmonostearate) on the phase behavior and morphology of flat sheet cellulose acetate (CA) membranes prepared from CA/N,N-dimethylformamide (DMF)/water system via phase inversion technique were investigated. The phase separation behavior of the investigated systems was studied by determination of cloud point curves of each system in presence of different surfactant concentrations. Cross-sectional morphologies of the prepared membranes were studied by scanning electron microscopy (SEM). It was found that even addition of small amount of investigated surfactant additives to the casting solution could affect the phase separation behavior of polymeric solutions and increased sub-structure porosity. On the other hand, denser membranes were synthesized due to decrease in rate of demixing process and preventing growth of nucleuses. Findings suggested addition of 0.2 wt % of studied surfactants increases the exchange rate of solvent and non-solvent causing formation of macrovoids and finger-like structures. Moreover, the effect of surfactant hydrophobicity on contact angle and water permeation flux of prepared membranes was studied. It was found that membrane derived from tween-60 exhibits the lowest flux and highest contact angle (77.2°).