Preparation of an asymmetric membrane via vapor induced phase separation for membrane distillation

Lately, membrane with asymmetric properties of its two sides has shown promising potential in direct contact membrane distillation (DCMD) application, based on the fact that the hydrophilic side of the asymmetric membrane can effectively mitigate wetting and fouling, while the hydrophobic layer facilitate transportation of water vapor to realize separation. We here report an asymmetric membrane prepared via a facile method of vapor induced phase separation (VIPS). Namely, hydrophobic polymer of poly(styrene-block-butadiene-block- styrene) (SBS) and hydrophilic cellulose acetate (CA) were co-dissolved in tetrahydrofuran, and then cast in an ethanol vapor atmosphere to evaporate solvent. After solvent evaporation (within 10 min), the obtained mem-brane was observed with scanning electron microscope (SEM) to indicate even microstructure of porous network matrix with nodular morphology, and its top surface consisted of SBS only, while microspheres of CA were found to be evenly embedded in the SBS matrix at the bottom side layer. The membrane was further treated with NaOH aqueous solution for CA deacetylation, which was confirmed by attenuated total reflection Fourier transform infrared spectroscopy, SEM and water contact angle (WCA) analyses. The deacetylated CA exposes OH groups to decrease the WCA of the bottom surface greatly. As a result, an asymmetric membrane with large difference in WCA between the top surface and the bottom surface has been obtained. The asymmetric membrane was employed for DCMD to separate an oily saline emulsion containing n-hexadecane, surfactant of Tween 80, and NaCl. The stable flux and high salt rejection of the asymmetric membrane indicate excellent resistance to wetting and fouling, which is attributed to the hydration layer formed by the OH groups of the deacetylated CA at the bottom surface of the membrane. The facile strategy reported in this work implies possible large scale production of the asymmetric membrane that is suitable for membrane distillation application.