Formation of Cellulose Acetate Membranes via Phase Inversion Using Ionic Liquid, [BMIM]SCN, As the Solvent

Ionic liquids have gained worldwide attention as green solvents in the past decade. This study explores, for the first time, the fundamental science and engineering of using ionic liquids as a new generation of solvents to replace traditional organic solvents for the fabrication of flat sheet membranes and hollow fiber membranes. The fundamentals and characteristics of membrane formation of cellulose acetate (CA) membranes have been investigated using 1-butyl-3-methylimidazolium thiocyanate ([BMIM]SCN) as the solvent via phase inversion in water. For elucidation, other solvents, i.e., N-methyl-2-pyrrolidinone (NMP) and acetone, were also studied. It is found that [BMIM]SCN has distinctive effects on phase inversion process and membrane morphology compared to NMP and acetone because of its unique nature of high viscosity and the high ratio of [BMIM]SCN outflow to water inflow. Membranes cast or spun from CA/[BMIM]SCN have a macrovoid-free dense structure full of nodules, implying the paths of phase inversion are mainly nucleation growth and gelation, followed possibly by spinodal decomposition. The recovery and reuse of [BMIM]SCN have also been demonstrated and achieved. The derived flat sheet membranes made from the recovered [BMIM]SCN show morphological and performance characteristics similar to those from the fresh [BMIM]SCN. It is believed that this study could enrich the understanding of membrane formation using environmentally benign ionic liquids.