Effect of Molecular Configuration of Additives on the Membrane Structure and Water Transport Performance for Forward Osmosis

In this work, we chose cis-trans isomers (trans2-butenedioic acid, TBA, and cis-2-butenedioic acid, CBA) as pore-forming additives to fabricate cellulose acetate (CA) forward osmosis (FO) membranes and explored the underlying correlations between additive molecular structure and membrane properties. The most significant fact to note is that the molecular configuration of the additive indeed leads to a considerable influence on membrane performance, although these additive molecules have the same constitution, conformation, molecular formula, and functional groups. CBA tends to form a more porous structure in the macro-void middle region (noticeable finger-like and loose sponge-like structures) and two perfectly dense top and bottom surfaces with higher hydrophilicity, which facilitates water transport with no sacrifice of salt rejection. On the basis of our observations, we confirm that there are some residual CBA molecules in the prepared membranes, while TBA totally transfers from the casting film to the coagulation bath by TGA, which contributes to the more hydrophilic surface and instantaneous demixing. Moreover, the additive transfer rate during phase inversion depends on the superiority of two interactions between additive-CA and additive-water from the point of hydrogen bonds. This work provides insights into the additive selection and membrane design for water reuse and desalination.