Surfactant-assisted interfacial polymerization for improving the performance of nanofiltration-like forward osmosis membranes

Surfactants modified the property and performance of polyamide layer during interfacial polymerization of amines and trimesoyl chloride. In this work, three types of similar chain length surfactants with different charge and hydrophile-lipophile (HLB) value was used as additive separately in fabrication of nanofiltration-like forward osmosis membrane. These surfactants are sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), and sorbitan monolaurate (Span 20). The negatively charged SDS interacted with amines and polyamide layer through electrostatic interaction and hydrogen bonding. It also facilitated the transport of amine into the organic phase by reducing the interfacial tension on the immiscible interface of water and n-hexane. Adding SDS into the amine solution could convert the membrane into more positively charged surface, and a membrane with low water flux. DTAB, a cationic surfactant only interacted with the polyamide layer through electrostatic interaction between its quaternary amine and carboxyl groups of polyamides. In addition, it also changed the membrane into more hydrophilic and negatively charged polyamide layer, leading to higher water flux and low salt reverse flux of TFCDTAB. On the other hand, Span 20, a neutral surfactant that is not completely soluble because of its low HLB value, only dispersed in the amine solution leading to poor membrane performance. In the forward osmosis test (pressure-retarded osmosis mode), TFCDTAB delivered the highest membrane performance using 1 M Na2SO4 draw solution and deionized water as feed: 63.23 ± 2.24 L·m−2·h−1 and the salt reverse flux of 17.83 ± 2.18 g·m−2·h−1. Therefore, a suitable surfactant is needed to regulate the performance of the membrane.