Effects of CTAC micelles on the molecular structures and separation performance of thin-film composite (TFC) membranes in forward osmosis processes

In this article, cetyltrimethylammonium chloride (CTAC) was used to produce novel thin-film composite (TFC) forward osmosis (FO) membranes through interfacial polymerization with 1,3,5-benzenetricarbonyl trichloride (TMC) and m-phenylenediamine (MPD). The effects of the CTAC concentrations on the structure of the polyamide skin layer were investigated by ATR-FTIR, XPS, WAXD, NMR and SEM analyses. FO performance of TFC membranes was detected using 2 M each of four different salt solutions (NaCl, Na2SO4, MgCl2 and MgSO4) as draw solutions. The results indicate that the skin layer is significantly affected by the CTAC micelles. CTAC micelles in aqueous solution have ionic interactions with MPD molecules and affect interfacial polymerization process of MPD and TMC. With an increase of the CTAC concentration, the linear molecular structure of the skin layer gradually increases and the more microcrystalline structure appears. The pure water flux of the TFC FO membranes has been negatively affected in FO process by the special polyamide structures as a result of CTAC; however, the rates of rejection and reverse selectivity of salts are significantly improved with the increase of the CTAC concentrations (maximum 99.9% of MgCl2), and they are all greater than 90% (MgSO4 > MgCl2 > Na2SO4 > NaCl).