Effects of acyl chloride monomer functionality on the properties of polyamide reverse osmosis (RO) membrane

In this work, three novel polyacyl chloride monomers: 2,4,4′,6-biphenyl tetraacyl chloride (BTAC), 2,3′,4,5′,6-biphenyl pentaacyl chloride (BPAC) and 2,2′,4,4′,6,6′-biphenyl hexaacyl chloride (BHAC) have been successfully synthesized. For the purpose of investigating the effects of the polyacyl chloride functionality on the reverse osmosis (RO) membrane properties, the thin film composite (TFC) RO membranes were prepared through interfacial polymerization of trimesoyl chloride (TMC), BTAC, BPAC, and BHAC with m-phenylenediamine (MPDA) respectively. The membrane properties including physicochemical properties and separation performances were evaluated by a combination of attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle (CA), streaming potential measurements as well as cross-flow reverse osmosis tests. The results reveal that the functionality of the acid chloride monomer strongly influences the properties of the RO membrane. As the functionality of the acid chloride monomer increased, the resulting membrane skin layer became more negatively charged, thinner and smoother. However, the change of hydrophilicity did not seem to follow any rule or trend, which could be ascribed to the cooperative effects of surface roughness and the carboxylic acid group content. In addition, all the four membranes exhibited close salt rejection rates according to the RO separation performance tests. However, with the increase of acid chloride functionality the permeate flux of the resulting RO membrane became lower, due to a combination of the increase in the carboxylic acid groups on the membrane surface, lower surface roughness, and lower mobility of the crosslinked polyamide chains.