Probing the nano- and micro-scales of reverse osmosis membranes—A comprehensive characterization of physiochemical properties of uncoated and coated membranes by XPS, TEM, ATR-FTIR, and streaming potential measurements

The performance of polyamide (PA) composite reverse osmosis (RO) membranes is essentially determined by a dense skin layer several hundred nanometers thick. Improved understanding of this critical thin layer will likely advance our understanding and control of membrane fouling and trace organics rejection. Unfortunately, the processes and exact chemistries for producing RO membranes are proprietary. In this study, XPS, ATR-FTIR, TEM microscopy, and streaming potential analysis were used to characterize commercial fully aromatic polyamide RO membranes. By using these techniques together in a systematic way, the presence of an aliphatic coating layer rich in –COH groups was confirmed for some commercial RO membranes. While the uncoated RO membranes had surface elemental compositions (measured by XPS) very close to the predicted values for polyamides based on the classical interfacial polymerization chemistry of trimesoyl chloride and 1,3-benzenediamine (m-phenylene-diamine), the coated membranes showed higher oxygen and lower nitrogen content. The typical layered structures (a polyamide layer on top of a polysulfone layer) and the rough ridge-and-valley features were clearly visible in TEM micrographs. For a coated membrane, a light-colored coating layer (lack of electron density) was visible, especially when a stained humic acid foulant layer was present to increase the contrast between the coating and the background. An intense OH stretching peak in conjunction with new aliphatic C–H stretching peaks were observed in ATR-FTIR spectra for coated membranes, where the aromatic C–H stretching peak normally present for uncoated membranes was overwhelmed. High-resolution XPS scans on carbon 1s peaks confirmed the presence of alcoholic –COH groups in the coating layer, where an additional peak with a binding energy shift of 1.5 eV was identified in addition to the two peaks present for uncoated membranes: 284.6 eV (aliphatic and aromatic carbon atoms) and ∼287.8 eV (carboxylic and amide bond carbon atoms). The measured zeta potential values of the coated membranes were significantly less negative than those of the uncoated ones, suggesting that the coating layer is likely neutral.