Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes

The physiochemical properties of reserve osmosis (RO) and nanofiltration (NF) polyamide (PA) membranes are largely determined by their PA chemistry and coatings, if any. Knowledge on such inherent relationship is critically needed in advancing membrane technology. This paper presents a consistent and in-depth characterization on diagnosing the chemistry of polyamide and the presence of any coating or modifying agent. Fourier-transform infrared (FTIR) and x-ray photoelectron spectra (XPS) of 17 commonly used commercial thin film composite polyamide RO and NF membranes are presented. The FTIR spectra for fully aromatic trimesoyl chloride and 1,3-benzenediamine based membranes had an amide II band (1541 cm−1) and an aromatic amide band (1609 cm−1) that were absent for the semi-aromatic membranes. Consistent with that, the XPS binding energy shift for carbon atoms in fully aromatic amide groups was higher than that for semi-aromatic ones likely due to the more electron withdrawing environment. An additional intermediate peak with a binding energy shift of 1.1-1.6 eV was present in the XPS spectra of C(1s) for some commercial RO and NF membranes. The additional peak, coupled with FITR analysis over the high wave number region and XPS elemental analysis, provided consistent evidence that these membranes were either coated with an additional coating layer or had a modified PA chemistry.