Depth heterogeneity of elemental composition, concentration of functional groups and degree of crosslinking in the active layers of reverse osmosis and nanofiltration membranes

We have studied the depth heterogeneity of the elemental composition, concentration of ionized functional groups and degree of polymer crosslinking in the active layer of the ESNA nanofiltration (NF) and FT30 reverse osmosis (RO) membranes. We studied the elemental composition of the bulk of the active layers using Rutherford backscattering spectrometry (RBS), and of the surface of the active layers using X-ray photoelectron spectroscopy (XPS). We also studied the concentration of functional groups by preparing membrane samples using the ion-probing procedure reported in our previous studies, and analyzing the prepared samples using RBS, XPS and time-of-flight secondary ion-mass spectrometry (TOF-SIMS). Our results show that the concentration of carboxylic groups and degree of polymer crosslinking in the active layer of the ESNA and FT30 membranes are higher near the top surface than in the bulk of the active layer. In contrast, amine groups were found to be preferentially located in the bottom ~ 100 nm of the FT30 active layer.