Reliable, bench-top measurements of charge density in the active layers of thin-film composite and nanocomposite membranes using quartz crystal microbalance technology

A reliable, user-friendly, bench-top method was developed and evaluated for the measurement of negative charge density in the active layers of thin-film composite and thin-film nanocomposite membranes. The method consists of isolating the active layer on a quartz crystal microbalance (QCM) sensor (i.e., AL+sensor sample), exposing the AL+sensor sample to an aqueous cesium solution at any pH of interest, and measuring with a QCM the mass of cesium ion that associates with the negative sites of the active layer. Results showed that QCM measurements of charge density in active layers were: (1) repeatable within 3% for tests performed with the same AL+sensor sample under the same experimental conditions; (2) reproducible within 3.8% for tests performed with the same AL+sensor sample when the ionic strength of cesium solutions was varied by 300%; (3) reproducible within 4% for active layers isolated from nearby locations of the same membrane sheet; and (4) consistent within 2.1% at pH=10.5 with results obtained using the previously reported Rutherford backscattering spectrometry method on non-isolated active layers. The results therefore demonstrate the robustness, repeatability, reproducibility, and accuracy of the QCM method. We also demonstrated that the ionization behaviors of the polyamide-based thin-film composite and nanocomposite membranes tested were similar: both membranes had bimodal pKa distributions and negative charge densities of ≈0.5 M at full ionization.