The North American Membrane Society (NAMS) held its 2021 Annual Meeting in Colorado from September 22-25. During the conference, Professor Jay Werber received the NAMS Young Membrane Scientist Award and gave a special oral presentation titled, Heterogeneous Ionization Behavior of Polyamide Thin-Film Composite Membranes for Reverse Osmosis and Nanofiltration.
Polyamide thin-film composite desalination membranes are the industry standard for reverse osmosis (RO) and nanofiltration (NF). An important property is the ionization behavior of dissociable groups—particularly carboxylic acids and amines that stem from unreacted components in interfacial polymerization—which impact properties such as water uptake, water permeability, and water/ion selectivity. In their landmark work, Coronell and co-workers developed a silver-binding/Rutherford Backscattering Spectroscopy (RBS) method to demonstrate that carboxylic acids follow two dissociation constants (pKa of ~5 and ~9). However, RBS is highly inaccessible and the mechanisms of acid ionization remained unknown. Professor Werber’s work seeks to address these issues. His team first demonstrated a quenching method wherein amines and alcohols were used to neutralize residual acid chloride groups during membrane synthesis. To quantify the ionized carboxyl content, they modified the silver-binding approach to include an acidic elution step, which allowed simple quantitation using the widely available inductively coupled plasma–mass spectrometry technique. Amine content was characterized similarly, using bromide as a counter-ion. By pairing these methods with controlled membrane synthesis techniques, they demonstrated that the two pKa values for carboxyl groups stem from their location in the film: surface-bound groups ionize at the lower pKa that is similar to the bulk solution value, while interior groups ionize at the higher pKa, which they show stems from nanoconfinement and the resulting lower dielectric constant. Furthermore, ionization behavior was similar at high salinities, suggesting that interior carboxyl functionalities are neutrally charged during typical desalination processes. Their work has important ramifications for structure-property-transport relationships in these widely used materials.