A facile method to quantify the carboxyl group areal density in the active layer of polyamide thin-film composite membranes

Polyamide thin-film composite (TFC) membranes are the industry standard for membrane-based desalination. Negatively-charged carboxyl groups in the polyamide selective layer play an important role in membrane performance, affecting ion permeation, fouling, and scaling. As such, simple and accurate quantitation of the carboxyl group density is needed. While several methods already exist, each has important drawbacks that limit its application. In this study, we develop a simple bind-and-elute method utilizing silver ion probes, with silver quantitation performed using inductively coupled plasma mass spectrometry. First, the efficacy of the binding, wash, and elution steps is verified, most notably using ion exchange resin with known binding capacity. The method is then used to characterize the carboxyl group density and ionization behavior of six commercial polyamide TFC membranes, with total densities ranging from 7.2 to 37 sites/nm² and ionization behaviors best described using two acid dissociation constants (pK_a values). Comparison with data for polyamide layers isolated using dimethylformamide (DMF) shows a 35%–65% decrease in carboxyl density, suggesting that DMF may dissolve uncrosslinked polyamide oligomers. Lastly, the method is used to characterize the effect of two membrane surface modifications, with the results supporting an alternative physical interpretation of the ionization behavior in which a carboxyl group's pK_a is dependent on whether it is located on the surface or buried within the polyamide network. The simplicity, accuracy, and accessibility of the developed method should allow for widespread usage in future studies involving membrane surface charge, as well as studies involving other charged interfaces.