Polyamide thin-film composite (TFC) membranes are state of the art 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, they each have important drawbacks that limit their 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, we verify the efficacy of the binding, wash, and elution steps, most notably using ion exchange resin with known binding capacity. We then use the method to characterize the carboxyl group density and ionization behavior of six commercial polyamide TFC membranes, finding that total densities range from 7.2 to 37 sites/nm2 and that ionization behaviors are best described using two acid dissociation constants (pKa values). Polyamide layers isolated using dimethylformamide (DMF) experienced 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 a physical interpretation of the ionization behavior in which a carboxyl group’s pKa 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.

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

254th National Meeting (2017)

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