Quantifying Carboxylic Acid Concentration in Model Polyamide Desalination Membranes via Fourier Transform Infrared Spectroscopy

Carboxylic acid groups impart hydrophilicity and ionizable moieties to polyamide membranes for desalination, hence influencing water and ion transport through the material. Model polyamide films were synthesized via molecular layer-by-layer (mLbL) deposition on planar substrates to study the formation process of these materials and overcome the chemical and topological inhomogeneity inherent to conventional interfacially polymerized polyamide membranes. The carboxylic acid content in these model films was characterized using Fourier transform infrared (FTIR) spectroscopy by quantifying the C═O band at 1718 cm–1. The concentration of carboxylic acid groups decreased as the thickness of the membrane increased, suggestive of an increase in cross-link density as the polyamide network develops. For the thinnest molecular layer-by-layer samples, the carboxylic acid concentration for films on gold was 0.35 mmol g–1, whereas analogous films on silicon had an acid content of 0.56 mmol g–1, indicating a clear influ...