Quantifying Carboxylic Acid Concentration in Model Polyamide Desalination Membranes via Fourier Transform Infrared Spectroscopy
Tawanda J. Zimudzi, Kathleen E. Feldman, Michael A. Hickner, Christopher Stafford
Carboxylic acid groups impart hydrophilicity and ionizable groups to polyamide membranes for desalination, hence influencing water and ion transport through the membrane. Model polyamide films were synthesized via molecular layer-by-layer deposition on planar substrates to overcome the chemical 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 free C=O band at 1718 cm-1. The concentration of carboxylic acids decreased as the thickness of the membrane increased, suggestive of an increase in crosslink density as the polyamide network develops. For the thinnest membranes studies, 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 influence of the substrate on the initial network formation. As the thickness of the membrane increased, the influence of the substrate and initial layer growth became less significant as the carboxylic acid concentration on both substrates reached a steady-state asymptote of 0.12 mmol g-1. We demonstrate that FTIR spectroscopy is a practical and accessible way to quantify the carboxylic acid content in polyamide membranes.
, Feldman, K.
, Hickner, M.
and Stafford, C.
Quantifying Carboxylic Acid Concentration in Model Polyamide Desalination Membranes via Fourier Transform Infrared Spectroscopy, Macromolecules, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=925076
(Accessed December 2, 2023)