Amide A band is a fingerprint for water dynamics in reverse osmosis polyamide membranes

Donatas Surblys, Taro Yamada, Bo Thomsen, Tomonori Kawakami, Isamu Shigemoto, Jun Okabe, Takafumi Ogawa, Masahiro Kimura, Yuji Sugita, Kiyoshi Yagi

Research output: Contribution to journalArticlepeer-review

11 Citations (Scopus)


Reverse osmosis membranes based on aromatic polyamide (ar-PA) are widely used in desalination of seawater, yet the microscopic mechanism of water diffusion through a polyamide layer remains elusive. Here, we study the structure and dynamics of polymer chains and water molecules in ar-PA in comparison to nylon 6 (one of aliphatic polyamides) under various water contents (0.0–15.9 wt%). The infrared (IR) difference spectrum between dry and moist ar-PA shows little change in amide A bands, in contrast to that of nylon 6 which yields a prominent dip. Theoretical analyses using molecular dynamics simulations and quantum electronic and vibrational calculations reveal that the dip in nylon 6 is caused by breaking of hydrogen bonds (HBs) among amide groups. The incoming water molecules that break amide-amide HBs are bound to polyamide chains nearby and diffuse slowly. On the other hand, the amide-amide HBs of ar-PA are kept upon hydration. Such polymer structure facilitates growth of large water clusters with more than 100 water molecules and rapid diffusion of water molecules. The amide A band serves as a fingerprint to characterize the water permeability of polyamide materials.

Original languageEnglish
Article number117705
JournalJournal of Membrane Science
Publication statusPublished - 2020 Feb 15


  • Amide bands
  • IR difference spectrum
  • Molecular dynamics simulations
  • Vibrational quasi-degenerate perturbation theory
  • Water diffusion


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