In situ study of diffusion and interaction of water and electrolyte solution in polyacrylonitrile (PAN) membrane using FTIR and two-dimensional correlation analysis

In the study, the diffusion process of water and CaCl₂ aqueous solution in polyacrylonitrile (PAN) membrane have been investigated using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and two-dimensional (2D) correlation analysis. In 2D ATR-FTIR spectra, the water ν(OH) band in PAN membrane during both water and CaCl₂ solution diffusion is split into three separate bands. However, the OH band appearing from bound water which forms hydrogen-bonding of CaCl₂ solution diffusion has a considerably lower wavenumber than that of pure water diffusion, which is due to association between Ca²⁺ ions and the CN groups in PAN. The sequential orders of the intensity changes for water and CaCl₂ solution diffusion in the region of ν(OH) and ν(CN) bands reveal the diffusion process and interaction of water and electrolyte in PAN membrane. That is, in the process of water diffusion in PAN membrane, water molecules first interact with –CN group in PAN to form weak hydrogen bond, then the moderate hydrogen-bonding interactions start gradually, and finally, the strong hydrogen-bonding interactions may be formed. When CaCl₂ solution diffuses in PAN membrane, the CN groups in PAN first associate with Ca²⁺ ions and then interact with water molecules to form hydrogen bond that grows gradually from weak to strong. Furthermore, the results are validated further by band fitting to calculate the ratio of each component band area and the proportion of component band to ν(OH) water band.