In Situ Fourier Transform Infrared Study of the Effects of Silica Mesopore Confinement on Hydration of Ionic Liquid 1-Butyl-3-methylimidazolium Chloride

The absorption of water by ionic liquid 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]) confined in silica thin films with accessible 8 nm cylindrical mesopores was investigated and compared to that of bulk ionic liquid. Transmission Fourier transform infrared spectra were collected in situ at room temperature while the relative humidity (RH) of the environment (RH of 0% to 87%) was changed. Pore confinement induces binding of imidazolium rings with the silica pore wall under dry conditions, but hydration disrupts hydrogen bonding of imidazolium with both silica and Cl^- as interpreted from peak shifts corresponding to C-H vibrations on the imidazolium ring. Molecular interactions with water are similar regardless of confinement, but [BMIM][Cl] exhibits significantly reduced water uptake at RH > 15% due to nanopore charge and structuring effects associated with confinement. Deconvolution of OH stretching bands shows that weakly coordinated water is promoted in confined [BMIM][Cl], which may affect mechanisms of solubilization and catalysis in confined ILs.