Effect of Alkyl Chain Length on Hygroscopicity of Nanoparticles and Thin Films of Imidazolium-Based Ionic Liquids

This work focuses on the interaction of water vapor with ionic liquids (ILs) consisting of [C_<i>n</i>MIM]⁺ (<i>n</i> = 2, 4, or 6) cations paired with Cl^– or BF₄^– anions to examine the effect of alkyl chain length on IL hygroscopicity. Tandem nanodifferential mobility analysis (TDMA) and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy were used to study IL nanoparticles and thin films. Studying IL nanoparticles overcomes kinetic limitations potentially present in bulk experiments as true IL–water vapor equilibrium is quickly established. Growth curves recorded in TDMA experiments showed steady uptake of water vapor with increasing RH. ILs containing Cl^– absorbed more water than those containing BF₄^– over the entire RH range, and IL hygroscopicity decreased with increasing alkyl chain length. The intensities of water stretching vibrations in IL thin films exposed to water vapor measured with ATR-FTIR were in qualitative agreement with the TDMA measurements. Water molar fractions for IL nanoparticles were calculated, and the performance of several water activity coefficient models were evaluated by fits to the experimental data. These combined experimental and modeling techniques help provide a more complete picture for these two families of ILs in the presence of water