Moisture Absorption in Ionic Liquid Films

The uptake of water vapor by ionic liquids (ILs) has been studied by various research groups, but there is little knowledge on how liquid confinement affects the process of moisture absorption. In this work, several imidazolium-based ILs deposited on quartz crystals were used as water vapor sensors in a high sensitivity quartz-crystal microbalance with dissipation. The process of water vapor absorption by the IL films was interpreted, taking into account the changes of the different harmonics of the fundamental frequency. For the most viscous ILs, such as [C2OHmim][BF4] and [C6mim][OAc], the decrease in viscosity was the dominant effect, while for the others, [C8mim][BF4], [C2mim][OAc], and [C4mim][OAc], the mass loading effect predominated. However, there are differences within the latter set: a gradual water adsorption and diffusion was observed in [C2mim][OAc] and [C4mim][OAc] films; in contrast, a discontinuity seems to have occurred between the formation of a water film on top of the [C8mim][BF4] film and its diffusion into the bulk. The kinetics of water absorption in the films of [C2mim][OAc] and [C4mim][OAc] was well described by a two-step model, and water diffusion coefficients in those films were obtained.