Water sorption by ionic liquids: Evidence of a diffusion-controlled sorption process derived from the case study of [BMIm][OAc]

The hydrophilicity of ionic liquids (ILs) could be exploited as desiccants or in sorption-driven heat transformation, yet reports of water sorption isotherms of pure ionic liquids are still scarce. Here, water sorption studies for the six pure hydrophilic ionic liquids with 1-ethyl-3-methylimidazolium [EMIm]+ or 1-butyl-3-methylimidazolium [BMIm]+ cations and acetate [OAc]−, formate [HCOO]− or lactate [Lac]− anions show a steady and high water uptake with a small hysteresis between absorption and desorption over the relative pressure range of p/p0 = 0 – 0.9 for the first time. Repeated static volumetric sorption measurements with different amounts or different surface area of the example IL [BMIm][OAc] reveal a slow and strongly diffusion-driven sorption process stemming from the absorption of surface-adsorbed water into the bulk IL. Extrapolation of the water uptake to very small IL amounts with negligible kinetic hindrance gives an equilibrium water uptake capacity of over 2100 mg/g(IL) at p/p0 = 0.9. Gravimetric control experiments at a relative humidity of 75.5 % confirm the time-dependence with the IL surface area at the gas–liquid-interface and the amount of the IL to reach the equilibrium uptake of 1040 mg/g at p/p0 = 0.75.