Aqueous
solutions of lithium halides are state-of-the-art sorbents
in sorption-based heat pump and heat storage devices, with the drawbacks
of high corrosivity and limited natural resources of lithium. Halide-free
ILs could be well-suited alternative sorbents and have been explored
with isothermal titration calorimetry showing superior thermochemical
performance. Here, we have explored the performance of selected halide-free
ILs in perfusion measurements monitoring the uptake of water vapor
and CO2 from an air stream with well-defined flow and humidity
as a model for open sorption processes. We have determined the saturation
vapor pressure of water in the corresponding water–IL mixtures
and moreover determined the diffusion coefficients of molecular species
in binary [EMIM][Ac]–water mixtures via pulsed-field-gradient
nuclear magnetic resonance (PFG NMR) measurements. These, together
with variation of the absorption rates with calorimetric monitoring,
indicate that diffusion is approximately 3 orders of magnitude faster
than absorption and that the fast absorption process is not diffusion
limited for [EMIM][Ac] in the used concentration range (D = 5.046 ± 0.021 × 10–11 m2/s; T = 25 °C). All measurements have been
carried out in comparison to LiCl which corroborates the superior
performance with regard to reducing water vapor pressure, heat of
dilution, and absorption rate of acetate-based ILs and [EMIM][Ac],
in particular.