Ionic liquids (ILs)
whose water solutions are thermoresponsive
provide an appealing route to harvest water from the atmosphere at
an energy cost that can be accessed by solar heating. IL/water solutions
that present a lower critical solution temperature (LCST), i.e., demix
upon increasing temperature, represent the most promising choice for
this task since they could absorb vapor during the night when its
saturation is highest and release liquid water during the day. The
kinetics of water absorption at the surface and the role of nanostructuring
in this process have been investigated by atomistic molecular dynamics
simulations for the ionic liquid tetrabutyl phosphonium 2,4-dimethylbenzenesulfonate
whose LCST in water occurs atTc= 36
°C for solutions of 50–50 wt % composition. The simulation
results show that water molecules are readily adsorbed on the IL and
migrate along the surface to form thick three-dimensional islands.
On a slightly longer time scale, ions crawl on these islands, covering
water and recreating the original surface whose free energy is particularly
low. At a high deposition rate, this mechanism allows the fast incorporation
of large amounts of water, producing subsurface water pockets that
eventually merge into the populations of water-rich and IL-rich domains
in the nanostructured bulk. Simulation results suggest that strong
nanostructuring could ease the separation of water and water-contaminated
IL phases even before macroscopic demixing.