Physical Chemistry

Water harvesting by thermoresponsive ionic liquids: A molecular dynamics study of the water absorption kinetics and of the role of nanostructuring

Authors

  • Nancy C. Forero-Martinez Institut fuer Physik, Johannes Gutenberg-Universitaet Mainz, Staudingerweg 9, 55128 Mainz, Germany ,
  • Robinson Cortes-Huerto Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ,
  • Lainey Ward School of Physics, University College Dublin, Dublin, Ireland ,
  • Pietro Ballone School of Physics, University College Dublin, Dublin, Ireland

Abstract

Ionic liquids (IL) whose water solutions are thermoresponsive provide an appealing route to harvest water from the atmosphere at an energy cost that can be filled 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 vapour 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 [P4444][DMBS] whose LCST in water occurs at $T=36$ $^{\circ}$C for solutions of 50-50 % composition. The simulation trajectories show that water molecules are readily adsorbed on the IL, and migrate along the surface to form 3D 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 high deposition rate, this mechanism allows the incorporation of large amounts of water in a short time, 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 the macroscopic demixing.

Content

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Supplementary material

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Water harvesting by thermoresponsive ionic liquids: A molecular dynamics study of the water absorption kinetics and of the role of nanostructuring
Miscellaneous figures cocerning: Clustering and nanostructuring of absorbed water; Snapshot from simulation.