We study the molecular-scale structure of the surface of Reline, a DES made from urea and choline chloride, using heterodyne-detected vibrational sum frequency generation (HD-VSFG). Reline absorbs water when exposed to the ambient atmosphere, and following structure-specific changes at the Reline/air interface is crucial and difficult. For Reline (dry, 0 wt %, w/w, water) we observe vibrational signatures of both urea and choline ions at the surface. Upon increase of the water content, there is a gradual depletion of urea from the surface, an enhanced alignment, and an enrichment of the surface with choline cations, indicating surface speciation of ChCl. Above 40% w/w water content, choline cations abruptly deplete from the surface, as evidenced by the decrease of the vibrational signal of the −CH2– groups of choline and the rapid rise of a water signal. Above 60% w/w water content, the surface spectrum of aqueous Reline becomes indistinguishable from that of neat water.

American Chemical Society (ACS), ACS
European Research Council (ERC) , European Union Horizon 2020
J. Phys. Chem. Lett.
Ultrafast Spectroscopy

Gera, R., Moll, C., Bhattacherje, A., & Bakker, H. (2022). Water-Induced Restructuring of the Surface of a Deep Eutectic Solvent. J. Phys. Chem. Lett., 13, 634–641. doi:10.1021/acs.jpclett.1c03907