Transition from molecular- to nano-scale segregation in a deep eutectic solvent - water mixture

The changes upon water addition in the nanostructure of a deep eutectic solvent (DES) formed by choline chloride (ChCl) and sesamol in 1:3 ratio have been studied by means of small- and wide-angle X-ray scattering (SWAXS), ATR-FTIR spectroscopy and molecular dynamics (MD) simulations. SWAXS data show that small inhomogeneities in the sub-nanometer scale are formed for water/DES molar ratios (W) up to 6, while after this threshold segregation to higher scale occurs up to the formation of nanoscale structures with size of about 70 Å. This behavior is different from that obtained for 1:2 ChCl:urea (reline) mixtures with water. 2D-correlation spectroscopy plots between SWAXS and FTIR spectra evidence that this pseudo-phase separation can be ascribed to the formation of nanometric water aggregates inside the DES for high W values. MD simulations show that, for high water concentration, water segregation from sesamol occurs by confining most of ChCl in the aqueous region.