Further Extending the Dilution Range of the “Solvent-in-DES” Regime upon the Replacement of Water by an Organic Solvent with Hydrogen Bond Capabilities

Aqueous dilutions of deep eutectic solvents (DESs) have lately allowed exploring new and more demanding applications where neat DESs are not able to perform well. However, the use of DES dilutions with nonaqueous hydrogen-bond-forming solvents remains basically unexplored. It is worth noting the obvious interest of using organic solvents in those cases where any reagent or byproduct is nonsoluble, nonmiscible, or unstable in water, the presence of water might alter the reaction kinetics (for instance, when water is a byproduct), or a cosolvent with low vapor pressure allows exploring reaction processes (high temperatures or solvothermal conditions, among others) not suitable for water. Herein, we investigated benzyl alcohol (BA) dilutions of RUChCl, a DES composed of resorcinol (R), urea (U), and choline chloride (ChCl). In particular, neutron scattering, nuclear magnetic resonance, and Brillouin experiments revealed how BA was accommodated within the hydrogen-bond (HB) complex structure of RUChCl for BA contents of up to 67 wt % whereas this HB complex structure basically disappeared for higher BA contents. This behavior somehow resembled that found in aqueous solutions, with two well-differentiated regimes—e.g., the "solvent-in-DES" and the "DES-in-solvent"—depending on the DES content. However, the "solvent-in-DES" regime was preserved for much higher solvent contents of BA than H2O—e.g., 60–65 versus 18–20 wt %, respectively. Interestingly, the specific BA dilution where transition from one regime to the other occurs was particularly well suited to develop a spinodal decomposition process when used as the precursor for the preparation of polymer resins (by polycondensation between R and p-phthalaldehyde).