Liquid Structure of Single and Mixed Cation Alkylammonium Bromide Urea Deep Eutectic Solvents

The liquid structures of three alkyl ammonium bromide and urea DESs, ethylammonium bromide:urea (1:1), butylammonium bromide:urea (1:1), and ethylammonium bromide/butylammonium bromide:urea (0.5:0.5:1), have been studied using small-angle neutron diffraction with H/D substituted sample contrasts. The diffraction data was fit using empirical potential structure refinement (EPSR). An amphiphilic nanostructure was found in all DESs due to cation alkyl chains being solvophobically excluded from charged domains, and due to clustering together. The polar domain was continuous in all three DESs, whereas the apolar domain was continuous for the butylammonium DES and in the mixed DES, but not the ethylammonium DES. This is attributed to solvophobic interactions being weaker for the short ethyl chain. Surprisingly, the urea also forms large clusters in all three DESs. In ethylammonium bromide:urea (1:1), urea-urea orientations are mainly perpendicular, but in butylammonium bromide:urea (1:1) and the mixed system in-plane and perpendicular arrangements are found. The liquid nanostructures found in this work, especially for the ethylammonium DES, are different from those found previously for the corresponding DESs formed using glycerol, revealing that the DES amphiphilic nanostructure is sensitive to the nature of the HBD (hydrogen bond donor).