This constancy in the number of hydrogen bonds while adding co-solvents is in agreement with that of the molecular dynamics study by Stumpe et al. In the supplementary information (Fig S8 ) we have provided the numbers of hydrogen bonds between choline and urea per choline cations. We find that with an increase in the reline wt% in the system the number of hydrogen bonds increases from 0.1 (in W-UC (2:1)) to 0.7 (in W-UC (8:4)). It accounts for the closer proximity and stronger interaction between choline and urea at higher concentrations.