On the not so anomalous water-induced structural transformations of choline chloride–urea (reline) deep eutectic system

Abstract

The choline chloride–urea binary mixture in the molar ratio (1 : 2), commonly known as reline, is an archetypal solvent among deep eutectic solvents (DES). Neutron diffraction (ND) and empirical potential structure refinement (EPSR) results provided evidence that reline exhibits a peculiar structural transformation upon water addition that manifests in a sudden dewetting of the choline cations at ∼51 wt% water and, therefore, a non-monotonic variation of the choline–water and choline–choline coordination. Here, we study, through molecular dynamics (MD), the influence of water on the structure of a choline chloride : urea : water DES (1 : 2 : ζ; ζ = 0 to 40), to gain additional insight into the molecular source of this peculiar structural transformation. Five different force fields were investigated. Our results show that the ND/EPSR non-monotonic behavior of the choline–choline coordination is qualitatively reproduced by those force fields that describe more accurately the dynamics of the DES, namely, the diffusion and viscosity coefficients. However, the apparent increase of the choline–choline coordination at ζ > 10 (41 wt%) is associated with the drain of a particular first coordination sphere, rather than with a dewetting of the choline cations, as predicted by ND/EPSR. Thus, a monotonic increase of the choline–water coordination is found instead, as well as the emergence of a tetrahedral hydrogen bond network of water, opposite to ND/EPSR. The highest rate of depletion upon initial hydration is found for urea around choline (∼0.9 urea molecules/ζ), whereas choline depicts the highest rate of hydration (∼2.5 water molecules/ζ). These rates decrease with ζ and a structural transition that stabilizes above 41 wt% is observed, consistent with ND/EPSR results. Despite limitations in the force fields, we argue that the experimental and simulation opposite pictures could be related to an underestimation of water's hydrogen bond network portrayed by the EPSR method. Overall, a smoother transition from a DES to an aqueous solution of the DES components is portrayed by MD, compared with ND/EPSR.