Decoding disorder: unravelling entropy effects in deep eutectic systems with neutron spectroscopy

The vibrational dynamics and phase behavior of deep eutectic systems (DES) comprising ammonium salts and urea have been investigated using a combination of computational chemistry with inelastic neutron scattering (INS) and isotopic substitution. The study explores the impact of cation (a)symmetry on the vibrational modes of ammonium salts and urea in eutectic mixtures. By employing deuterated urea in tetraalkylammonium-based DES mixtures, selective observation of the INS spectra of individual components is achieved. Subtracting INS spectra for isotopically substituted urea mixtures provided separate "views" of the salt and urea contributions, highlighting changes in intermolecular interactions. Two types of organization of urea molecules are derived from the INS bands of urea in the mixture with symmetric and asymmetric cations. This is consistent with the view that the cation's asymmetry affects mostly the urea side of the solid-liquid equilibrium diagram and deep eutectic behavior stems from the deviation of the urea solid-liquid equilibrium line from ideality.