Exploring asymmetry induced entropy in tetraalkylammonium-urea DES systems: what can be learned from inelastic neutron scattering?

In this work, inelastic neutron scattering (INS) spectroscopy is used to investigate the impact of entropic factors on the behaviour of deep eutectic solvents (DES). Periodic density functional theory calculations (DFT) provide a reliable assignment of the vibrational modes of pure compounds. This assignment guides the analysis of INS spectra of binary mixtures - with particular attention to methyl torsional modes. Deviations from ideality in the mixtures of tetraalkylammonium salts with urea are readily determined through a simplified thermodynamic approach. This study reports and discusses the relationship between the cation's asymmetry, the INS spectra of the eutectic mixture and its deviation from ideality. Contrary to the majority of systems studied so far, the deep eutectic system comprised of [N]Cl and urea appears to owe its deviation from ideality to entropic rather than enthalpic factors.