Evidence of Molecular Heterogeneities in Amide-Based Deep Eutectic Solvents

The liquid structure of five different amide-based deep eutectic solvents (DESs) as a function of the chemical structure of the hydrogen bond acceptor (HBA) was investigated by linear Fourier transform infrared (FTIR) and two-dimensional infrared (2DIR) spectroscopies. Linear FTIR spectroscopy shows that the amide band of the DESs is not significantly affected by the chemical structure and symmetry of the HBA cation. However, its excitonic nature does not allow us to draw further conclusions. Analysis of the ¹³C amide line shapes derived from the 2DIR spectra reveals that the different DESs do not show appreciable differences in the level of disorganization. The vibrational dynamics, derived from the photon echo experiments on the ¹³C amide, shows that there is a fast component with a time scale of ∼1 ps irrespective of the HBA. The ultrafast dynamics is assigned to hydrogen bond making and breaking between amides. In addition, a slow dynamical component is observed in the time evolution of the photon echo signal. This contribution appears to be correlated with the asymmetry and polarity of the moieties of the HBA. The overall dynamics is rationalized in terms of a microscopic heterogeneous structure of the DESs, where the heterogeneities create domains that slow the hydrogen bond making and breaking. Molecular dynamics simulations provide additional support for our modeling of the data. In addition, the presence of nanoscopic heterogeneities is consistent with the observation of an endortherm at 23 °C in the differential scanning calorimetry thermogram, which evidenced a phase transition at 23 °C, even though the tested DESs have a melting temperature below -40 °C.