Low-frequency collective dynamics in deep eutectic solvents of acetamide and electrolytes: A femtosecond Raman-induced Kerr effect spectroscopic study

In this study, we have investigated the ion concentration dependent collective dynamics in two series of deep eutectic solvent (DES) systems by femtosecond Raman-induced Kerr effect spectroscopy, as well as some physical properties, e. g. , shear viscosity (η), density (ρ), and surface tension (γ). The DES systems studied here are 0. 75CH3CONH2 + 0. 25f KSCN + (1 - f) NaSCN and 0. 78CH3CONH2 + 0. 22f LiBr + (1 - f) LiNO3 with f = 0, 0. 2, 0. 4, 0. 6, 0. 8, and 1. 0. γ of these DES systems shows near insensitivity to f, while ρ shows a moderate dependence on f. Interestingly, η exhibits a strong dependence on f. In the low-frequency Kerr spectra, obtained via the Fourier transform of the collected Kerr transients, a characteristic band at ∼70 cm (-1) is clear in 0. 78CH3CONH2 + 0. 22f LiBr + (1 - f) LiNO3 DES especially at the larger f. The band is attributed to the intermolecular hydrogen bond of acetamide. Because of less depolarized Raman activities of intermolecular/interionic vibrational motions, which are mostly translational (collision-induced or interaction-induced) motions, of spherical ions, the intermolecular hydrogen-bonding band is clearly observed. In contrast, the intermolecular hydrogen-bonding band is buried in the other intermolecular/interionic vibrational motions, which includes translational and reorientational (librational) motions and their cross-terms, in 0. 75CH3CONH2 + 0. 25f KSCN + (1 - f) NaSCN system. The first moment (M1) of the intermolecular/interionic vibrational band in these DES systems is much higher than that in typical neutral molecular liquids and shows a weak but contrasting dependence on the bulk parameter √γ/ρ. The time constants for picosecond overdamped Kerr transients in both the DES systems, which are obtained on the basis of the analysis fitted by a triexponential function, are rather insensitive to f for both the DES systems, but all the three time constants (fast: ∼1-3 ps; intermediate: ∼7-20 ps; and slow: ∼100 ps) are different between the 0. 78CH3CONH2 + 0. 22f LiBr + (1 - f) LiNO3 and 0. 75CH3CONH2 + 0. 25f KSCN + (1 - f) NaSCN systems. These results indicate that the intermolecular/interionic interactions in DES systems is strongly influenced by the ionic species present in these DES systems.