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 (eta), density (rho), and surface tension (gamma). The DES systems studied here are [0.75CH(3)CONH(2) + 0.25{fKSCN + (1 - f)NaSCN}] and [0.78CH(3)CONH(2) + 0.22{fLiBr + (1 - f)LiNO3}] with f = 0, 0.2, 0.4, 0.6, 0.8, and 1.0. gamma of these DES systems shows near insensitivity to f, while rho shows a moderate dependence on f. Interestingly, eta 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 similar to 70 cm(-1) is clear in [0.78CH(3)CONH(2) + 0.22{fLiBr + (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 [0.75CH(3)CONH(2) + 0.25{fKSCN + (1 - f) NaSCN}] system. The first moment (M-1) 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 root gamma/rho. 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: similar to 1-3 ps; intermediate: similar to 7-20 ps; and slow: similar to 100 ps) are different between the [0.78CH(3)CONH(2) + 0.22{fLiBr + (1 - f) LiNO3}] and [0.75CH(3)CONH(2) + 0.25{fKSCN + (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. (C) 2014 AIP Publishing LLC.