Publication Date:
2014-10-08
Description:
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 [0.75CH 3 CONH 2 + 0.25{ f KSCN + (1 – f )NaSCN}] and [0.78CH 3 CONH 2 + 0.22{ f LiBr + (1 – f )LiNO 3 }] 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 [0.78CH 3 CONH 2 + 0.22{ f LiBr + (1 – f )LiNO 3 }] 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{ f KSCN + (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 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 [0.78CH 3 CONH 2 + 0.22{ f LiBr + (1 – f )LiNO 3 }] and [0.75CH 3 CONH 2 + 0.25{ f 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.
Print ISSN:
0021-9606
Electronic ISSN:
1089-7690
Topics:
Chemistry and Pharmacology
,
Physics
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