Low-frequency spectra of reline and its mixtures with water: A comparative study based on femtosecond Raman-induced Kerr effect spectroscopy and molecular dynamics simulations

In this study, we investigated the low-frequency spectra below 650 cm(-1) of aqueous reline solutions with various concentrations via femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and molecular dynamics (MD) simulations. The density, surface tension, viscosity, and electrical conductivity of the aqueous reline solutions were also measured. In the low-frequency spectrum, the peak frequency due to intermolecular vibrations of the aqueous reline solutions proportionally shifted to lower frequency with increasing the water content (C-H2O; wt%). The density of state (DOS) spectra of aqueous reline solutions obtained via MD simulations also showed a red shift of the low-frequency spectrum upon increasing C-H2O. A decomposition analysis of the DOS spectra revealed that the peaks corresponding to all components, that is, choline cation, urea, chloride, and water, showed a lower frequency shift in the DOS spectrum with increasing C-H2O. The plot of the peak frequency of the low-frequency spectrum vs the bulk parameter root gamma/rho (gamma: surface tension; rho: density) in aqueous reline solutions showed two relations at a turning point C-H2O = 10 wt%, which can be attributed to a change in the microscopic structure of the aqueous reline solution from a bulk-like reline structure to reline clusters.