Exploring the Impact of Glycerol on Terahertz Absorption Spectra of Its Aqueous Mixtures Using Polarizable Models

Impact of glycerol on the terahertz (THz) absorption spectra of its aqueous mixtures is explored through the dissection of the calculated spectra into various intra- and interspecies components using polarizable models for both water and glycerol. The water librational peak exhibits a systematic blue shift with increasing glycerol mole fraction, which arises from the effects of severely hindered rotational motion of water molecules in the presence of glycerol. The C-C-O skeletal deformation of the glycerol moiety also lies close to the frequency range of the water librational mode. This is further supported by the dissection of the THz spectrum into water-water, glycerol-glycerol, and glycerol-water contributions. A distinct absorption feature emerges near ∼155 cm^-1 in the difference absorption spectrum at higher glycerol concentrations, corresponding to intramolecular torsional mode of glycerol. The total THz response is also dissected into permanent dipole, induced dipole, and permanent-induced dipole correlation components, which offer a detailed description of the origin of various peaks. The spectral features at ∼155 cm^-1 and also at the hydrogen bond stretch region of ∼200 cm^-1 are primarily attributed to the induced dipole correlation contributions, highlighting the critical role of induced polarization effects and the significance of using explicit polarizable force fields. Furthermore, we divided the water around glycerol into three categories and calculated the THz spectra of distinct water populations of glycerol-bound, glycerol-proximal, and bulk water molecules, which provide molecular-level insights into water population heterogeneity that shapes the intermolecular dynamics of glycerol-water mixtures. Analysis of hydration water spectra reveals that glycerol-bound water predominantly drives the blue shift in the water librational region.