Terahertz Dynamics in the Glycerol-Water System

TL;DR

This study uses terahertz spectroscopy to explore the vibrational dynamics of glycerol-water mixtures across different regimes, revealing how water content influences molecular mobility, vibrational features, and structural transitions.

Contribution

It provides new insights into the vibrational and structural dynamics of glycerol-water systems, especially regarding the effects of water clustering and temperature on the boson peak and molecular mobility.

Findings

Infrared absorption relates to reduced Raman intensity and density of states.

The glass transition temperature correlates with the onset of anharmonic effects.

Water clustering occurs around 5 wt., affecting vibrational dynamics.

Abstract

The model glass-former glycerol and its aqueous mixtures were investigated with terahertz-time domain spectroscopy (THz-TDS) in the frequency range of 0.3--3.0\,THz at temperatures from 80--305\,K. It was shown that the infrared absorption coefficient measured with THz-TDS can be theoretically related to the reduced Raman intensity ($\propto \alpha/\omega^2$) and the reduced density of states ($\propto \alpha/\omega^3$) and the agreement with experimental results confirms this. The data were further used to investigate the behaviour of model glasses in the harmonic (below the glass transition temperature $T_{\text{g}}$), anharmonic (above $T_{\text{g}}$), and liquid regime. The onset temperature of the molecular mobility as measured by the infrared active dipoles, $T_{\text{g}}$, was found to correlate with the onset of anharmonic effects, leading to an apparent shift of the boson peak and obscuring it at elevated temperatures. The influence of clustered and unclustered water on the dynamics, the boson peak, and the vibrational dynamics was also investigated. A change in structural dynamics was observed at a water concentration of approximately 5\,wt.\%, corresponding to a transition from isolated water molecules distributed homogeneously throughout the sample to the presence of small water clusters and an increased number of water-water hydrogen bonds which lower the barriers on the potential energy surface.