Slow rheological mode in glycerol and glycerol-water mixtures

Glycerol-water mixtures were studied at molar concentrations ranging from $x_\text{gly} = 1$ (pure glycerol) to $x_\text{gly}=0.3$ using shear mechanical spectroscopy. We observed a low frequency mode in neat glycerol, similar to what is usually reported for monohydroxy alcohols. This mode has no dielectric counterpart and disappears with increased water concentration. We propose that the hydrogen-bonded network formed between glycerol molecules is responsible for the observed slow mode and that water acts as a plasticizer for the overall dynamics and as a lubricant softening the hydrogen-bonding contribution to the macroscopic viscosity of this binary system.