Is there a water content threshold for cluster formation in glycerol

The stability of freeze-dried formulations due to residual water is a topic of significant interest. Glycerol, a cryoprotectant that is widely used in freeze drying, is mixed with water in different concentrations and measured with terahertz time-domain spectroscopy over a wide range of temperatures. Here, the question of whether there is a threshold for water clustering or a gradual change in the relative amount of unclustered water upon increased water content is addressed. When plotting the absorption coefficient versus temperature, four distinct temperature regions are identified that are characteristic with the three relaxation regimes of disordered organic molecular materials with temperature. The onset temperature of molecular mobility can be extracted from the terahertz experimental data. Rather than a gradual change we find evidence from this data for a distinct threshold at a water concentration of 5 wt.%.