Water dynamics in n-propylene glycol aqueous solutions.

The relaxation dynamics of dipropylene glycol and tripropylene glycol (nPG—n=2,3) water solutions on the nPG-rich side has been studied by broadband dielectric spectroscopy and differential scanning calorimetry in the temperature range of 130–280 K. Two relaxation processes are observed for all the hydration levels; the slower process (I) is related to the α relaxation of the solution whereas the faster one (II) is associated with the reorientation of water molecules in the mixture. Dielectric data for process (II) at temperatures between 150 and 200 K indicate the existence of a critical water concentration (x_c) below which water mobility is highly restricted. Below x_c, nPG-water domains drive the dielectric signal whereas above x_c, water-water domains dominate the dielectric response at low temperatures. The results also show that process (II) at low temperatures is due to local motions of water molecules in the glassy frozen matrix. Additionally, we will show that the glass transition temperatures (T_g) for aqueous PG, 2PG, and 3PG solutions do not extrapolate to ∼136 K, regardless of the extrapolation method. Instead, we find that the extrapolated T_g value for water from these solutions lies in the neighborhood of 165 K. [ABSTRACT FROM AUTHOR]