Hydrogen-bond network formation of water molecules and its effects on the glass transitions in the ethylene glycol aqueous solutions: failure of the Gordon-Taylor law in the water-rich range and absence of the Tg = 115 K rearrangement process in bulk pure water

Enthalpy relaxation processes proceeding in ethylene glycol (EG) aqueous solutions [(EG)x(H2O)1-x] within silica-gel nanopores were studied by adiabatic calorimetry. While the x = 0.25 solution within pores with diameter of 52 nm showed a glass transition at Tg = 139 K, ageing of the solution at 160 K caused a phase separation to reveal glass transitions at Tg = 145 and 160 K for EG-rich and water-rich regions, respectively: the water molecules are understood to form a more developed hydrogen-bond network, and consequently force the EG molecules in between the water-rich regions. The Tg = 160 K is in good agreement with the Tg value of the internal (not interfacial) water confined within pores with thickness of 1.1 nm. The ageing further remarkably diminished the Tg = 115 K glass transition. This indicates that, while the molecules responsible for the glass transition are the mobile water ones forming a lower number of hydrogen bonds than four, the fraction of such water molecules is reduced in association with the development of the network and the glass transition is absent in bulk pure water. When the same x = 0.25 solution was confined within 1.1- and 12 nm pores, the water molecules developed a hydrogen-bond network in the pore centre due to the presence of the pore wall and pushed the EG molecules onto the pore surface even at higher temperatures: the water-rich region gave Tg = 155 K close to 160 K. It is concluded that the hydrogen-bond network inherent to water structure is developed/collapsed remarkably in the range near x = 0; consequently, the composition dependence of Tg in the bulk system deviates sharply in the range from the Gordon-Taylor empirical law followed for large x > 0.2.