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

Abstract

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 thex = 0.25 solution within pores with diameter of 52 nm showed a glass transition atTg = 139 K, ageing of the solution at 160 K caused a phase separation to reveal glass transitions atTg = 145 and 160 K for EG-rich and water-rich regions, respectively: the water moleculesare understood to form a more developed hydrogen-bond network, andconsequently force the EG molecules in between the water-rich regions. TheTg = 160 K is in goodagreement with the Tg value of the internal (not interfacial) water confined within poreswith thickness of 1.1 nm. The ageing further remarkably diminished theTg = 115 K glass transition. This indicates that, while the molecules responsible for the glasstransition 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 thenetwork and the glass transition is absent in bulk pure water. When the samex = 0.25 solution was confined within 1.1- and 12 nm pores, the water molecules developed ahydrogen-bond network in the pore centre due to the presence of the pore wall and pushed theEG molecules onto the pore surface even at higher temperatures: the water-rich region gaveTg = 155 K close to 160 K. It is concluded that the hydrogen-bond network inherentto water structure is developed/collapsed remarkably in the range nearx = 0; consequently, the composition dependence ofTg inthe bulk system deviates sharply in the range from the Gordon–Taylor empirical law followed for largex > 0.2.