The glass transition and fragility of supercooled confined water

In this paper we discuss the glass transition and fragility of supercooled confined water. We have used vermiculite clay, a molecular sieve and purple membrane as host materials in order to obtain confinements that are severe enough to avoid crystallization of the water, even in the (for bulk water) experimentally inaccessible temperature range 150-235 K. The clay and membrane have a 2D structure with a water layer thickness of 6 and 9 Angstrom, respectively, whereas the molecular sieve contains cylindrical pores with a diameter of 10 Angstrom. We show that one dielectric relaxation process, observed in the deeply supercooled regime, is very similar for the three investigated systems, as well as for water confined in other types of host materials, suggesting that it is relevant, although not identical, also for bulk water. At the lowest temperatures this process follows an Arrhenius temperature dependence with a relaxation time of 100 s at a temperature of approximately 130 K. However, an interesting crossover to a non-Arrhenius behaviour seems to occur at T > 185 K. We discuss the possible origins of the observed dielectric relaxation process and its relation to the present controversy regarding the glass-transition temperature of hyperquenched water, as well as a proposed fragile to strong transition of supercooled water.