Dynamics of supercooled water in confined geometry

As with most liquids, it is possible to supercool1,2,3,4 water; this generally involves cooling the liquid below its melting temperature (avoiding crystallization) until it eventually forms a glass. The viscosity and related relaxation times (τ) of glass-forming liquids typically show non-Arrhenius temperature (T) dependencies. Liquids with highly non-Arrhenius behaviour in the supercooled region are termed ‘fragile’. In contrast, liquids whose behaviour is close to the Arrhenius law (ln τ ∝ 1/T) are termed ‘strong’ ( ref. 5). A unique ‘fragile–strong’ transition around 228 K has been proposed6 for supercooled water; however, experimental studies of bulk supercooled water in this temperature range are generally hampered because crystallization occurs. Here we use broad-band dielectric spectroscopy to study the relaxation dynamics of supercooled water in a wide temperature range, including the usually inaccessible temperature region. This is possible because the supercooled water is held within a layered vermiculite clay—the geometrical confinement and presence of intercalated sodium ions prevent7 most of the water from crystallizing. We find a relaxational process with an Arrhenius temperature dependence, consistent with the proposed strong nature of deeply supercooled bulk water. Because water that is less supercooled has been established6 as highly fragile, our results support the existence of a fragile–strong transition.