Amorphous Phases, and the Fragile to Strong Transition

Water undergoes a “fragile to strong” dynamic crossover. The definition of “fragile” was introduced by C. Austen Angell; it concerns the viscosity of a liquid, as it approaches the glass transition. “Strong” liquids show Arrhenius behavior for viscosity and have an extended transition, while “fragile” liquids show non-Arrhenius behavior, and a relatively rapid transition to the glassy state. Water is one of the molecules showing such behavior.

There are also changes in the types of vibrational excitations that the different phases can support. At the lowest temperatures, the vibrations are largely local, while at higher temperatures longer wavelength vibrations, stretching over more of the crystal, become important. Proteins may not function below about 225 K, as the water is effectively too cold to move; the water exists in a glassy state, so the proteins are also trapped. Around 228 K, a network of hydrogen bonds can form, allowing longer wavelength vibrations; the low temperature localized vibrations may be similar to those in confined water, where the water is also restricted to local connections, but no long range network is possible.

It is not entirely clear that this is a phase transition, as it may be partly out of equilibrium, and the free energy change is not necessarily zero at the crossover. However, it does occur at a fixed temperature, so it does behave like a phase transition in that respect. The transition has been studied by simulations and modeling, by differential scattering calorimetry, X-ray diffraction, by NMR, and by neutron scattering; while these are not the only studies on this phenomenon, these are sufficient to demonstrate that it exists. It is sometimes important in biological systems, and is likely to be found in confined water, rather than bulk. Not all forms of confinement are biological, but there is no doubt that it is important in determining the vibrations in biological molecules, which do not function at temperatures below the transition.