Appearance of a fractional Stokes-Einstein relation in water and a structural interpretation of its onset

The Stokes-Einstein relation has long been regarded as one of the hallmarks of transport in liquids. It predicts that the self-diffusion constant D is proportional to (tau/T)(-1), where tau is the structural relaxation time and T is the temperature. Here, we present experimental data on water confirming that, below a crossover temperature T-x approximate to 290 K, the Stokes-Einstein relation is replaced by a 'fractional' Stokes-Einstein relation D similar to (tau/T)(-zeta) with zeta approximate to 3/5 (refs 1-6). We interpret the microscopic origin of this crossover by analysing the OH-stretch region of the Fourier transform infrared spectrum over a temperature range from 350 down to 200 K. Simultaneous with the onset of fractional Stokes-Einstein behaviour, we find that water begins to develop a local structure similar to that of low-density amorphous solid H2O. These data lead to an interpretation that the fractional Stokes-Einstein relation in water arises from a specific change in the local water structure. Computer simulations of two molecular models further support this interpretation.