Appearance of a fractional stokes-einstein relation in water and a structural interpretation ofits onset

Abstract

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 (/T)1, where is the structural relaxation time and T is the temperature. Here, we present experimental data on water confirming that, below a crossover temperature T× 290 K, the Stokes-Einstein relation is replaced by a fractional Stokes-Einstein relation D(/T) with 3/5 (refs1, 2, 3 4, 5, 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 H 2 O. 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. © 2009 Macmillan Publishers Limited.