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 (τ/T)−1, where τ 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 ∼ (τ/T) with ζ ≈ 3/5 (refs 1–6). We interpret the microscopic origin of this crossover by analysing the OHstretch 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. We first present our experimental results on water confined in MCM-41-S nanotubes. We measure the self-diffusionD by nuclear magnetic resonance, and we measure the translational relaxation time τ by using incoherent, quasi-elastic neutron scattering1,2 (QENS). Thus, the Stokes–Einstein relation,