Giant dynamic isotope effect in supercooled water

Abstract

Light and heavy water show similar anomalies in their thermodynamic and dynamic properties, yet attempts to interrelate them face several challenges. While a simple temperature shift apparently helps in collapsing data for both isotopes, it lacks a clear justification and requires additional ad hoc scaling factors. The quantum effect on the hydrogen bond complicates the picture. Here we tackle this issue by investigating the decoupling between shear viscosity η and translational self-diffusion Ds. To this end, we have measured shear viscosity data for heavy water supercooled down to 244 K, together with data for light water with improved accuracy. The isotope effect on viscosity is extreme for water: the viscosity ratio almost doubles when cooling from room temperature to 244 K. Viscosity and self-diffusion are coupled at high temperature, with an apparent hydrodynamic radius in good agreement for both isotopes, whereas, at low temperature, decoupling occurs with a different degree for the two isotopes. The low temperature apparent hydrodynamic radii are reconciled by a +7 K temperature shift of the light water data, without the need for a scaling factor. Our results suggest that this isotopic temperature shift mirrors that of water's thermodynamic and structural anomalies in the supercooled region.

Shear viscosity and Stokes-Einstein violation in supercooled light and heavy water

“…19) are in good agreement with R h for both isotopes above 300 K. The violation of the SE relation appears here as a decrease of R h when temperature decreases. However, the low temperature values should not be interpreted as a physical radius, and are discussed in more details in Ref [73]. In particular, it is shown there that the low-temperature R h values for D 2 O are equal to those for H 2 0, shifted by +7 K in temperature.…”