Isotope effect on the anomalies of water: a corresponding states analysis

TL;DR

This study uses a corresponding states analysis to explain isotope effects in water's anomalies, revealing a unified framework that aligns thermodynamic and dynamic data for light and heavy water, and suggests a link to the liquid-liquid transition.

Contribution

It introduces a coherent analysis based on a liquid-liquid critical point to unify isotope effects in water's anomalies, improving upon simple temperature shift methods.

Findings

Collapse of thermodynamic data for isotopes using the analysis

Reduced isotope effect ratio in dynamic properties after analysis

Decoupling of viscosity and diffusion collapses with the method

Abstract

Light and heavy water show similar anomalies in thermodynamic and dynamic properties, with a consistent trend of anomalies occurring at higher temperature in heavy water. Viscosity also increases faster upon cooling in heavy water, causing a giant isotope effect, with a viscosity ratio near 2.4 at 244 K. While a simple temperature shift apparently helps in collapsing experimental data for both isotopes, it lacks a clear justification, changes value with the property considered, and requires additional ad hoc scaling factors. Here we use a corresponding states analysis based on the possible existence of a liquid-liquid critical point in supercooled water. This provides a coherent framework which leads to the collapse of thermodynamic data. The ratio between dynamic properties of the isotopes is strongly reduced. In particular, the decoupling between viscosity $\eta$ and self-diffusion $D$, measured as a function of temperature $T$ by the Stokes-Einstein ratio $D\eta/T$, is found to collapse after applying the corresponding states analysis. Our results are consistent with simulations and suggest that the various isotope effects mirror the one on the liquid-liquid transition.