Calorimetric Signature of Deuterated Ice II: Turning an Endotherm to an Exotherm

Abstract

Calorimetric studies on ice II reveal a surprising H 2 O/D 2 O isotope effect. While the ice II to ice Ic transition is endothermic for H 2 O, it is exothermic for D 2 O samples. The transition enthalpies are +40 and −140 J/mol, respectively, where such a sign change upon isotope substitution is unprecedented in ice research. To understand the observations we employ force field calculations using two water models known to perform well for H 2 O ice phases and their vibrational properties. These simulations reveal that the isotope effect can be traced back to zero-point energy. q-TIP4P/F fares better and is able to account for approximately three-fourths of the isotope effect, while MB-pol only catches approximately one-third. Phonon and configurational entropy contributions are necessary to predict reasonable transition enthalpies, but they do not have an impact on the isotope effect. We suggest to use these calorimetric isotope data as a benchmark for water models.

Enthalpy Change from Pure Cubic Ice Ic to Hexagonal Ice Ih

“…In terms of enthalpy difference, numerous calorimetric studies are available that have scrutinized the transition from different stacking-disordered ices I sd to I h , as listed in Table . The reported literature values for the enthalpy change scatter by an order of magnitude, ranging from less than −10 to ca. −160 J mol –1 .…”

Exploring High-Pressure Transformations in Low-Z (H2, Ne) Hydrates at Low Temperatures

“…Between 4.6(1) and 1.90(4) GPa the sample developed into single phase C 1 which at atmospheric pressure turned into pure ice II (empty C 1 ). Ice II is known to be recoverable at LN 2 temperature and is metastable at 200 K due to its low entropy (H-ordering) [49,50]. In experiment b CS-II HH was observed metastably up to 2.2( 1) GPa (at 95 K).…”

New Insights into the Volume Isotope Effect of Ice Ih from Polarizable Many-Body Potentials

“…Nuclear quantum effects can manifest themselves quite prominently in macroscopic thermodynamic properties, like for example phase transition enthalpies, 1 negative thermal expansion (NTE), or density change at low temperatures upon substitution of a light by a heavier isotope. 2 The latter is called the volume isotope effect (VIE) and originates from the zero-point energy of the lattice vibrations (phonons).…”