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

Calorimetric
studies on ice II reveal a surprising H2O/D2O isotope effect. While the ice II to ice Ic transition
is endothermic for H2O, it is exothermic for D2O 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 H2O 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.