Low-Temperature Thermodynamic Study of the Metastable Empty Clathrate Hydrates Using Molecular Simulations

The thermodynamics of metastable empty sI-clathrate hydrates are probed over broad temperature and pressure ranges, 100 <= T (K) <= 220 and 1 <= p (bar) <= 5000, respectively, by large-scale simulations and compared with experimental data at 1 bar. The whole p-V-T surface obtained is fitted by the universal form of the Parsafar and Mason equation of state with an accuracy of 99.7-99.9%. Framework deformation brought about by the applied temperature follows a parabolic law, and there is a critical temperature above which the isobaric thermal expansion becomes negative, ranging from 194.7 K at 1 bar to 166.2 K at 5000 bar. That response to the applied (p, T) field is analyzed in terms of angle and distance descriptors of a classical tetrahedral structure and observed to occur essentially by means of angular alteration for (p, T) > (2000 bar, 200 K). The length of the hydrogen bonds responsible for framework integrity is insensitive to the thermodynamic conditions and its average value is (r) over bar ((O-H)) = 0.25 nm.