Simulation of the CO2 hydrate-water interfacial energy: The mold integration-guest methodology.

ABSTRACT

The growth pattern and nucleation rate of carbon dioxide hydrate critically depend on the precise value of the hydrate-water interfacial free energy. There exist in the literature only two independent experimental measurements of this thermodynamic magnitude one obtained by Uchida et al. [J. Phys. Chem. B 106, 8202 (2002)], 28(6) mJ/m2, and the other by Anderson and co-workers [J. Phys. Chem. B 107, 3507 (2003)], 30(3) mJ/m2. Recently, Algaba et al. [J. Colloid Interface Sci. 623, 354 (2022)] have extended the mold integration method proposed by Espinosa and co-workers [J. Chem. Phys. 141, 134709 (2014)] to deal with the CO2 hydrate-water interfacial free energy (mold integration-guest or MI-H). Computer simulations predict a value of 29(2) mJ/m2, in excellent agreement with experimental data. The method is based on the use of a mold of attractive wells located at the crystallographic positions of the oxygen atoms of water molecules in equilibrium hydrate structures to induce the formation of a thin hydrate slab in the liquid phase at coexistence conditions. We propose here a new implementation of the mold integration technique using a mold of attractive wells located now at the crystallographic positions of the carbon atoms of the CO2 molecules in the equilibrium hydrate structure. We find that the new mold integration-guest methodology, which does not introduce positional or orientational information of the water molecules in the hydrate phase, is able to induce the formation of CO2 hydrates in an efficient way. More importantly, this new version of the method predicts a CO2 hydrate-water interfacial energy value of 30(2) mJ/m2, in excellent agreement with experimental data, which is also fully consistent with the results obtained using the previous methodology.