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

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.