Hypothesis: The interfacial free energy between hydrate and water phases is a key thermodynamic parameterthat governs both nucleation kinetics and crystal growth of gas hydrates. In these strategic materials—crystallineinclusion compounds where hydrogen-bonded water cages encapsulate small guest molecules such as methane(CH4) and carbon dioxide (CO2) -this interfacial energy plays a crucial role in determining phase stabilityand formation pathways. Given the significance of gas hydrates in energy storage, CO2 sequestration, andclimate-related processes, accurately determining their interfacial energies is essential for advancing bothfundamental understanding and technological applications. Despite its importance, the hydrate–water interfacialenergy remains poorly constrained due to substantial experimental uncertainties and the limitations of indirectestimation methods. For example, reported experimental values for CH4 hydrate span a wide range from 28 to40 mJ/m2. Interestingly, some studies suggest these values are comparable to the interfacial free energy of thehexagonal ice (ice Ih)--water interface, approximately 32 mJ/m2, hinting at potential analogies between clathratehydrate and ice interfaces.Calculations: In this work, we present a direct molecular simulation of the CH4 hydrate–water interfacial freeenergy using two novel and independent extensions of the mold integration method. These extensions arespecifically designed to induce the formation of a thin, planar hydrate–water interface and to compute thereversible work required to create it. For this purpose, we employ the TIP4P/Ice force field—one of the mostreliable water models available—known for accurately reproducing the melting temperature of ice Ih underambient conditions. Findigns: Our results show that the interfacial free energy of CH4 hydrate is significantly higher than that of CO2hydrate, offering a natural explanation for their markedly different nucleation behaviors. This aligns with priorpredictions based on Classical Nucleation Theory, as well as advanced sampling techniques such as TransitionPath Sampling and Transition Interface Sampling. Notably, our computed value for CH4 hydrate approaches∼ 40mJ∕m2, in agreement with the upper bound of existing experimental estimates, while values for CO2 hydrateand ice Ih remain closer to ∼ 30mJ∕m2. This direct, theory-independent determination provides new insights intothe molecular mechanisms underlying hydrate formation and offers robust benchmarks for predictive modelingand the design of hydrate-based technologies.
Zerón et al. (Mon,) studied this question.