Assessing the Competition between Bulk and Interfacial Nucleation of H2S Hydrates via Molecular Dynamics

Understanding the mechanisms by which gas hydrates nucleate from aqueous solutions is essential for predicting and controlling their formation. Experimental observations have suggested that hydrate nucleation occurs preferentially at interfaces between the aqueous phase and hydrate-forming reservoirs. However, recent simulation studies have raised questions regarding the general validity of this assumption. Here, we investigate the spontaneous nucleation of H2S hydrates at 500 bar using molecular dynamics simulations employing the TIP4P/Ice water model and a four-site H2S potential, with the aim of clarifying the competition between bulk and interfacial nucleation pathways. To ensure rigorous control of the thermodynamic conditions, we first determine the three-phase coexistence temperature (T3) from the intersection of the aqueous solubility and hydrate-solution equilibrium curves; our calculated value of T3 = 314(3) K shows excellent agreement with an experimentally inferred value of approximately 316 K. Then, under deeply supercooled conditions, we directly compare nucleation kinetics across two distinct configurations: an interfacial system with an aqueous solution in contact with an H2S reservoir and a standalone bulk solution at identical saturation. Our results show that nucleation rates in bulk systems are comparable to those obtained in the presence of an interface, indicating that the interface does not promote hydrate nucleation under the conditions explored. This behavior is consistent with our previous findings for CO2 hydrates and suggests that bulk nucleation can be the prevailing pathway under deep supercooling conditions.