Hydrate Nucleation in Water Nanodroplets: Key Factors and Molecular Mechanisms

Clathrate hydrate nucleation in heterogeneous systems, such as the water-in-oil emulsions found in pipeline environments, is of considerable technological importance and scientific interest. While there has been a number of experimental studies investigating hydrate nucleation in water-in-oil emulsions, there have been essentially no molecular simulations to provide important molecular insights into the hydrate nucleation process. Here, we report extensive molecular dynamics simulations of gas hydrate nucleation to examine nucleation behavior in water nanodroplets immersed in a non-aqueous liquid, probing key factors impacting nucleation, including guest species, guest compositions, size of the nanodroplets, and temperature. The nucleation behavior with pure-guest (i.e., H₂S, C₃H₈, and CO₂) and H₂S-containing mixtures, where the second guest species is one of C₃H₈, CH₄, C₂H₆, and CO₂, has been studied. For the various systems examined in this study, we find that H₂S always tends to initiate hydrate formation, with the only exception being the H₂S/CO₂ mixture, where the relatively high solubility of H₂S compared to the other guest species is identified as an important factor for the current systems. Three different sizes of water nanodroplets at different temperatures are used to examine hydrate nucleation with pure-H₂S guest systems, where the observed mechanism of hydrate nucleation within nanodroplets exhibits behavior similar to that found in bulk counterparts. Within water nanodroplets, the hydrate nucleation process features the initial formation of amorphous solids, which can then be annealed into more recognizable hydrate-like structures. Detailed cage analyses provide insights into the impacts of temperature and the size of the water nanodroplet on the initial location and the induction time of hydrate nucleation. Our simulations improve the understanding of the molecular mechanism of clathrate hydrate nucleation in water-in-oil emulsions, thus helping the development of hydrate-related applications and exploitation.