Molecular Insights into Guest and Composition Dependence of Mixed Hydrate Nucleation

Multicomponent crystals, for which natural gas hydrates are important examples, have drawn considerable attention because of their scientific and industrial importance. By performing large-scale molecular dynamics simulations on hydrate nucleation of several common natural gases (CH4, C2H6, C3H8, and H2S) and their mixtures, we aim to elucidate the roles of gas molecules and their compositions on mixed hydrate nucleation. We find that at a fixed temperature and solution composition, the induction time (tau) sequence for pure guest hydrate nucleation is tau(C2H6) < tau(C3H8) < tau(CH4) < tau(H2S). A relatively small number of guest molecules with a shorter induction time for pure guest hydrate nucleation can promote the formation of initial nuclei of a mixed hydrate. Detailed cage analysis shows that small guests such as CH4 can induce the formation of more standard cages for mixture systems with either C2H6 or C3H8 systems, thereby reducing the formation of nonstandard cages and impacting the nucleation kinetics. Enrichment of CH4 in the nucleated cluster of C2H6/CH4 and C3H8/CH4 mixed hydrates was also observed. Four cage type subsets, i.e., 5(12)6(a), 4(1)5(10)6(b), 4(2)5(8)6(c), and 4(3)5(6)6(d) (with a ranging from 0, 2, 3, 4; b from 2 to 6; c from 0 to 6; and d from 0 to 6) are identified as dominating the complete-cage populations for the hydrates formed in this study. Moreover, four-membered water rings appear to play an increasingly larger role in nucleated clusters as the size of guest molecules becomes larger.