Free energy landscape and molecular pathways of gas hydrate nucleation

被引:66
作者
Bi, Yuanfei [1 ]
Porras, Anna [1 ]
Li, Tianshu [1 ]
机构
[1] George Washington Univ, Dept Civil & Environm Engn, Washington, DC 20052 USA
基金
美国国家科学基金会;
关键词
METHANE HYDRATE; DYNAMICS SIMULATIONS; ICE NUCLEATION; CRYSTAL NUCLEATION; SAMPLING METHOD; CO2; HYDRATE; WATER; LIQUIDS; GROWTH; SIZE;
D O I
10.1063/1.4961241
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Despite the significance of gas hydrates in diverse areas, a quantitative knowledge of hydrate formation at a molecular level is missing. The impediment to acquiring this understanding is primarily attributed to the stochastic nature and ultra-fine scales of nucleation events, posing a great challenge for both experiment and simulation to explore hydrate nucleation. Here we employ advanced molecular simulation methods, including forward flux sampling (FFS), p(B) histogram analysis, and backward flux sampling, to overcome the limit of direct molecular simulation for exploring both the free energy landscape and molecular pathways of hydrate nucleation. First we test the half-cage order parameter (H-COP) which we developed for driving FFS, through conducting the p(B) histogram analysis. Our results indeed show that H-COP describes well the reaction coordinates of hydrate nucleation. Through the verified order parameter, we then directly compute the free energy landscape for hydrate nucleation by combining both forward and backward flux sampling. The calculated stationary distribution density, which is obtained independently of nucleation theory, is found to fit well against the classical nucleation theory (CNT). Subsequent analysis of the obtained large ensemble of hydrate nucleation trajectories show that although on average, hydrate formation is facilitated by a two-step like mechanism involving a gradual transition from an amorphous to a crystalline structure, there also exist nucleation pathways where hydrate crystallizes directly, without going through the amorphous stage. The CNT-like free energy profile and the structural diversity suggest the existence of multiple active transition pathways for hydrate nucleation, and possibly also imply the near degeneracy in their free energy profiles among different pathways. Our results thus bring a new perspective to the long standing question of how hydrates crystallize. Published by AIP Publishing.
引用
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页数:10
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