Free energy landscape and molecular pathways of gas hydrate nucleation

Abstract

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 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 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.

Formation of inclusion type silicon phases induced by inert gases

“…To identify clathrate-like Si atoms in liquid, we employ the half-cage order parameter (H-COP) 21,22 . H-COP was developed to distinguish hydrate-like water molecules in liquid on the basis of topological analysis of the tetrahedral network, by identifying half polyhedral cages of water molecules, namely, the unique building blocks of clathrate hydrates.…”

“…This typically occurs when the nucleation barrier becomes sufficiently small, for example, under high supercooling conditions, so that the spontaneous crossing becomes frequent within the typical temporal and spatial scale of direct simulation. Alternatively, one may use an advanced sampling method, such as the forward flux sampling (FFS) method 20 , to explore crystallization under more realistic conditions 21,22 , but the computational cost may also be exceedingly demanding for the scope of this study.…”

“…The crystallization event is detected by a gradual decrease of potential energy, which is a typical signature of a disorder-order transition. Subsequent analysis of the structure using the half-cage order parameter 21,22 , which was developed to distinguish hydrate-like water, indeed shows that clathrate forms spontaneously. As shown in Fig.…”

Forward-flux sampling with jumpy order parameters

“…Here, Q is the configuration space of the underlying system, and A and B are two of its local free energy minima. In recent years, FFS has been used for studying a wide range of rare-event-driven phenomena such as evaporation [18][19][20], coalescence [21], wetting [22], magnetic switching [23], protein folding [24], DNA hybridization [25,26], phase separation in active systems [27], protein aggregation [28] and crystal nucleation [29][30][31][32][33][34][35][36][37][38][39][40][41][42].…”

Bridging solution properties to gas hydrate nucleation through guest dynamics

“…The mechanism of gas hydrate nucleation has received considerable attention in recent times. [7][8][9][10][11][12][13][14][15] The first gas hydrate nucleation hypothesis, known as the labile cluster hypothesis (LCH), was proposed by Sloan and coworkers. 7 This hypothesis imagined that water cages formed around the guest molecules in solution and then tended to combine to form the crystal.…”