Local structuring nucleation hypothesis

Molecular simulation methods have been applied to investigate the nucleation mechanism of gas hydrates in the bulk water phase (Baez and Clancy, 1994), and more recently at the water-hydrocarbon interface (Radhakrishnan and Trout, 2002 Moon et al., 2003). The recent simulations performed at the water-hydrocarbon interface provide support for a local structuring nucleation hypothesis, rather than the previously described labile cluster model. 

Thermal fluctuations cause a group of guest (CO2) molecules to be arranged in a configuration similar to that in the clathrate hydrate phase. The structure of water molecules around locally ordered guest molecules is perturbed compared to that in the bulk. The thermodynamic perturbation of the liquid phase is due to the finite temperature of the system. This process is stochastic. 

The number of guest molecules in a locally ordered arrangement exceeds that in the critical nucleus. Guest-guest and host-host cluster order parameters take on values that are very close to the clathrate hydrate phase, which results in formation of a critical nucleus. 

FIGURE 3.12 Methane-methane radial distribution functions calculated from successive 0.9 ns portions of the simulation, indicating ordering of the methane molecules during hydrate nucleation. (Reproduced from Moon, C., Taylor, P.C., Rodger, P.M., J. Am. Chem. Soc., 125, 4706 (2003). With permission from the American Chemical Society.) 

Despite the formation of clathrate-like clusters and complete 512 cages during these simulations, the increased ordering observed from the radial distribution functions and local phase assignments resulted in the authors concluding that their simulation results are consistent with a local order model of nucleation, and therefore do not support the labile cluster model. 

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A hypothesis picture of hydrate growth at a crystal is shown in Figure 3.21, modified from Elwell and Scheel (1975). This conceptual picture for crystal growth may be combined with either the labile cluster or local structuring hypotheses for nucleation. 

We find that the nucleation proceeds via "the local structuring mechanism/ " i.e., a thermal fluctuation causing the local ordering of CO2 molecules leads to the nucleation of the clathrate, and not by the labile cluster hypothesis, one current conceptual picture. The local ordering of the guest molecules induces ordering of the host molecules at the nearest- and next-to-nearest-neighbor shells, which are captured by a three-body host-host order parameter, f these thermodynamic fluctuations lead to the formation of the critical nucleus. 

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