The cages, dynamics, and structuring of incipient methane clathrate hydrates

Abstract

Interest in describing clathrate hydrate formation mechanisms spans multiple fields of science and technical applications. Here, we report findings from multiple molecular dynamics simulations of spontaneous methane clathrate hydrate nucleation and growth from fully demixed and disordered two-phase fluid systems of methane and water. Across a range of thermodynamic conditions and simulation geometries and sizes, a set of seven cage types comprises approximately 95% of all cages formed in the nucleated solids. This set includes the ubiquitous 5(12) cage, the 5(12)6(n) subset (where n ranges from 2-4), and the 4(1)5(10)6(n) subset (where n also ranges from 2-4). Transformations among these cages occur via water pair insertions/removals and rotations, and may elucidate the mechanisms of solid-solid structural rearrangements observed experimentally. Some consistency is observed in the relative abundance of cages among all nucleation trajectories. 5(12) cages are always among the two most abundant cage types in the nucleated solids and are usually the most abundant cage type. In all simulations, the 5(12)6(n) cages outnumber their 4(1)5(10)6(n) counterparts with the same number of water molecules. Within these consistent features, some stochasticity is observed in certain cage ratios and in the long-range ordering of the nucleated solids. Even when comparing simulations performed at the same conditions, some trajectories yield swaths of multiple adjacent sI unit cells and long-range order over 5 nm, while others yield only isolated sI unit cells and little long-range order. The nucleated solids containing long-range order have higher 5(12)6(2)/5(12) and 5(12)6(3)/4(1)5(10)6(2) cage ratios when compared to systems that nucleate with little long-range order. The formation of multiple adjacent unit cells of sI hydrate at high driving forces suggests an alternative or addition to the prevailing hydrate nucleation hypotheses which involve formation through amorphous intermediates.

Some current challenges in clathrate hydrate science: Nucleation, decomposition and the memory effect

“…For instance, in simulations it is quite commonly observed that in addition to the thermodynamically stable phase that is observed during synthetic procedures other metastable hydrate phases are formed [46,47,36], in agreement to the appearance of mixed phases in modeling studies. In other experiments where nucleation and growth takes place rather slowly (min/h), cage populations can be measured as a function of time.…”

“…2 [35]. Recent work demonstrated that the stable sI clathrate hydrate structure can be obtained from some trajectories of the homogeneous nucleation simulation [36,37]. Increased curvature of the water-gas interface was shown to increase the rate of clathrate hydrate nucleation [30].…”

Clathrate structure-type recognition: Application to hydrate nucleation and crystallisation

“…5,28,29 Radhakrishnan and Trout defined a further order parameter based on the number of guests at a suitable distance from the central one, i.e., within the domain of the first peak of the guest-guest pair correlation function of hydrates. 4 Further, geometric hydrate-liquid recognition criteria have focussed on identification of hydrate-like cavities, either polyhedral (i.e., 5 12 , 5 12 6 2 , 5 12 6 3 , or 5 12 6 4 ) 10,16,30,31 or halfpolyhedral (i.e., 5 6 , 5 6 6 1 , or 5 6 6 2 ). 32 Chakraborty et al defined Voronoi tessellation of the space centred around guest molecules.…”

Evidence from mixed hydrate nucleation for a funnel model of crystallization

“…Ice nuclei are almost solely comprised of double-diamond and hexagonal water cages as illustrated in figures 5 and 6 of ref. 19, whereas hydrate nuclei contain many more different types of water cages (25,26,35). From the perspective of combinatorics, there are many more possible hydrogen-bond structures for hydrate nuclei than there are for ice nuclei.…”