Cross-nucleation between clathrate hydrate polymorphs: Assessing the role of stability, growth rate, and structure matching

Abstract

Cross-nucleation is a phenomenon where a new crystal nucleates and grows upon the surface of a different polymorph. Previous studies indicate that faster growth rate of the new crystal is a necessary but not sufficient condition for cross-nucleation. The thermodynamic stability of the different polymorphs can also affect cross-nucleation by modulating the rates of crystal growth. The interplay between thermodynamic stability of the polymorphs involved, the growth rate of the crystals, and the need for creation of an interfacial transition layer that seamlessly connects the two structures has not yet been fully elucidated. Predicting cross-nucleation is particularly challenging for clathrate hydrates, for which there are sometimes several polymorphs with similar stability and for which growth rates are not known. In this work, we use molecular dynamics simulations to investigate which factor (stability, growth rate, or formation of interfacial transition layer) controls cross-nucleation between the four known Frank-Kasper clathrate hydrate polymorphs: sI, sII, TS, and HS-I. We investigate the growth and cross-nucleation of these four hydrates filled with a set of guest molecules that produce different order of stabilities for the four crystal structures. We determine that the growth rate of sII clathrate is the fastest, followed by TS, HS-I, and sI. We find that cross-nucleation into or from sII clathrates is preceded by the formation of an interfacial transition layer at the seed crystal/liquid interface because sII does not share a crystal plane with sI, HS-I, or TS. Cross-nucleation between the latter three can occur seamlessly and is determined only by their growth rates. Our results indicate that nucleation of an interfacial transition layer between non-matching polymorphs can control cross-nucleation or lack thereof under conditions of small driving force. Under conditions of sufficient supercooling clathrate hydrate polymorphs cross-nucleate into the fastest growing phase even if that new phase is less stable and does not share a common crystal plane with the initial polymorph.

Nucleation of crystals that are mixed composites of all three polymorphs in the Gaussian core model

“…Despite its importance, the mechanism by which one polymorph is selected preferentially during crystallization is currently poorly understood, and the subject of active research. 1,[5][6][7][8][9]12 Here, our focus is on nucleation, the first stage of crystallization, during which a microscopically small nucleus of the crystal phase is formed in the metastable fluid phase. It is well known that what happens at nucleation can determine important physical properties of much larger crystals.…”

Communication: Structural interconversions between principal clathrate hydrate structures

“…The sI layer is now ready for continued growth as sI hydrate or further interconversions to another hydrate structure such as HS-I. 12,19,20 Another view of this structural interconversion can be found in the supplementary material. 47 The lattice parameter, 12 Å, of a cubic sI hydrate matches the 12.2 × 10.6 Å 2 repeat dimensions of the (101 0) plane of a sH hydrate very well in one dimension while having a discrepancy of over 10% in the other dimension.…”

“…We note that these observations were made directly from molecular dynamics trajectories, and hence, other possible transition pathways may exist and be identified in future studies. Possible structural interconversions between other less common gas hydrate phases, for example, HS-I, 12,20 TS, 19,21 and filled ice structures under high pressures 40,41 or those involving other related mineral structures such as silica crystalline structures, 42,43 are also worthy of further investigation. Similar to the direct structural interconversion between the sH and sII crystals observed here, crossnucleation between crystalline structures that share compatible crystallographic planes has been previously reported, such as for the face-centered cubic and the hexagonal close-packed structures 44 and the cubic and hexagonal ice structures.…”

“…The cross-nucleation between sI, sII, and two other clathrate hydrate structures, HS-I and TS, has been further investigated by Molinero and coworkers. 19,21 While these studies have provided insights into the mechanism and conditions for the cross-nucleation among these clathrate hydrate structures, the structural interconversions between sH and other hydrate structures have remained unclear, which are arguably more intriguing since sH contains tetragons (of 4 3 5 6 6 3 cages) that are absent in either sI or sII crystals, or the above-mentioned less common structures of HS-I and TS.…”

“…Cross-nucleation, which is a phenomenon where a new crystal forms upon the surface of a different crystalline phase, is important to many fields such as pharmaceutical research, solid-state physics and chemistry, and material sciences. 18,19 The cross-nucleation between sI and sII hydrate phases has been observed in molecular dynamics (MD) simulation investigation of crystal growth of methane hydrates. 20 This structural interconversion utilizes intermediate 5 12 6 3 cages, which are not native to either sI or sII crystals but can connect the hydrogenbonded networks of sI and sII crystals.…”