Can Amorphous Nuclei Grow Crystalline Clathrates? The Size and Crystallinity of Critical Clathrate Nuclei

Abstract

Recent studies reveal that amorphous intermediates are involved in the formation of clathrate hydrates under conditions of high driving force, raising two questions: first, how could amorphous nuclei grow into crystalline clathrates and, second, whether amorphous nuclei are intermediates in the formation of clathrate crystals for temperatures close to equilibrium. In this work, we address these two questions through large-scale molecular simulations. We investigate the stability and growth of amorphous and crystalline clathrate nuclei and assess the thermodynamics and kinetic factors that affect the crystallization pathway of clathrates. Our calculations show that the dissociation temperature of amorphous clathrates is just 10% lower than for the crystals, facilitating the formation of metastable amorphous intermediates. We find that, at any temperatures, the critical crystalline nuclei are smaller than critical amorphous nuclei. The temperature dependence of the critical nucleus size is well described by the Gibbs-Thomson relation, from which we extract a liquid-crystal surface tension in excellent agreement with experiments. Our analysis suggests that at high driving force the amorphous nuclei may be kinetically favored over crystalline nuclei because of lower free energy barriers of formation. We investigated the role of the initial structure and size of the nucleus on the subsequent growth of the clathrates, and found that both amorphous and sI crystalline nuclei yield crystalline clathrates. Interestingly, growth of the metastable sII crystal polymorph is always favored over the most stable sI crystal, revealing kinetic control of the growth and indicating that a further step of ripening from sII to sI is needed to reach the stable crystal phase. The latter results are in agreement with the observed metastable formation of sII CO(2) and CH(4) clathrate hydrates and their slow conversion to sI under experimental conditions.

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

“…The coarse-grained mW water was used in previous studies to investigate the stability, growth, and nucleation of clathrate hydrates. 10,29,52,[54][55][56][57][60][61][62] In this work, we show that, under conditions of identical driving force, the sII clathrate crystal is the fastest growing polymorph followed by the TS, HS-I, and sI crystals. We find that nucleation and growth of a distinct interfacial transition layer at the seed/liquid interface is necessary in order for cross-nucleation to occur from or to sII and any of TS, HS-I, and sI, but not among the latter three.…”

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

“…For methane hydrates, using biased MD (owing to the larger free energy barriers involved for nucleation vis-à-vis the more hydrophilic, electrostatically interacting, and dipolar H 2 S in H 2 S clathrate), a marked kinetic preference for sII-like nano-crystallites is evident, despite sI being the thermodynamically stable form. 25 At first glance, this may seem somewhat surprising, but this exploration of other shapes, or indeed (quasi-) amorphous structures during nucleation has been observed, 9,10,[13][14][15][16]25 owing to relatively accessible free-energy barriers in between different cage motifs and polymorphic environments.…”

“…1,2 A unit cell in sII hydrate consists of 136 water molecules forming sixteen small 5 12 cages and eight large hexadecahedral 5 12 6 4 cages. 1,2 In recent years, the molecular simulation of clathrate hydrates has revealed much about their structural properties, 3 and Monte Carlo 4 and molecular dynamics (MD) simulations, whether brute force (i.e., standard) [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] or biased, [22][23][24][25] of hydrate formation have revealed a propensity for either (approximately) sI or sII polymorphs to form, despite one form typically being most thermodynamically stable for a given guest type (and temperature/pressure). Given that such simulation can provide molecular-level insights into hydrate-formation dynamics, this allows for probing the structural nature of the growing hydrate a) Authors to whom correspondence should be addressed.…”

CH-Stretching Vibrational Trends in Natural Gas Hydrates Studied by Quantum-Chemical Computations

“…[19][20][21] Theoretically, the nucleation and growth process of methane hydrate have been simulated and explored on the atomic and molecular level through molecular dynamics simulations. [23][24][25][26][27][28][29][30] The interaction between host water cages and guest molecules, 32,41 stability, diffusion and vibrations of guest molecules in water cavities of clathrate hydrates, 31,34,[38][39][40][42][43] and phase transition between ice and methane clathrates have been studied by firstprinciple calculations. 37,44 Experimentally, Raman spectra of hydrocarbon hydrates have demonstrated that the CH stretching frequency of the guest molecule is commonly (but not always) lower when in a large cage than when in a small cage [13][14][15] .…”