Molecular Understanding of Homogeneous Nucleation of CO₂ Hydrates Using Transition Path Sampling

Abstract

Carbon dioxide hydrate is a solid built from hydrogen-bond stabilized water cages that encapsulate individual CO 2 molecules. As potential candidates for reducing greenhouse gases, hydrates have attracted attention from both the industry and scientific community. Under high pressure and low temperature, hydrates are formed spontaneously from a mixture of CO 2 and water via nucleation and growth. Yet, for moderate undercooling, i.e., moderate supersaturation, studying hydrate formation with molecular simulations is very challenging due to the high nucleation barriers involved. We investigate the homogeneous nucleation mechanism of CO 2 hydrate as a function of temperature using transition path sampling (TPS), which generates ensembles of unbiased dynamical trajectories across the high barrier between the liquid and solid states. The resulting path ensembles reveal that at high driving force (low temperature), amorphous structures are predominantly formed, with 4 1 5 10 6 2 cages being the most abundant. With increasing temperature, the nucleation mechanism changes, and 5 12 6 2 becomes the most abundant cage type, giving rise to the crystalline sI structure. Reaction coordinate analysis can reveal the most important collective variable involved in the mechanism. With increasing temperature, we observe a shift from a single feature (size of the nucleus) to a 2-dimensional (size and cage type) variable as the salient ingredient of the reaction coordinate, and then back to only the nucleus size. This finding is in line with the underlying shift from an amorphous to a crystalline nucleation channel. Modeling such complex phase transformations using transition path sampling gives unbiased insight into the molecular mechanisms toward different polymorphs, and how these are determined by thermodynamics and kinetics. This study will be beneficial for researchers aiming to produce such hydrates with different polymorphic forms.

Reduced phase stability and faster formation/dissociation kinetics in confined methane hydrate

“…For bulk methane hydrate, it has been shown that nucleation can proceed through direct crystallization from a solid nucleus or through transformation of an amorphous phase first appearing in the solution (46,47). The observation of these two processes-either one-step crystallization or two-step crystallization through the formation of an amorphous phase-was shown to depend on temperature with the two molecular mechanisms competing at moderate undercooling (32).…”

The Seven Deadly Sins: when computing crystal nucleation rates, the devil is in the details

“…In contrast, path sampling-based techniques such as transition path sampling (TPS) [42][43][44] , transition interface sampling (TIS) [45][46][47] and forward flux sampling (FFS) 10,20,21,48,49 all allow for the direct calculation of nucleation rates. Interestingly, until quite recently not many examples of any of these methodologies being applied to the calculation of J could be found in the literature, most likely due to the high computational cost associated with converging these algorithms.…”

“…Forward-Flux Sampling (FFS) in particular has become the principle workhorse of many rare event studies 10,143 . Alternative techniques such as transition interface sampling (TIS) are also used in the context of nucleation 43 , but generally acknowledged to be somewhat more complex to implement. The aimless shooting approach to path sampling is a third example 144 , used in a number of nucleation studies 122 .…”

Homogenous nucleation rate of CO2 hydrates using transition interface sampling

“…At high driving force, the CO 2 hydrates are known to nucleate into amorphous polymorphs in which the 4 1 5 10 6 2 cage type is most abundant. 8,9 At higher pressures (6-18 kbar), another stable crystalline hydrate structure can form, characterized by helicoidal channels. 10 Precise knowledge of the molecular kinetics of formation of these (synthetic) hydrates allows a better control of this process.…”

“…The height of this barrier is determined by the liquid-solid surface tension and the driving force. The barrier height can easily be many times the thermal energy, rendering the nucleation an unlikely, or rare, event 9,[12][13][14] to observe computationally. Only due to rare thermal fluctuations, a nucleus of a size corresponding to the free energy maximum, the so-called critical nucleus, emerges.…”