Propane and Water: The Cooperativity of Unlikely Molecules to Form Clathrate Structures

A Molecular Dynamics Study on Xe/Kr Separation Mechanisms Using Crystal Growth Method

The separation of xenon/krypton gas mixtures is a valuable but challenging endeavor in the gas industry due to their similar physical characteristics and closely sized molecules. To address this, we investigated the effectiveness of the hydrate-based gas separation method for mixed Xe-Kr gas via molecular dynamics (MD) simulations. The formation process of hydrates facilitates the encapsulation of guest molecules within hydrate cages, offering a potential strategy for gas separation. Higher temperatures and pressures are advantageous for accelerating the hydrate growth rate. The final occupancy of guest molecules and empty cages within 512, 51264, and all hydrate cages were thoroughly examined. An increase in the pressure and temperature enhanced the occupancy rates of Xe in both 512 and 51264 cages, whereas elevated pressure alone improved the occupancy of Kr in 51264 cages. However, the impact of temperature and pressure on Kr occupancy within 512 cages was found to be minimal. Elevated temperature and pressure resulted in a reduced occupancy of empty cages. Predominantly, 51264 cages were occupied by Xe, whereas Kr showed a propensity to occupy the 512 cages. With increasing simulated pressure, the final occupancy of Xe molecules in all cages rose from 0.37 to 0.41 for simulations at 260 K, while the final occupancy of empty cages decreased from 0.24 to 0.2.

Mesomorphology of clathrate hydrates from molecular ordering

Clathrate hydrates are crystals formed by guest molecules that stabilize cages of hydrogen-bonded water molecules. Whereas thermodynamic equilibrium is well described via the van der Waals and Platteeuw approach, the increasing concerns with global warming and energy transition require extending the knowledge to non-equilibrium conditions in multiphase, sheared systems, in a multiscale framework. Potential macro-applications concern the storage of carbon dioxide in the form of clathrates, and the reduction of hydrate inhibition additives currently required in hydrocarbon production. We evidence porous mesomorphologies as key to bridging the molecular scales to macro-applications of low solubility guests. We discuss the coupling of molecular ordering with the mesoscales, including (i) the emergence of porous patterns as a combined factor from the walk over the free energy landscape and 3D competitive nucleation and growth and (ii) the role of molecular attachment rates in crystallization-diffusion models that allow predicting the timescale of pore sealing. This is a perspective study that discusses the use of discrete models (molecular dynamics) to build continuum models (phase field models, crystallization laws, and transport phenomena) to predict multiscale manifestations at a feasible computational cost. Several advances in correlated fields (ice, polymers, alloys, and nanoparticles) are discussed in the scenario of clathrate hydrates, as well as the challenges and necessary developments to push the field forward.

A Clathrate Hydrate Structure Hidden in Plain Sight

Clathrate hydrates have diverse crystal structures, and among them, the three (sI, sII, and sH) most prevalent ones cover nearly all known structures, while the norm is to consider other structures only when specific guest molecules are present. Here we report the observation of a hidden clathrate structure: the tetragonal structure (TS-I) in commonly formed gas hydrates, as evidenced from molecular dynamics simulations. We show that when two (or more) sI crystal grains with different growth directions come into contact or when the growth of a sI crystal encounters geometrical frustration, the TS-I results as a cocrystal. We give evidence that TS-I may also play an important role in the combination and/or transition between sI and sII. These results imply that this previously neglected structure may be commonly present whenever sI or sII is formed. This hidden structure must be identified, experimentally and in simulations; confining the possible structures may hinder an in-depth understanding of clathrate hydrates.

Molecular behavior of hybrid gas hydrate nucleation: separation of soluble H2S from mixed gas

Soluble H₂S widely exists in natural gas or oil potentially corroding oil/gas pipelines. Furthermore, it can affect the hydrate formation condition, resulting in pipeline blockage; the nucleation mechanism from mixed gas including H₂S is still largely unclear. Molecular dynamics simulations were performed to reveal the effects of different initial mixed H₂S/CH₄ compositions on the hydrate nucleation and growth process. The geometric details of the nanobubbles and gas composition in the nanobubbles were analyzed; the size of the nanobubbles was found to decrease from 3.4 nm to 1.4 nm. With the increase in the initial H₂S proportion, the diameter of the nanobubbles decreased; more guest molecules were dissolved in the water, which improved the initial concentration of guest molecules in the water. A multi-site nucleation process was observed, and separate hydrate clusters could grow independently until the simulation box limited their growth due to high local H₂S concentration as a potential nucleation location. When the initial proportion of mixed gas approaches, H₂S preferred to occupy and stabilize the incipient cage. Moreover, 5¹², 4¹5¹⁰6², and 5¹²6² cages accounted for approximately 95% of the first hydrate cage. Nucleation rates were shown to increase from 4.62 × 10²⁴ to 9.438 × 10²⁶ nuclei cm^-3 s^-1. The present high subcooling and H₂S concentration provided a high driving force to promote mixed hydrate nucleation and growth. The proportion of cages occupied by H₂S increased with increasing initial H₂S proportion, but the largest enrichment factor of 1.38 occurred at 10% initial H₂S/CH₄ mixed gas.

Homogenous nucleation rate of CO2 hydrates using transition interface sampling

Carbon dioxide and water can form solid clathrate structures in which water cages encapsulate the gas molecules. Such hydrates have sparked much interest due to their possible application in CO₂ sequestration. How the solid structure forms exactly from the liquid phase via a homogenous nucleation process is still poorly understood. This nucleation event is rare on the molecular timescale even under moderate undercooling or supersaturation conditions because of the large free energy barrier toward crystallization, rendering a brute force simulation of hydrate nucleation unfeasible for moderate undercooling or supersaturation. Here, we perform transition interface sampling simulations to quantify the homogenous nucleation rate for CO₂ hydrate formation using accurate atomistic force fields at 500 bars for three different temperatures between 260 and 273 K. Collecting more than 100 000 pathways comprising roughly two milliseconds of simulation time, we computed a nucleation rate in the amorphous phase of ∼10²¹ nuclei s^-1 cm^-3 for a temperature of 260 K and a rate of ∼10¹² nuclei s^-1 cm^-3 for a temperature of 265 K. For a temperature of 273 K, we find that the hydrate forms an sI crystalline phase with a rate of order of ∼10¹ nuclei s^-1 cm^-3. We compare these rates to classical nucleation theory estimates as well as experiments, and to nucleation rate estimates for methane hydrates and discuss possible causes of the observed differences. Our findings shed light on the kinetics of this important clathrate and should assist in future hydrate formation investigation.

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

Carbon dioxide hydrate is a solid built from hydrogen-bond stabilized water cages that encapsulate individual CO2 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 CO2 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 CO2 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 4151062 cages being the most abundant. With increasing temperature, the nucleation mechanism changes, and 51262 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.