In this study, experimental data on methane hydrate phase equilibria containing electrolytes, sodium chloride (NaCl), potassium chloride (KCl), and ammonium chloride (NH4Cl) were measured for ...concentrations up to about 10wt%. The concentration of the aqueous salt solution in the system containing hydrate and salt solution is continuously changed during the hydrate formation and dissociation processes, so applying the isochoric method with the continuously temperature ramping to measure hydrate phase equilibria in the presence of salts may be unsuitable for precise and accurate measurements. An isochoric method, using a step-wise increase of temperature with sufficient equilibration time at every step, was introduced for measuring the hydrate equilibrium conditions for these systems containing electrolytes. To compare the results from the isochoric method, measurements using a high-pressure differential scanning calorimetry (DSC) were also performed using the same step-wise increase of temperature method. The results from both isochoric and DSC methods showed good agreement. The effects of the cation in the electrolyte on the hydrate inhibition were identified through the measurements, showing that hydrate inhibition strength by the sodium cation was slightly stronger than that of potassium and ammonium cations.
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Many unanswered questions still exist at the molecular level to understand the nucleation process and mechanism of clathrate hydrates, especially for larger guest molecules that would result in the ...structure II crystal. Here, we report on molecular dynamics simulations for propane and water to describe the molecular mechanism leading to a structure II system. Through a large number (30) of long (5 μs) and coupled annealing (20 μs) simulations, we detail the prenucleation, nucleation, growth, and annealing of propane clathrate hydrate structures at 250 K and 1800 bar. The results demonstrate the equal importance of the empty and occupied cages in the nucleation of propane hydrates. The critical nucleus size is identified to be eight cages. While separate distinct clusters may exist during the prenucleation period, only one survives to grow beyond the critical nucleus size, with the others remaining subcritical. From the annealing simulations, it is clear that solid rearrangement is a very slow process, and 20 μs is still not long enough to capture long-range ordering resembling the structure II crystal. These results, along with the developed analysis method, have a significant impact in advancing our understanding of the nucleation process for unlike molecules.
Type II clathrate hydrates are prevalent in numerous applications including gas production, gas storage, and seawater desalination. However, less focus has been given to understanding the molecular ...mechanism for the nucleation of type II clathrate hydrates, especially for mixed systems with dissimilar-sized molecules. Through a number of systematic molecular dynamic simulations for systems with different initial methane/propane content, we report here detailed molecular resolution into the mechanism of nucleation and crystal growth of these mixed clathrate structures. In addition to identifying the critical nucleus size for the nucleation, we determine the different uptake rates of methane and propane molecules into the solid phase, which were found to be uncorrelated between the solution composition of these species and the occupancy of the first cage to form or the occupancy in the growing solid clathrate phase. The evolution of the cages formed in the solid is tracked, and the coexistence of type I and type II structures is captured. The results show that the type I patterns appear earlier than type II even though thermodynamically the latter is preferred. The molecular details revealed here, along with new methods developed in this study, advance our understanding of the nucleation and crystal growth of clathrate structures, bringing new insights into the molecular events, structure, and ordering in the presence of molecules enclathrated in different-sized cages.
Clathrate hydrates are a class of ordered structures that are stabilized via the delicate balance of hydrophobic interactions between water and guest molecules, of which the space-filling network of ...hydrogen-bonded (H-bonded) water molecules are closely related to tetrahedrally close-packed structures, known as Frank-Kasper (FK) phases. Here we report an alternative way to understand the intricate structures of clathrate hydrates, which unveils the diverse crystalline H-bonded networks that can be generated via assembly of one common building block. In addition to the intrinsic relations and pathways linking these crystals, we further illustrate the rich structural possibilities of clathrate hydrates. Given that the topological dual relations between networks of clathrate hydrates and tetrahedral close-packed structures, the descriptors presented for clathrate hydrates can be directly extended to other ordered materials for a more thorough understanding of their nucleation, phases transition, and co-existence mechanisms.
Clathrate hydrates have steadily emerged as an important field in the areas of flow assurance, energy storage and resource, and environment. To better understand the role of hydrates in all of these ...areas, knowledge developed in laboratory experiments must be effectively transferred to address the challenges related to hydrate formation, dissociation, agglomeration, and stability. This paper highlights the recent hydrate literature focusing on the thermodynamics, kinetics, structural properties, particle properties, rheological properties, and molecular mechanisms of formation. The foundation for continued understanding and development of hydrates in engineering practice will rely on laboratory measurements utilizing traditional and innovative tools capable of probing time-dependent and time-independent properties.
Position and orientation of water protons need to be specified when the molecular simulation studies are performed for clathrate hydrates. Positions of oxygen atoms in water are experimentally ...determined by X-ray diffraction analysis of clathrate hydrate structures, but positions of water hydrogen atoms in the lattice are disordered. This study reports a determination of the water proton coordinates in unit cell of structure I (sI), II (sII), and H (sH) clathrate hydrates that satisfy the ice rules, have the lowest potential energy configuration for the protons, and give a net zero dipole moment. Possible proton coordinates in the unit cell were chosen by analyzing the symmetry of protons on the hexagonal or pentagonal faces in the hydrate cages and generating all possible proton distributions which satisfy the ice rules. We found that in the sI and sII unit cells, proton distributions with small net dipole moments have fairly narrow potential energy spreads of about 1 kJ∕mol. The total Coulomb potential on a test unit charge placed in the cage center for the minimum energy∕minimum dipole unit cell configurations was calculated. In the sI small cages, the Coulomb potential energy spread in each class of cage is less than 0.1 kJ∕mol, while the potential energy spread increases to values up to 6 kJ∕mol in sH and 15 kJ∕mol in the sII cages. The guest environments inside the cages can therefore be substantially different in the sII case. Cartesian coordinates for oxygen and hydrogen atoms in the sI, sII, and sH unit cells are reported for reference.
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.
Despite the industrial implications and worldwide abundance of gas hydrates, the formation mechanism of these compounds remains poorly understood. We report direct molecular dynamics simulations of ...the spontaneous nucleation and growth of methane hydrate. The multiple-microsecond trajectories offer detailed insight into the process of hydrate nucleation. Cooperative organization is observed to lead to methane adsorption onto planar faces of water and the fluctuating formation and dissociation of early hydrate cages. The early cages are mostly face-sharing partial small cages, favoring structure II; however, larger cages subsequently appear as a result of steric constraints and thermodynamic preference for the structure I phase. The resulting structure after nucleation and growth is a combination of the two dominant types of hydrate crystals (structure I and structure II), which are linked by uncommon 5¹²6³ cages that facilitate structure coexistence without an energetically unfavorable interface.
Molecular dynamics simulations were performed for CO2 dissolved in water near silica surfaces to investigate how the hydrophilicity and crystallinity of solid surfaces modulate the local structure of ...adjacent molecules and the nucleation of CO2 hydrates. Our simulations reveal that the hydrophilicity of solid surfaces can change the local structure of water molecules and gas distribution near liquid-solid interfaces, and thus alter the mechanism and dynamics of gas hydrate nucleation. Interestingly, we find that hydrate nucleation tends to occur more easily on relatively less hydrophilic surfaces. Different from surface hydrophilicity, surface crystallinity shows a weak effect on the local structure of adjacent water molecules and on gas hydrate nucleation. At the initial stage of gas hydrate growth, however, the structuring of molecules induced by crystalline surfaces are more ordered than that induced by amorphous solid surfaces.
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