A sound knowledge of the effects of clay surfaces and salt ions on CH 4 hydrate formation is vital to understand the formation of natural gas hydrate resources and develop hydrate-based technologies, such as seawater desalination. Herein, microsecond simulations are conducted to investigate CH 4 hydrate formation from fresh water and NaCl solution in kaolinite nanopores and the outside bulk phase. Simulation results show that the gibbsite surface shows strong affinity for water and salt ions, while the siloxane surface exhibits certain affinity for CH 4 and hence affects hydrate formation. With two siloxane surfaces in the nanopore, hydrate formation is severely inhibited by the significantly lowered aqueous CH 4 concentration, caused by surface adsorption of CH 4 to form nanobubbles; salt ions greatly enhance such an inhibitory effect by promoting surface nanobubble formation and suppressing CH 4 dissolution from nanobubbles. However, with one siloxane surface and one gibbsite surface presented in the nanopore, no nanobubble forms on the siloxane surface, no matter salt ions are present or not, and the inhibitory effects caused by the siloxane surface are not observed. In contrast, gibbsite surfaces are more beneficial to hydrate formation, and salt ions have little effect as most ions adsorb to the surface. Hydrate formation outside of the nanopores is obviously inhibited by salt ions, especially at high salt concentrations. Some interesting phenomena are observed, such as stochastic formation of large sI and sII hydrate domains, which then blocks the nanopore throat and hinders the mass transfer of CH 4 and water, and sustained growth of large hydrate solids outside of the nanopore causes decomposition of small hydrate solids in the nanopore. Additionally, densification of salt ions in solution and formation of ion-containing hydrate cages are systematically analyzed to figure out the surface effects of kaolinite on the desalination process. These molecular insights are of scientific and engineering significance to sustainable chemistry related to exploitation of natural gas hydrate and hydrate-based seawater desalination technology.
Hydrate formation of a natural gas mixture is fascinating. Whereas both pure methane and ethane form a structure I (sI) hydrate, their mixture may form a structure II (sII) hydrate at certain compositions. Here, we investigated the underlying mechanisms of the methane–ethane mixture hydrate structural transition using an sII-hydrate–water–hydrocarbon three-phase interface system. The results indicate that sII hydrate formation is a function of methane concentration with a maximum at a mole concentration of 83–90%. In contrast, a significant amount of the sI hydrate forms at concentrations below 75% or equal to 100%. In addition, we unveil the mechanism of the rarely discussed structure conversion from sII to sI. The 15-hedron (mainly 51263) cages, which may originate from both aqua and the 16-hedron (51264) cage transition, play a vital role as a transitional bridge in the conversion at the transition interface. Owing to the specific structure of 15-hedron cages, the 14-hedron (51262) cages can grow, and thus, the sI hydrate grows. Furthermore, the methane occupancy in the 14-hedron cages and the concentration for the energetically favorable formation of the sII structure were quantified based on the formation energies for hydrates with methane and ethane molecules trapped inside. We show that the adsorption preference of guest molecules at the interfaces or cages can be attributed to the difference between the formation energy prediction and the reaction process results. Our findings have both academic and engineering significance for sustainable chemistry related to flow assurance, gas separation, gas transportation, and natural hydrate development in reservoirs with mixed gas hydrate structures.
With the development of microscopy and sensor techniques, it becomes evident that nonswelling clays show swelling behavior under CO 2 − water mixture environments at high pressures and temperatures. The examples include Illite, muscovite, and kaolinite-rich rock samples. Here, we investigated the underlying mechanisms of kaolinite swelling induced by CO 2 and water using molecular simulations and low-pressure gas adsorption experiments. The results suggest the cooperative adsorption behavior of CO 2 and water on contact with kaolinite micropores, which have distinct wettabilities on the two adjoining interlayer surfaces. Even if clay-bound water exists, CO 2 can enter the micropores to induce swelling. The measured micropore volume, simulated equilibrium stable interlayer distance with pure water, and that with CO 2 −water mixture were used in the swelling estimation, which shows good agreement with our experiments. The CO 2 and water molecule distributions inside the interlayer micropores verify the importance of the wettabilities of the kaolinite surfaces in this cooperative adsorption behavior. The result extends the traditional understanding of the swelling mechanism, i.e., cation hydration and subsequent osmotic processes. In addition to earlier observations of kaolinite swelling behavior with potassium acetate, our study indicates the significance of the subtle balance of the noncovalent interactions between CO 2 , water, and the kaolinite Janus surfaces.
The formation of clathrate hydrates in pipelines is potentially threatening to exploration and gas transportation in the petroleum industry. To reduce such risks, various surfactants have been explored as anti-agglomerants to prevent the aggregation of hydrate particles. However, the anti-agglomeration mechanisms are not yet fully understood. In this study, modified atomic force microscopy is first developed to investigate the effects of two surfactants, namely, 1-naphthaleneacetic acid and dodecylbenzenesulfonic acid, on the surface of a tetrahydrofuran (THF) hydrate. The surfactants have remarkable effects in terms of changing the grain size and decreasing the grain boundary depth and surface roughness of the THF hydrate. In addition, the surfactants reduce the quasi-liquid layer (QLL) thickness of the THF hydrate and the adhesion forces between the hydrate and the microsphere probe. This phenomenon may be caused by the changes induced by surfactant molecules on the water–guest molecule structure near the gas/water interface. Thus, hydrate growth is enhanced in the QLL. The adhesion forces decrease linearly with QLL thickness after adding the surfactants. These findings indicate that surfactants reduce the adhesion forces by reducing the QLL thickness at higher temperatures. The effects of surfactants on the QLL thickness and surface morphology of the hydrate are investigated, providing critical information on the cohesive behaviors among hydrate particles or between hydrate particles with other materials. Our work provides new insights into the underlying mechanism of how surfactants prevent hydrate aggregation, which is crucial in hydrate-related safety management.
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