Abstract
Natural gas hydrate is a promising future energy source, but it also poses a huge threat to oil and gas production due to its ability to deposit within and block pipelines. Understanding the atomistic mechanisms of adhesion between the hydrate and solid surfaces and elucidating its underlying key determining factors can shed light on the fundamentals of novel antihydrate materials design. In this study, large-scale molecular simulations are employed to investigate the hydrate adhesion on solid surfaces, especially with focuses on the atomistic structures of intermediate layer and their influences on the adhesion. The results show that the structure of the intermediate layer formed between hydrate and solid surface is a competitive equilibrium of induced growth from both sides, and is regulated by the content of guest molecules. By comparing the fracture behaviors of the hydrate–solid surface system with different intermediate structures, it is found that both the lattice areal density of water structure and the adsorption of guest molecules on the interface together determine the adhesion strength. Based on the analysis of the adhesion strength distribution, we have also revealed the origins of the drastic difference in adhesion among different water structures such as ice and hydrate. Our simulation indicates that ice-adhesion strength is approximately five times that of lowest hydrate adhesion strength. This finding is surprisingly consistent with the available experimental results.
Highlights
Natural gas hydrate is considered to be a future energy source
In almost all the studies involving solid surfaces, a hydrate nucleus can eventually form a stable conglutination by forming an intermediate layer or connecting with some functional groups at solid surfaces.[18−20] This microscopic propensity is the physical basis for the deposition of hydrates
The selection of such a surface can introduce into the system an ability to promote ice nucleation “bottom-up” in addition to the “top-down” ability of hydrates to induce IML growth.[28]. This arrangement can generate the extreme structure of the IML in the competitive growth of ice and hydrate, and, more significantly, shows the difference in adhesion caused by the change in the structure of the IML. The difference of these models is reflected in the different content of guest molecules in the IML between the hydrate crystal and solid surface
Summary
Natural gas hydrate is considered to be a future energy source. It is conservatively estimated that the energy stored in natural gas-hydrate sediments is about twice that of conventional fossil fuels on the earth.[1,2] Substantial natural gas hydrates have been found on the continental shelf and in the permafrost regions, and have aroused widespread research interest all over the world.[3−5] On the other hand, as a metastable phase of water, the hydrate can exist stably under high pressure and low temperature environmental conditions. Natural gas hydrate has an ice-like appearance macroscopically and with a cage-like structure on the microscopic level, and some small molecules such as methane, carbon dioxide, hydrogen, and other small hydrocarbons are often trapped in cage cavities formed by water molecules as guests.[3] Under normal, low-temperature conditions, hexagonal ice (Ih) is always a relatively stable thermodynamic phase than the empty clathrate hydrate structure.[11] It is almost impossible to form an empty clathrate crystal structure in the pure water phase, which means that guest molecules are indispensable, at least at the beginning of hydrate nucleation.[12] high concentrations of solvated guest molecules often trigger rare hydratenucleation events. The undesired formation of Received: August 31, 2021 Revised: November 3, 2021 Published: November 16, 2021
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