For the explicit coexistence simulations, instead, we employed larger systems, composed of 2300, 2238, and 2787 molecules for the sI/liquid, HP/liquid, and ice VI/liquid cases, respectively. The single phase simulations, namely, sI clathrate, HP hydrate, ice VI, and CO 2 ice I, were composed of 432, 493, 640, and 864 molecules, respectively. taking care of rearranging the position of the hydrogen atoms in the crystalline lattice in order to satisfy (as much as possible) the ice rules, as both ice VI and ice XVII are hydrogen-disordered. The initial structures of sI hydrate, ice VI, and ice XVII were generated using the Genice program, 41 41. using a real space mesh with a spacing of 0.15 nm and a relative accuracy of the potential energy, calculated at the cutoff of 1 nm, of 10 −5 and 10 −3 for the Coulomb and dispersion interactions, respectively. Both the electrostatic and the dispersion interactions were calculated using the smooth variant of the Particle Mesh Ewald (sPME) method 40 40. The equations of motion were integrated using the leapfrog algorithm with a time step of 1 fs, and the molecular structure of water and CO 2 was kept rigid using the SETTLE 38 38. When simulating the water/CO 2 liquid mixture, we adopt the slab configuration, and in this case, we keep the box edges parallel to the liquid-liquid interface fixed, and we allow only the remaining box edge to relax. The pressure coupling scheme adopted is the anisotropic one, where the diagonal elements of the pressure tensor are relaxed independently from each other, if one solid phase is present in the system. (1 ps relaxation time) and Parrinello-Rahman barostat 37 37. simulating the isothermal-isobaric ensemble by imposing the temperature and pressure equilibrium values by means of the Nosé–Hoover thermostat 35,36 35. Lindahl, International Conference on Exascale Applications and Software ( Springer, 2014), pp. All simulations were performed using the GROMACS molecular dynamics simulation package, version 2018.2, 33,34 33. The parameters of the models are summarized in Table I. In this work, we employ molecular dynamics simulations to check the stability of the CO 2 sI clathrate and ice XVII-based HP hydrate at the boundaries with the CO 2-saturated, water-rich liquid phase and with the mixture of water ice VI and dry ice (CO 2 ice I). to probe those regions of the phase diagram, which are difficult to investigate experimentally. The question of the stability field boundaries of this new type of hydrate is still open, and computer simulations can provide a viable alternative 21–30 21. showed that the water network associated with these new types of hydrates is in fact ice XVII (also known as S χ) and is characterized by open, helicoidal channels, which give the structure a chiral nature. ), only to decompose into dry ice and ice VI above 10 kbar.
In 2010, however, Hirai and co-workers found that at pressure higher than 6 kbar, the CO 2 clathrate hydrate, which starts at low pressure in the sI form, changes into a previously unknown form of hydrate (named by Hirai high-pressure phase, or HP 19 19. possibly passing through the tetragonal structure (ST). In most cases, clathrates that are in the sI or sII form, at low pressures, transform upon compression to the sH and, eventually, to the methane hydrate-III form (MH-III), 17 17. several additional structures have been identified, the most common being denoted as the cubic structure I (sI), 14 14. Since the first determination by von Stackelberg and Müller of the crystal structure of a clathrate hydrate using X-Ray diffraction, 13 13. showing how these structures keep prompting new exciting findings, more than 200 years after their discovery. Hydrogen hydrates have also been used as alternative routes to obtain new ice polymorphs, as in the case of ices XVI and XVII, 9,10 9. whereas there are still important open questions regarding the role of clathrates in the climate change feedback.
as well as a possible means of CO 2 sequestration and long-time storage, 7 7. much of the interest in clathrates revolves around the possibility of using them as energy source, 6 6. While clathrate formation is of high practical relevance as it is responsible for severe pipeline clogging, 4,5 4. making it possibly the largest source of hydrocarbon on Earth. ), where methane hydrate is stable below 300 m, 3 3.
Naturally occurring clathrate hydrates, mainly biogenic methane hydrate, are present in large amounts on the sea floor (estimates vary between 100 and 63.000 Gt of carbon 2 2. Clathrate hydrates, also known as gas hydrates, are a class of host-guest materials where guest molecules reside within, and help stabilize, crystalline cages formed by the hydrogen-bond network of water molecules.
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