Phase stability of the ice XVII-based CO2 chiral hydrate from molecular dynamics simulations

Abstract

We computed the phase diagram of CO2 hydrates at high pressure (HP), from 0.3 to 20 kbar, by means of molecular dynamics simulations. The two CO2 hydrates known to occur in this pressure range are the cubic structure I (sI) clathrate and the HP hydrate, whose water framework is the recently discovered ice XVII. We investigated the stability of both hydrates upon heating (melting) as well as the phase changes upon compression. The CO2-filled ice XVII is found to be more stable than the sI clathrate and than the mixture of ice VI and dry ice at pressure values ranging from 6 to 18 kbar and in a wide temperature range, although a phenomenological correction suggests that the stability should more realistically range from 6.5 to 13.5 kbar. Our simulation results support the current hypothesis that the HP hydrate is stable at temperatures above the melting curve of ice VI.

Homogenous nucleation rate of CO2 hydrates using transition interface sampling

“…Smaller guest molecules such as hydrogen prefer structure II, a different thermodynamically stable polymorph, where 5 12 6 4 is the larger cage. At high driving force, the CO 2 hydrates are known to nucleate into amorphous polymorphs in which the 4 1 5 10 6 2 cage type is most abundant. 8,9 At higher pressures (6-18 kbar), another stable crystalline hydrate structure can form, characterized by helicoidal channels.…”

“…8,9 At higher pressures (6-18 kbar), another stable crystalline hydrate structure can form, characterized by helicoidal channels. 10 Precise knowledge of the molecular kinetics of formation of these (synthetic) hydrates allows a better control of this process. Hydrates form from solution by nucleation, a process that is described by classical nucleation theory (CNT).…”

Molecular Interaction and Orientation of HOCl on Aqueous and Ice Surfaces

“…The contradictory results obtained in the above studies originate from different methods used in the studies. Classical MD simulation generally performs calculations for large systems at ambient temperatures, which thus can consider the realistic ice surface and thermal effects. , However, the introduced artificial force field cannot describe the interactions of HOCl with ice accurately. In comparison, quantum calculations can describe the potential between HOCl and water. , However, they are performed at 0 K, and in many studies, only small systems are considered, which thus reveals HOCl–H 2 O interaction in the gas phase.…”

“…Classical MD simulation generally performs calculations for large systems at ambient temperatures, which thus can consider the realistic ice surface and thermal effects. 21,22 However, the introduced artificial force field cannot describe the interactions of HOCl with ice accurately. In comparison, quantum calculations can describe the potential between HOCl and water.…”