Characterizing key features in the formation of ice and gas hydrate systems

Abstract

Crystallization in liquids is critical to a range of important processes occurring in physics, chemistry and life sciences. In this article, we review our efforts towards understanding the crystallization mechanisms, where we focus on theoretical modelling and molecular simulations applied to ice and gas hydrate systems. We discuss the order parameters used to characterize molecular ordering processes and how different order parameters offer different perspectives of the underlying mechanisms of crystallization. With extensive simulations of water and gas hydrate systems, we have revealed unexpected defective structures and demonstrated their important roles in crystallization processes. Nucleation of gas hydrates can in most cases be characterized to take place in a two-step mechanism where the nucleation occurs via intermediate metastable precursors, which gradually reorganizes to a stable crystalline phase. We have examined the potential energy landscapes explored by systems during nucleation, and have shown that these landscapes are rugged and funnel-shaped. These insights provide a new framework for understanding nucleation phenomena that has not been addressed in classical nucleation theory. This article is part of the theme issue ‘The physics and chemistry of ice: scaffolding across scales, from the viability of life to the formation of planets’.

Molecular Insights into Guest and Composition Dependence of Mixed Hydrate Nucleation

“…The interactions between different species were determined by the Lorentz–Berthelot combining rules. These parameters can accurately reflect the phase equilibria of gas hydrates, ,,, and they have been widely used in previous studies. ,,,,,,,,,,,,− The simulation configurations were generated with our previously established methods, , where homogeneous solutions contain 2944 water molecules and a total of 150 guest molecules. While the overall guest concentration remained fixed at 0.048 mole fraction, relative ratios of the two guests varied from 0 to 100% for each guest.…”

Coarse-Grained Model for the Hydrothermal Synthesis of Zeolites

“…The first one uses the averaged bond-orientational order parameter q̅ 6 of the three-coordinated T particles to distinguish atoms in zeolite Z1 as those with q̅ 6 > 0.2 and their neighbors within 1.45 nm. ,, The second approach considers the solidity of the T particles to identify zeolites. A similar approach has been previously used for the identification of ice and clathrates . We find that T particles that move less than 0.8 nm in 0.5 ns are part of the zeolite framework.…”

“…A similar approach has been previously used for the identification of ice and clathrates. 82 We find that T particles that move less than 0.8 nm in 0.5 ns are part of the zeolite framework. We verify our assignment computing q ̅ 6 and the displacements along the same isothermal trajectory.…”

In Situ X-Ray Diffraction Study on Hydrate Formation at Low Temperature in a High Vacuum

“…At 150 K, diffraction peaks of SI CO 2 hydrate started to appear (Figure a) but vanished at temperatures higher than 210 K. In the present study, within the time scale of the X-ray diffraction experiment, the clathrate hydrate formation proceeded directly, i.e., from water molecule and the gaseous guests, with no new and unidentifiable crystalline structures. There is no evidence for the existence of speculative nascent or pre-(proto)­hydrate structures as mentioned previously. , …”

“…There is no evidence for the existence of speculative nascent or pre-(proto)hydrate structures as mentioned previously. 22,23 The counting time used in the X-ray diffraction experiments was not intended to obtain quantitative results. The primary purpose is to use the characteristic X-ray diffraction pattern as a fingerprint to identify the existence and structure of the gas hydrate.…”