Can clathrates heterogeneously nucleate ice?

Abstract

Methane hydrates can be preserved at ambient pressure, beyond their region of thermodynamic stability, by storing them at temperatures from 240 to 270 K. The origin of this anomalous self-preservation is the formation of an ice coating that covers the clathrate particles and prevents further loss of gas. While there have been several studies on self-preservation, the question of what is the mechanism by which ice nucleates on the decomposing clathrate hydrates has not yet been fully explained. Here, we use molecular simulations, thermodynamic analysis, and nucleation theory to investigate possible scenarios for the nucleation of ice: heterogeneous nucleation at the clathrate/vapor or clathrate/liquid interfaces and homogeneous nucleation from supercooled water. Our results indicate that clathrates cannot heterogeneously nucleate ice and that ice nucleation is due to the cooling of water at the decomposing clathrate/liquid interface, which suffices to trigger homogeneous ice nucleation. We find that the (111) face of the sII structure clathrate can bind to the (111) plane of cubic ice or the basal plane of hexagonal ice through domain matching, resulting in a weak binding that—while insufficient to promote heterogeneous ice nucleation—suffices to produce epitaxy and alignment between these crystals. We use thermodynamic relations, theory, and the contact angles of ice at the (111) sII clathrate/liquid interface to determine—for the first time—the interfacial free energy of this most favorable ice-clathrate interface, 59 ± 5 mJ/m2. We discuss the implications of our results for the feasibility of heterogeneous nucleation of gas clathrates at ice/vapor interfaces.

Slow Propagation of Ice Binding Limits the Ice-Recrystallization Inhibition Efficiency of PVA and Other Flexible Polymers

“…The equilibrium melting temperature of hexagonal ice in the mW model is T m = 273 ± 0.5 K . The mW model has been extensively validated to reproduce the structure, ,− thermodynamics, ,,,− interfacial properties, ,,,− and phase transitions of water ,,,,,,− as well to model the interactions between ice binding molecules at the ice–water interface. ,,,,− ,,, …”

Is Ice Nucleation by Organic Crystals Nonclassical? An Assessment of the Monolayer Hypothesis of Ice Nucleation

“…To assess the ice nucleating ability of an ordered monolayer at the best ice nucleating face of PD, we extract and minimize a configuration from a large simulation cell in which ice is forming over the (010) organic crystal surface, then make the region of the monolayer rigid, melt the ice to leave a rigid well-ordered monolayer (which still has some defects, see Figure S3) in contact with supercooled liquid water at 260 and 270 K, and bias the formation of ice through a harmonic restraint in Q 6 with target value of the order parameter per water molecule Q 6 ° = 0.03 and force constant of the harmonic restraint 100 kcal mol –1 , following the procedures of ref . The application of the restraint in Q 6 allows the formation of ice even if the process is uphill in free energy …”

“…The global bond-order parameter Q 6 is defined as ,, the harmonic restraint function coupled with LAMMPS molecular dynamics package by a code developed by Clemens Moritz and Andreas Singraber. This implementation has been previously applied to various ice-liquid interface and clathrate hydrate interfaces. ,,,, …”

“…This implementation has been previously applied to various ice-liquid interface and clathrate hydrate interfaces. 44,53,56,111,138 We characterize the structure of interfacial water through Q 6 interfacial , which is Q 6 computed only over the subset that encompass the OH groups of the organic molecules at the interface and the water molecules within 0.35 nm of the plane of these OH groups.…”

“…The application of the restraint in Q 6 allows the formation of ice even if the process is uphill in free energy. 138 The ice that forms on the rigid ITL in contact with liquid is dispersed over the surface, resulting in a very low contact angle of ice with the ITL-covered surface (Figure 9 and Movies S2− S5). The spread of ice over the ITL indicates that ice binds very strongly to it.…”