Pressure-induced amorphization of methane hydrate

Abstract

Pressure-induced amorphization of methane hydrate has been investigated by molecular dynamics simulation. In accord with experimental results of Tulk et al., a crystalline → amorphous transition was confirmed at 3.3 GPa where the water lattice collapsed around the encaged methane. Thermal annealing at 5.5 GPa allows the water to adopt a lower energy conformation with a denser structure. In both structures, methane molecules are immobilized and maintain long-range correlation as in the crystal. Consequently, both phases are predicted to revert back to the crystalline form upon decompression. Pressure-induced amorphization is a nonequilibrium process due to a mechanical instability; the interconversion between kinetically stable amorphous phases is a relaxation effect.The two-liquid hypothesis of water 1 was based on observing low-temperature pressure amorphization of ice into a highdensity form (HDA) 2 and subsequent conversion to a lowdensity form (LDA) upon heating. 3 It is reckoned that HDA formation occurs via thermodynamic melting. 4 Further, if the HDA → LDA transformation is first order, extrapolation of the phase boundary of the purportedly distinct phases into the liquid region will terminate at a critical point; 1,4 this would support the view that liquid water structure arises from the coexistence of low-and high-density liquid phases. However, this proposition, along with underlying mechanisms of pressure-induced amorphization, has been the subject of much controversy; a different viewpoint on pressure-induced amorphization arises from considering it as violating Born's stability criteria, 5,6 with ice structure collapse into a metastable frustrated disordered structure. 7 Apart from theoretical evidence, the latter explanation has gained support from recent neutron experiments where softening of shear lattice vibrations in ice was enhanced with increasing pressure. 8,9 However, it is also interesting to consider mechanisms of pressureinduced amorphization in other water-based structures; recently, pressure-induced amorphization in tetrahydrofuran (THF) clathrate hydrates has been studied by x-ray and Raman techniques 10 and dielectric spectroscopy. 11 In many respects, clathrate hydrates' properties are very similar to ice. Indeed, it was first postulated in 1984 that THF hydrates would amorphize under pressure, 2 as has been observed. 10,11 Very recent experiments by Tulk et al., 12 however, have shown that the frameworks in methane hydrates collapse under pressure; it was not possible to quench-recover the amorphized structure and, without exception, the original crystalline structures were recovered upon decompression. This suggests strongly that pressure amorphization does not constitute thermodynamic melting. A similar observation has been made for the silica analog of clathrate hydrate, clathrasils. 13 With the advance of high-pressure diffraction techniques, it has become possible recently to record diffraction patterns of pressure-collapsed structures. Significantly, in situ warming of the exp...

Structural investigation of three distinct amorphous forms of Ar hydrate

“…A similar hindrance of rotational motion of molecular guests was found in MD simulations of PIA of CS-II THF hydrate 17 and CS-I CH 4 hydrate. 25 Actual PIA implies the “collapse” of D cages. 24 Fig.…”

“…PIA is initiated by the deformation of the 6-membered rings of the large H cages. 17,24,25 When approaching PIA, the translational motion of Ar atoms inside the cages decreased drastically. A similar hindrance of rotational motion of molecular guests was found in MD simulations of PIA of CS-II THF hydrate 17 and CS-I CH 4 hydrate.…”

“…Hitherto this has been only reported for amorphous CH 4 and THF hydrate. 17,25 In this work we extend such a study to amorphous Ar hydrate, for which there are several unique aspects. First, since the composition of the CS-II crystalline precursor is well characterized, the composition of the amorphous Ar hydrate is also known.…”

Pressure-Induced Densification of Ice Ih under Triaxial Mechanical Compression: Dissociation versus Retention of Crystallinity for Intermediate States in Atomistic and Coarse-Grained Water Models

“…Both potential are proven in terms of reasonable thermodynamics for ice, e.g., with accurate melting points of ∼270 K. , Naturally, TIP4P-Ice is expected to be more accurate (at the very least kinetically), , due to explicit long-range dipolar and Coulombic electrostatic interactions, particularly at solid–liquid interfaces . Indeed, one interesting matter concerns a detailed comparison of mW versus TIP4P-Ice in terms of sudden pressurization-response behavior, as regards characterization of (probably metastable intermediates’) structural, energetic and dynamical properties, as well as their likelihood of reversibility within nanoseconds to ice I h , given that refs , found that for both pressure-amorphised ice and methane hydrate, removal of pressure led to recovery of a defective, near-crystalline state, i.e., the transformations were near-reversible. We performed MD for ∼70–130 ns at 240 K (owing to be a near-optimal temperature for potential interesting propagation of ice-stacking faults, if present), under pressures of up 30 kbar, on systems ranging in size from ∼40,000 to 1 million molecules for TIP4P-Ice and at the latter size for mW (given greater computational speed).…”

“…Certainly, the strong tetrahedral biasing in mW appears to be reinforced by the smaller interplanar spacing (e.g., 3.79 versus 4.15 Å at 10 kbar, cf. Figure ) under higher-pressure conditions, with the three-body nature of the Stillinger–Weber potential not capturing the atomistic realism of explicit Bernal–Fowler (ice-rule) hydrogen bonds under severe compaction, and the tendency for these to collapse into the amorphous , or liquid state. Indeed, at elevated pressures, nuclear quantal effects become all the more important in high-pressure ices (whether crystalline or amorphous), or condensed aqueous phases in general; French and Redmer have taken great care in developing a thermodynamic potential for high-pressure ices, while Sugimura et al have highlighted the delicate intricacies of hydrogen-bond symmetrization in high-pressure ice polymorphs and Ikeda has very recently shown that path-integral simulation is needed to define pressure more accurately for high-pressure-ice dynamics .…”