(580e) Methane Hydrate Nucleation: Choreography and Cadence From Molecular Simulations

Methane hydrates are ice-like solids that form at low temperatures and high pressures, with methane molecules encaged in a hydrogen-bonded water lattice.  They have been of long-standing interest to the oil and gas industry because of their tendency to plug pipelines, as a resource in the vast seafloor hydrate sediment deposits, and recently in the containment of the Gulf oil spill.  The control of hydrate formation in these and other situations can be guided by moleular level knowledge of the hydrate nucleation process.  The nucleation of hydrates is a rare event that occurs in nanoseconds at a random location on a nanometer length scale, and thus a molecular understanding of the process has been difficult to obtain experimentally or by simulation.  By employing large-scale microsecond long molecular dynamics simulations, we captured information about the mechanism of spontaneous hydrate nucleation, revealing an intricate cooperative dance between methane and water molecules [1].

Cooperative organization is observed to lead to methane adsorption onto planar faces of water and the fluctuating formation and dissociation of early hydrate cages.  The early cages are mostly face-sharing partial small cages, favoring structure II, however larger cages subsequently appear due to steric constraints and thermodynamic preference for the structure I phase.  The resulting structure after nucleation and growth is a combination of the two dominant types of hydrate crystals (structure I and structure II), which are linked by uncommon 51263 cages facilitating structure coexistence without an energetically unfavorable interface.  Knowledge of the phase boundary, necessary for determining the subcooling driving force, was calculated by thermodynamic integration for the molecular potentials used [2].  Analysis of trajectories under different driving forces and geometries allowed an estimate of the nucleation rate as a function of methane concentration.

References

1. Walsh M. R., Koh C. A., Sloan E. D., Sum A. K. and Wu D. T. Microsecond Simulations of Spontaneous Methane Hydrate Nucleation and Growth, Science, 326, 1095-1098, 2009. 
2. Jensen L., Thomsen K., von Solms N., Wierzchowski S., Walsh M. R., Koh C. A., Sloan E. D., Wu D. T. and Sum A. K. Calculation of Liquid Water-Hydrate-Methane Vapor Phase Equilibria from Molecular Simulations, J. Phys. Chem. B, 114, 5775-5782, 2010.