Amorphous precursors in the nucleation of clathrate hydrates.

The nucleation and growth of clathrate hydrates of a hydrophobic guest comparable to methane or carbon dioxide are studied by molecular dynamics simulations of two-phase systems. The crystallization proceeds in two steps: First, the guest molecules concentrate in "blobs", amorphous clusters involving multiple guest molecules in water-mediated configurations. These blobs are in dynamic equilibrium with the dilute solution and give birth to the clathrate cages that eventually transform it into an amorphous clathrate nucleus. In a second step, the amorphous clathrate transforms into crystalline clathrate. At low temperatures, the system can be arrested in the metastable amorphous clathrate phase for times sufficiently long for it to appear as an intermediate in the crystallization of clathrates. The "blob mechanism" unveiled in this work synthesizes elements of the labile cluster and local structuring hypotheses of clathrate nucleation and bears strong analogies to the two-step mechanisms of crystallization of proteins and colloids.

[1]  E. D. Sloan,et al.  A molecular mechanism for gas hydrate nucleation from ice , 1991 .

[2]  A. Soper,et al.  Mechanisms of gas hydrate formation and inhibition , 2002 .

[3]  J. Vatamanu,et al.  Unusual crystalline and polycrystalline structures in methane hydrates. , 2006, Journal of the American Chemical Society.

[4]  K. Refson,et al.  Lifetimes of cagelike water clusters immersed in bulk liquid water: a molecular dynamics study on gas hydrate nucleation mechanisms. , 2004, The Journal of chemical physics.

[5]  E. D. Sloan,et al.  Microsecond Simulations of Spontaneous Methane Hydrate Nucleation and Growth , 2009, Science.

[6]  Deniz Erdemir,et al.  Nucleation of crystals from solution: classical and two-step models. , 2009, Accounts of chemical research.

[7]  J. Vatamanu,et al.  Observation of two-step nucleation in methane hydrates. , 2010, Physical chemistry chemical physics : PCCP.

[8]  D. Staykova,et al.  Structural studies of gas hydrates , 2003 .

[9]  S. Satija,et al.  Hydrate formation at the methane/water interface on the molecular scale. , 2010, Langmuir : the ACS journal of surfaces and colloids.

[10]  V. Molinero,et al.  Ice crystallization in water's "no-man's land". , 2010, The Journal of chemical physics.

[11]  Doroteya K. Staykova,et al.  Formation of Porous Gas Hydrates from Ice Powders: Diffraction Experiments and Multistage Model , 2003 .

[12]  E. D. Sloan,et al.  Mechanisms and Kinetics of Hydrate Formation , 1994 .

[13]  D. Quigley,et al.  Gas hydrate nucleation and cage formation at a water/methane interface. , 2008, Physical chemistry chemical physics : PCCP.

[14]  S. Choi,et al.  Molecular dynamics study of methane hydrate formation at a water/methane interface. , 2008, The journal of physical chemistry. B.

[15]  D. Frenkel,et al.  Enhancement of protein crystal nucleation by critical density fluctuations. , 1997, Science.

[16]  E. D. Sloan,et al.  The next generation of hydrate prediction IV A comparison of available hydrate prediction programs , 2004 .

[17]  J. Ripmeester,et al.  Nucleation and Growth of Hydrates on Ice Surfaces: New Insights from 129Xe NMR Experiments with Hyperpolarized Xenon , 2001 .

[18]  Kai Welke,et al.  Freezing, melting and structure of ice in a hydrophilic nanopore. , 2010, Physical chemistry chemical physics : PCCP.

[19]  P. Rodger,et al.  Molecular dynamics study of gas hydrate formation. , 2003, Journal of the American Chemical Society.

[20]  V. Molinero,et al.  Growing correlation length in supercooled water. , 2009, The Journal of chemical physics.

[21]  Liam C. Jacobson,et al.  Thermodynamic stability and growth of guest-free clathrate hydrates: a low-density crystal phase of water. , 2009, The journal of physical chemistry. B.

[22]  Liam C. Jacobson,et al.  A methane-water model for coarse-grained simulations of solutions and clathrate hydrates. , 2010, The journal of physical chemistry. B.

[23]  C. Gutt,et al.  The carbon dioxide-water interface at conditions of gas hydrate formation. , 2009, Journal of the American Chemical Society.

[24]  V. Molinero,et al.  Is it cubic? Ice crystallization from deeply supercooled water. , 2011, Physical chemistry chemical physics : PCCP.

[25]  Yigang Zhang,et al.  Effect of Methane Adsorption on the Lifetime of a Dodecahedral Water Cluster Immersed in Liquid Water: A Molecular Dynamics Study on the Hydrate Nucleation Mechanisms , 2007 .

[26]  Angelo Bifone,et al.  Monitoring Xenon Clathrate Hydrate Formation on Ice Surfaces with Optically Enhanced 129Xe NMR , 1995 .

[27]  Steve Plimpton,et al.  Fast parallel algorithms for short-range molecular dynamics , 1993 .

[28]  P. Vekilov.  Two-step mechanism for the nucleation of crystals from solution , 2005 .

[29]  B. Trout,et al.  A new approach for studying nucleation phenomena using molecular simulations: Application to CO2 hydrate clathrates , 2002 .

[30]  P. Bomans,et al.  The Initial Stages of Template-Controlled CaCO3 Formation Revealed by Cryo-TEM , 2009, Science.

[31]  Valeria Molinero,et al.  Water modeled as an intermediate element between carbon and silicon. , 2009, The journal of physical chemistry. B.