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.

We use molecular dynamics simulations with the monatomic water (mW) model to investigate the phase diagram, metastability, and growth of guest-free water clathrates of structure sI and sII. At 1 atm pressure, the simulated guest-free water clathrates are metastable with respect to ice and stable with respect to the liquid up to their melting temperatures, 245+/-2 and 252+/-2 K for sI and sII, respectively. We characterize the growth of the sI and sII water crystals from supercooled water and find that the clathrates are unable to nucleate ice, the stable crystal. We computed the phase relations of ice, guest-free sII clathrate, and liquid water from -1500 to 500 atm. The resulting phase diagram indicates that empty sII may be the stable phase of water at pressures lower than approximately -1300 atm and temperatures lower than 275 K. The simulations show that, even in the region of stability of the empty clathrates, supercooled liquid water crystallizes to ice. This suggests that the barrier for nucleation of ice is lower than that for clathrates. We find no evidence for the existence of the characteristic polyhedral clathrate cages in supercooled extended water. Our results show that clathrates do not need the presence of a guest molecule to grow, but they need the guest to nucleate from liquid water. We conclude that nucleation of empty clathrates from supercooled liquid water would be extremely challenging if not impossible to attain in experiments. We propose two strategies to produce empty water clathrates in laboratory experiments at low positive pressures.

Understanding the microscopic mechanism of nucleation of clathrate hydrates is important for their use in hydrogen storage, CO(2) sequestration, storage and transport of natural gas, and the prevention of the formation of hydrate plugs in oil and gas pipelines. These applications involve hydrate guests of varied sizes and solubility in water that form different hydrate crystal structures. Nevertheless, molecular studies of the mechanism of nucleation of hydrates have focused on the single class of small hydrophobic guests that stabilize the sI crystal. In this work, we use molecular dynamics simulations with a very efficient coarse-grained model to elucidate the mechanisms of nucleation of clathrate hydrates of four model guests that span a 2 orders of magnitude range in solubility in water and that encompass sizes which stabilize each one a different hydrate structure (sI and sII, with and without occupancy of the dodecahedral cages). We find that the overall mechanism of clathrate nucleation is similar for all guests and involves a first step of formation of blobs, dense clusters of solvent-separated guest molecules that are the birthplace of the clathrate cages. Blobs of hydrophobic guests are rarer and longer-lived than those for soluble guests. For each guest, we find multiple competing channels to form the critical nuclei, filled dodecahedral (5(12)) cages, empty 5(12) cages, and a variety of filled large (5(12)6(n) with n = 2, 3, and 4) clathrate cages. Formation of empty dodecahedra is an important nucleation channel for all but the smallest guest. The empty 5(12) cages are stabilized by the presence of guests from the blob in their first solvation shell. Under conditions of high supercooling, the structure of the critical and subcritical nuclei is mainly determined by the size of the guest and does not reflect the cage composition or ordering of the stable or metastable clathrate crystals.

Methane is the prototypic hydrophobic molecule; it has an extremely low solubility in liquid water that leads to phase segregation. On the other hand, at moderate pressures and room temperature, water and methane form hydrate clathrate crystals with a methane to water ratio up to a 1000 times higher than the saturated aqueous phase. This apparent dichotomy points to a subtle balance between the strong water-water hydrogen bonding, responsible for the hydrophobic effect, and water-methane attraction. Capturing these nuances with molecular models requires an appropriate balance of intermolecular interactions. Here we present such a coarse-grained molecular model of water and methane that represents each molecule by a single particle interacting through very short-range interaction potentials. The model is based on the monatomic model of water mW [Molinero, V.; Moore, E. B. J. Phys. Chem. B 2009, 113, 4008] and is between 2 and 3 orders of magnitude more computationally efficient than atomistic models with Ewald sums. The coarse-grained model of this study reproduces the solubility and hydration number of methane in liquid water, the surface tension of the water-methane interface and the equilibrium melting temperature of methane hydrate clathrates with structures sI and sII. To the best of our knowledge this is the first force-field, atomistic or coarse-grained, that reproduces these range of properties of liquid and solid phases of water and methane, making it an efficient and accurate model for the study of the mechanisms of nucleation and growth of clathrates. We expect that the results of this work will also be useful for the modeling of the hydrophobic assembly in aqueous solutions and the development of coarse-grained models of biomolecules with explicit solvation.

Recent studies reveal that amorphous intermediates are involved in the formation of clathrate hydrates under conditions of high driving force, raising two questions: first, how could amorphous nuclei grow into crystalline clathrates and, second, whether amorphous nuclei are intermediates in the formation of clathrate crystals for temperatures close to equilibrium. In this work, we address these two questions through large-scale molecular simulations. We investigate the stability and growth of amorphous and crystalline clathrate nuclei and assess the thermodynamics and kinetic factors that affect the crystallization pathway of clathrates. Our calculations show that the dissociation temperature of amorphous clathrates is just 10% lower than for the crystals, facilitating the formation of metastable amorphous intermediates. We find that, at any temperatures, the critical crystalline nuclei are smaller than critical amorphous nuclei. The temperature dependence of the critical nucleus size is well described by the Gibbs-Thomson relation, from which we extract a liquid-crystal surface tension in excellent agreement with experiments. Our analysis suggests that at high driving force the amorphous nuclei may be kinetically favored over crystalline nuclei because of lower free energy barriers of formation. We investigated the role of the initial structure and size of the nucleus on the subsequent growth of the clathrates, and found that both amorphous and sI crystalline nuclei yield crystalline clathrates. Interestingly, growth of the metastable sII crystal polymorph is always favored over the most stable sI crystal, revealing kinetic control of the growth and indicating that a further step of ripening from sII to sI is needed to reach the stable crystal phase. The latter results are in agreement with the observed metastable formation of sII CO(2) and CH(4) clathrate hydrates and their slow conversion to sI under experimental conditions.