Nucleation Pathways of Clathrate Hydrates: Effect of Guest Size and Solubility

Abstract

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.

Evaluating polymeric inhibitors of ethane clathrate hydrates

“…Modeling shows that natural gas clathrate hydrates form along a pathway wherein dissolved guest molecules (i.e., natural gas) quasi-equilibrate with an enriched guest separated phase (i.e., the nuclei). This phase changes irreversibly to become a structured natural gas clathrate hydrate (Jacobson et al, 2010) This modeling is in contrast to the labile cluster model where cages spontaneously enclathrate solvated guest molecules and assemble to form the hydrates (Sloan and Fleyfel, 1991), but it is similar to the model of Radhakrishnan and Trout (Radhakrishnan and Trout, 2002). Taking the most recent modeling work of Jacobson and coworkers as highly accurate (Jacobson et al, 2010), one way to inhibit natural gas clathrate hydrate would be to change the surface energy at the interface of the nuclei and the bulk solution, such that the surface area (along with the volume) of the nuclei decreases to disappearance.…”

“…This phase changes irreversibly to become a structured natural gas clathrate hydrate (Jacobson et al, 2010) This modeling is in contrast to the labile cluster model where cages spontaneously enclathrate solvated guest molecules and assemble to form the hydrates (Sloan and Fleyfel, 1991), but it is similar to the model of Radhakrishnan and Trout (Radhakrishnan and Trout, 2002). Taking the most recent modeling work of Jacobson and coworkers as highly accurate (Jacobson et al, 2010), one way to inhibit natural gas clathrate hydrate would be to change the surface energy at the interface of the nuclei and the bulk solution, such that the surface area (along with the volume) of the nuclei decreases to disappearance. Another possible way to inhibit the natural gas clathrate hydrates would be to add into the system a polymer attracted to the nuclei core (i.e., a polymer more hydrophobic than the bulk solution) to disrupt the structuring process (Rodger, 2011).…”

“…To date, this has been accomplished with molecular dynamic modeling (Jacobson et al, 2011). Modeling shows that natural gas clathrate hydrates form along a pathway wherein dissolved guest molecules (i.e., natural gas) quasi-equilibrate with an enriched guest separated phase (i.e., the nuclei).…”

Evidence from mixed hydrate nucleation for a funnel model of crystallization

“…3B. Previous work has indicated that gas hydrate nucleation proceeds with the formation of solvent-separated guest molecules and water molecule rearrangement to yield cages (33). Therefore, the early reductions in the MSD values of the water molecules, without commensurate changes to the tetrahedral ordering of the aqueous phase, may be indicative of early stage ordering of guest molecules in the aqueous phase.…”

Entropic Description of Gas Hydrate Ice-Liquid Equilibrium via Enhanced Sampling of Coexisting Phases

“…MD gREM simulations, the first for a bulk water crystal/liquid transition, were performed for 2944 waters in 54 replicas under pressure of 1 atm and T 0 = 330 K, and initially in perfect β ice configurations in symmetry SI, using the mW coarse-grained model [17] with periodic boundary conditions. Although β ice is never the thermodynamically stable state, the barrier to stable hexagonal α ice insures that it will not be sampled under these conditions and the metastable β ice-liquid transition is well defined in the restricted configuration space.…”