Clathrate Hydrate Formation: Dependence on Aqueous Hydration Number

The formation of methane-ethane (C1-C2) clathrate hydrate was studied with high-resolution, solid-state (13)C NMR and density functional theory techniques. The (13)C NMR experiments yield a number of significant findings: (1) the hydration number of C2(aq) is 26, (2) the initial quantity of C2-5(12)6(2) sI hydrate cages outnumber C1-5(12) cages at 274 K, (3) C1-C2 sII hydrate forms at a C1-C2 gas phase composition where only sI hydrate is thermodynamically stable, (4) the initial composition of C1-C2 sII hydrate at 268 K contains less than the original amount of C 1, (5) a quasi-liquid water layer solvating both C I and C2 exists at 268 K, (6) any C1(qll) and C2(qll) present at 253 K is too small to be detected, (7) the initial amounts of C1-C2 sI and sII hydrates formed at 253 K are much smaller than those formed at 268 and 274 K, and (8) Cl(aq), C2(aq) and C1(qll), C2(qll) facilitate the formation of C1-C2 sI and sII clathrate hydrate at 268 and 274 K, respectively. On the basis of these experimental observations, a model is developed that states that the aqueous hydration number of the most water-soluble clathrate hydrate former controls the structure of the clathrate hydrate that forms during the initial stages of the clathrate hydrate formation reaction. For methane-ethane clathrate hydrate, this means that ethane in a water liquid phase or quasi-liquid layer eliminates or adds two water molecules to its hydration shell to form the ethane-filled 5(12)6(2) or 5(12)6(4) cage building blocks of structure I or structure II clathrate hydrate, respectively. Density functional theory computations on methane-filled 5(12), ;5(12)6(2), and 5(12)6(4) and ethane-filled 5(12)6(2), 5(12)6(3), and 5(12)6(4) clathrate hydrate cages yield the stabilization energy of the gas-filled cages and provide theoretical evidence consistent with the experimentally based clathrate hydrate formation model. The proposed model is found to explain the results of other clathrate hydrate formation reactions.