Nucleation at the interface hypothesis

Gas molecules are transported to the interface. Long (1994) notes that the gas impingement rate is 1022 molecules/(cm2s) at the normal temperatures and pressures of hydrate formation. Kvamme (1996) indicates this step is transport of molecules through a stagnant boundary. 

Gas adsorbs on the aqueous surface. While both Long and Kvamme list adsorption as a separate step before either surface diffusion or clustering of the water, adsorption may occur in a partially completed cavity. 

The gas migrates to a suitable location for adsorption through surface diffusion. At this location the water molecules form first partial, and then complete cages around the adsorbed species. 

Labile clusters join and grow on the vapor side of the surface until a critical size is achieved. This can occur either by the addition of water and gas molecules to existing cavities, via the joining of cavities along the interface (as indicated in the cluster aggregation mechanism) or both. 

As noted in Section 2.1.2.1, the outside of hydrate cavities are never smooth, but have hydrogen atoms pointing outward that serve as positive attractions for other molecules and cavities, just as the cavity oxygen atoms with no outwardly pointing hydrogen atoms serve as negative charges for further attachments. 

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