Nucleation Mechanisms of Clathrate Hydrates

Conclusions

A simple universal model of formation of clathrate crystal nuclei in solution is suggested. The model is applicable equally to all clathrate materials: gas hydrates, cage zeolites (clathrasils) and clathrate compounds of semiconductor elements. The basic feature of the model is joining of polyhedral cage precursors, formed around guest molecules, by means of direct, or intermediated by one host molecule, bonds between precursors. The type of precursor cage formed in solution is determined strictly by the size of guest molecules. This defines the first link of the causal chain connecting the size of guest molecule and the clathrate structure formed. We name such cage precursors ‘structure-forming units‘ (SFU). For the structure I clathrates there are two SFU’s: small and large cage precursors, depending on the size of guest molecule, and, correspondingly, two routes of crystal growth. For other structures SFU is always the large cage. It is shown that for any clathrate structure there forms in solution correct dimer cluster of cage precursors corresponding to a pair of neighboring (without common faces) cages in the real clathrate crystal and no other (false) configurations forms. Consideration of initial growth stages after formation of the dimer clusters shows that growth in all cases can continue by usual mechanism of attaching single host molecules from solution with formation of new cages in the cluster. This growth is, in principle, unlimited and does not create defects. This means that structural information contained in cluster, even on the level of dimers, is sufficient for formation of the clathrate crystal by successive attachment of individual host molecules.