A complex clathrate hydrate structure showing bimodal guest hydration

Abstract

Interactions between hydrophobic groups in water¹, as well as biomolecular hydration more generally^2,3,4, are intimately connected to the structure of liquid water around hydrophobic solutes. Such considerations have focused interest on clathrate hydrates: crystals in which a hydrogen-bonded network of watermolecules encages hydrophobic guest molecules with which the water interacts only by non-directional van der Waals forces. Three structural families of clathrate hydrates have hitherto been recognized: cubic structure I (2M_S·6M_L·46H₂O) (ref. 5), cubic structure II (16M_S·8M _L·136H₂O) (ref. 5) and hexagonal structure H (M_L·3M_S·2M _S·34H₂O) (refs 6, 7) hydrates (here M_L and M_S are the hydrophobic guest sites associated with large and small cavities, respectively). Here we report a new hydrate structure: 1.67 choline hydroxide·tetra- n-propylammonium fluoride·30.33H₂O. This structure has a number of unusual features; in particular the choline guest exhibits both hydrophobic and hydrophilic modes of hydration. Formally the structure consists of alternating stacks of structure H and structure II hydrates, and might conceivably be found in those settings (such as seafloor deposits over natural-gas fields) in which clathrate hydrates form naturally.