No Need for Pores?

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Science  10 Jun 2005:
Vol. 308, Issue 5728, pp. 1521
DOI: 10.1126/science.308.5728.1521b

It is usually assumed that molecular diffusion through solid materials proceeds by means of pores that are wide enough to allow passage of molecules. Thallapally et al. cast doubt on this assumption by showing that water can diffuse through a seemingly nonporous crystal. They determined the structures of calixarene crystals before and after the crystals had been immersed in water for 8 hours. Calixarenes are macrocyclic compounds that can accommodate small molecules; in this present case, in the cleft of a pincer-like configuration. Despite the absence of discernable channels in the crystals, the post-immersion crystals contain one water per host molecule; the lattice structure is otherwise unchanged. The authors rule out crystal dissolution and regrowth because the calixarene is not soluble, even in boiling water, and the same crystal was studied before and after immersion. They conclude that concerted movements of calixarenes might allow the water molecules to diffuse through the crystal until they reach a cavity of suitable size. — JFU

Angew. Chem. Int. Ed. 10.1002/anie.200500749 (2005).

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