Cyclodextrin intermolecular voids

Of the 7.57 water molecules per asymmetric unit, four are fully ordered and one, W(5), shows two positions at 0.64 and 0.36 occupation. These water molecules are located in intermolecular voids, and the remaining 2.57 waters are statistically distributed over four sites in the a-cyclodextrin cavity (see Fig. 18.6 b). Associated with this disorder in the a-cyclodextrin cavity is a round shape of the macrocycle, where in contrast to the other two a-cyclodextrin hydrate forms, all six intramolecular interglucose 0(2) 0(3 ) hydrogen bonds are formed. Because hydrogen atoms attached to the disordered water molecules in the 7.57 hydrate could... 

Figure 3. Schematic representations of the possible modes of controlling the permeability through an ordered layer of a long alkyl chain derivative of cyclodextrin by host-guest complexation. (a) Control of permeation through intermolecular voids, (b) Control of permeation through intramolecular channels. (Adapted from ref 14.)...

Another point to be noted is the fact that a dramatic increase in the voltammogram area upon a decrease in the surface pressure was observed for both of the two bulky markers, regardless of whether the marker is hydrophilic ([Mo(CN)g] ) or hydrophobic ([Co(phen)3]2+). These results indicate that the main factor controlling the permeability through e intramolecular channel is the steric bulkiness rather than the hydrophobicity of the marker. Such an aspect seems to be characteristic of the cyclodextrin monolayer and contrasts to the properties of monolayers of simple alkane derivatives, in which the permeability (through intermolecular voids) is controlled mainly by the hydrophobicity and not the steric bulkiness of the marker (30). 

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