Cucurbit[7]uril: surfactant host-guest complexes in equilibrium with micellar aggregates

In order to compare the formation of host-guest complexes between β-cyclodextrin (β-CD) or cucurbit[7]uril (CB7) and cationic surfactants we studied the hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC). The selected surfactants allowed the length of the hydrocarbon chain to be varied between 6 and 18 carbon atoms. Contrary to the expected behaviour, the values of the binding constants between CB7 and surfactants are independent of the alkyl chain length of the surfactant. In the case of β-CD, however, a clear dependence of the binding constant on the hydrophobic character of the surfactant was observed. The values obtained with CB7 are significantly higher than those obtained with β-CD and these differences are explained to be a consequence of electrostatic interactions of the surfactants with the portals of CB7. It was found that a small percentage of uncomplexed CB7 was in equilibrium with the cationic micelles and this percentage increased on increasing the hydrophobic character of the surfactant.