Differences in Cucurbit[7]uril: Surfactant Complexation Promoted by the Cationic Head Group

The formation of host/guest complexes between cucurbit[7]uril (CB7) and tetradecyltrialkylammonium bromide surfactants with trimethyl (C₁₄ TMA⁺ ), triethyl (C₁₄ TEA⁺ ), and tripropyl (C₁₄ TPA⁺ ) head groups was studied. Complexation was studied by ESI-MS, isothermal titration calorimetry (ITC), and NMR spectroscopy. The structure of the complexes is strongly dependent on the surfactant head group. Both 1:1 and 2:1 complexes are formed with all cationic surfactants. The magnitude of the K_1:1 binding constants decreases with the alkyl length of the head group, whereas that of the K_2:1 binding constants is independent of the nature of the surfactant head group. Distinct structures were found for the 1:1 and 2:1 complexes. C₁₄ TMA⁺ and C₁₄ TPA⁺ surfactants form 1:1 inclusion complexes that comprise the head group close to the CB7 portal and 2:1 external complexes with both host molecules near the head group. The 1:1 complexes between C₁₄ TEA⁺ and CB7 showed that the head group was centered inside the CB7 cavity, whereas in the 2:1 complex the second molecule of CB7 was located at the end of the main alkyl chain.