The effect of the head-group spacer length of 12-s-12 gemini surfactants in the host–guest association with β-cyclodextrin

NMR spectroscopy has been used to study and characterize the interactions in solution between β-CD and alkyl-α,ω-bis(dodecyldimethyl ammonium bromide) gemini surfactants with the following head-group spacer lengths: 2, 4, 6, 8, and 10. The application of the method of continuous variation gives as a result that 1:1 and 2:1 (β-cyclodextrin-gemini) complexes are formed; the association stoichiometry is dependent on the spacer chain length, varying from 1.5 (for s = 2) to 1.8 (for s = 10). Assuming a two-step mechanism, the binding constants have been computed. In general, the overall binding constant slightly increases with an increase of the number of methylene groups in the spacer. The 1H NMR spectra of the N-(CH3)2 groups in β-cyclodextrin/gemini mixed solutions are split into two peaks for 12-10-12, suggesting that the gemini spacer can thread the β-cyclodextrin so that the latter is positioned between the gemini head-groups. Inspection of the ROESY spectra allowed the establishment of several spatial proximities between the protons from the β-CD and the gemini and for a spacer length of 10, the data indeed indicate that complexes are formed with the CD molecule positioned between the two charged head groups with the spacer passing through the CD molecule.

The schematic figure represents the differences in the association mechanism of β-CD and geminis with different spacer lengths.Figure optionsDownload high-quality image (85 K)Download as PowerPoint slideResearch highlights
► 1H NMR study of interactions between gemini surfactants and β-cyclodextrin. Each tail of the dimeric surfactant threads one molecule of cyclodextrin.
► The surfactant–cyclodextrin complexes are very stable (ca. −45 kJ/mol).
► The binding of the second cyclodextrin is non-cooperative.
► The spacer with 10 methylene groups can thread the cyclodextrin.