Scrutinizing ITC-study on the formation of inclusion complexes of nonionic surfactant Triton X-100 and cyclodextrins

The formation of host-guest-complexes of Triton X-100 (1), a nonionic surfactant comprising of an oligo ethylene glycol chain and a bulky hydrophobic tail, with alpha-, beta- and gamma-Cyclodextrin (alpha-, beta- and gamma CD) has been investigated by isothermal titration calorimetry (ITC). Especially, the interaction of beta CD with Triton X-100 has been subject-matter of a series of analytical studies though the results are contradictorily. Equilibrium constants diversify in the range of 200-200000 M-1. Among these, even an isothermal titration calorimetric study is reported, indicating an association constant of 9100 M-1. In contrast, the findings we report in the present paper approve an exceptionally high association constant reported just recently. Moreover, the stoichiometry of the formed complexes and the binding sites were investigated. beta CD and gamma CD interact with the octylphenyl residue of 1. In contrast, alpha CD forms pseudorotaxanes by threading onto the oligo ethylene glycol-part of the surfactant molecule.