Studies on Interactions of β-Cyclodextrin with a Nonionic Surfactant Triton N-101

The complexation ofβ-CD with a nonionic surfactant Triton N-101 (TN) has been studied by surface tension measurement and UV-visible spectroscopy. The surface tension curves and apparent critical micelle concentrationsof TN in the presenceofβ-CDare higher than that in the absenceofβ-CD, which indicates that surface activity of TN decrease and formation of micelle is delayed because of the complexation ofβ-CD with TN. To clearly explicate the influences ofβ-CD-TN complexation on changes of surface tension and formation of micelle, the surface tension of fixed concentrations of TN mixed withβ-CD is measured and the influences ofβ-CD on fixed concentrations of TN are discussed. When the concentration of TN is below its CMC, itssurface tension increases withβ-CD concentrations, and then remainsat amaximum. But when the concentrationof TN isabove itsCMC, itssurface tension remains the same asγCMC(surface tensionswhen TN concentrations are above its CMC) firstly, and then it increases withβ-CD concentration, finally remains at a maximum. The association constantsKaofβ-CD-TN system at 298 K are determined by the numerical method based on surface tension measurements that we developed. By measuring surface tensionsofβ-CD-TN systems, the plotsof calculatedS0-S value as a function ofS0/SorS0/S-1 give good straight line, which verified that, as assumed above, the supramolecularsystemstoichiometry is 1∶1 for the inclusion complexation.Kaofβ-CD-TN system at 298 K is calculated from the slope of the straight line and Gibbs free energy change -ΔG0values ofβ-CD-TN systems have also been obtained. The resultsshow thatKadependsonβ-CD concentration, the higher ofβ-CD concentration, the larger ofKavalues. Spectrophotometric titrations have been performed to show the interaction of this supramolecular system. As can be seen from the experiment results, whenβ-CD concentration is lower, the UV absorbance strength at 200, 222, 275 nm decreases with increasingβ-CDconcentration. But when the concentrationofβ-CDreachesat 8.0mmol/L,the absorbance at 275 nm shift to shorter wavelength. On the basis of this observation, we deduce that the hydrophobic chain of TN enterβ-CD cavity mainly whenβ-CD concentration is lower; But whenβ-CD concentration is increased, the phenyl moietyof TN islocated in amore hydrophobic environment, i.e., within the cyclodextrin cavity, rather than in the bulk aqueous solution. The models of the inclusion complexes are also designed based on experimental results.