Thermodynamic and kinetic characterization of host-guest association between bolaform surfactants and alpha- and beta-cyclodextrins.

The thermodynamics and kinetics of formation of host-guest complexes between a series of bolaform surfactants of type C n Me 6 (2+)2Br (-) ( n = 8, 10, and 12) and alpha-cyclodextrin and beta-cyclodextrin were studied with the aid of isothermal titration calorimetry (ITC) at 298.15 and 308.20 K. The association constant, the enthalpy, and the entropy of formation were determined. The obtained thermodynamic parameters are compared with parameters for the micelle formation of a related cationic surfactant. The difference in magnitude and sign between the parameters of the alpha-CD and beta-CD complexes is discussed based on the curvature of the cavity of the CD. We suggest that the water molecules inside the alpha-CD cavity are not able to maintain their hydrogen bond network. Upon complex formation these water molecules are expelled and reform their hydrogen bond network. The situation is different in the larger beta-CD cavity where water has the possibility of a more extensive hydrogen bonding. The kinetics for alpha-CD is slow, associated with high activation energies for both association and dissociation of the complex. The rates increased with a decrease in the number of methylene groups in the hydrocarbon chain. The slow kinetics is argued to originate from the fact that the charged headgroup needs to be pushed through a relative nonpolar cavity. A comparison is made with the Born energy.