Calorimetric study on the interaction of didecyldimethylammonium and decyltrimethylammonium cations with native cyclodextrins in water

The interaction of two decylammonium surfactants, didecyldimethylammonium bromide (D) and decyltrimethylammonium bromide (S), with native α-, β-, and γ-cyclodextrins (CDs) was systematically examined using the isothermal titration calorimetry (ITC). The measured heat data at five temperatures ranging from (283.15 to 318.15) K were treated simultaneously allowing the estimation of thermodynamically consistent temperature dependence of the equilibrium constant, the enthalpy and heat capacity for the formation of inclusion complexes. The heat data for D/α-CD interaction were analysed by the sequential binding model with 1:1, 1:2 and 1:3 (D:α-CD) stoichiometries, while other surfactant-CD combinations were examined using only 1:1 and 1:2/2:1 stoichiometries. Thermodynamic quantities for complexation are discussed in terms of structural features and their temperature dependence. The enthalpy-entropy compensation analysis of selected surfactant-CD 1:1 complexes obeys a linear relationship. Due to entropical stabilization caused to a high extent by alkyl chain desolvation, the >C10 bearing ions were found not to follow the patterns previously published on binding of other guests. The prediction of the binding thermodynamic properties of D/CD from the S/CD interaction was tested and found surprisingly accurate.