Thermodynamics of formation of ?-cyclodextrin inclusion complexes with four series of surfactant homologs

We report on a titration microcalorimetric study of the formation of ?-cyclodextrin inclusion complexes with the members of series of anionic, cationic, and nonionic surfactant homologs containing even numbers of carbon atoms in the alkyl chains. n-Alkyltrimethylammonium bromides (CxTAB, x = 6a16), sodium n-alkyl sulfates (SCxS, x = 6a12), sodium n-alkanesulfonates (SCxSN, x = 6a12), and N,N-dimethylalkylamine-N-oxides (DCxAO, x = 8a12) were selected for use. The stoichiometry, the binding constant, and the changes in enthalpy, entropy, and Gibbs free energy of the complexation reactions were determined at 298 K. It was found in each case that the complexation is favored by both the enthalpy and the entropy changes and that the thermodynamics of the process is affected far more strongly by the length of the alkyl chain than by the nature of the headgroup.