Thermodynamic and Spectroscopic Study of a Molecular Rotaxane Containing a Bolaform Surfactant and β-Cyclodextrin

A thermodynamic and proton NMR spectroscopy study of a bolaform type surfactant, docosane 1,22-bis(trimethylammonium bromide), has been carried out in water and in the presence of β-cyclodextrin (β-CD) at 298 K. Density and sound velocity data for the aqueous solutions of the bolaform in both systems were analyzed to calculate the molar apparent and partial volumes and adiabatic compressibilities. For the binary system, the molar partial compressibilities and volumes of the bolaform in water as a function of concentration have been obtained. Compressibility data indicate that the surfactant, both in monomer or in micelle form, is partially folded. For the ternary system, a remarkable increase of the thermodynamic properties of the surfactant is observed at infinite dilution with respect to the value in water and a shift of the critical micelle concentration in an extension that points to complexes of predominantly 2:1 stoichiometry. The values of the transfer properties of the bolaform at infinite dilution, discussed in terms of a simple model which takes into account the balance between the released water from the cavity and the methylene groups of the substrate that enter into the macrocycle, prove the formation of a molecular rotaxane in which three β-CDs are threaded by one molecule of surfactant under conditions of excess of β-CD, which turns to 2:1 when the surfactant concentration increases. 1 H NMR in D 2 O experiments have been performed in order to elucidate the molecular structure of the rotaxane in solution. Analyses of the induced chemical shifts corroborate the thermodynamic results and prove that the β-CD is located preferentially on the surfactant chain, being the cationic heads scarcely involved in the complex.