Thermodynamic properties of sodium n-perfluoroalkanoates in water and in water plus cyclodextrins mixtures

Densities and heat capacities of water-substrate and water-substrate-cyclodextrin mixtures were determined at 298 K. The substrates studied are sodium n-perfluoroalkanoates, (CF)(n)COONa, (from sodium perfluoroacetate to sodium perfluorooctanoate) and the cyclodextrins are hydroxypropyl-alpha-cyclodextrin (HP-alpha-CD), hydroxypropyl-beta-cyclodextrin (HP-beta-CD), and hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD). From experimental data of (CF)(n)COONa in water, the standard partial molar properties were determined. Moreover, the properties of micellization of the surfactants were determined. HP-beta-CD hardly affects the thermodynamic properties of CF3COONa while it strongly does those of the other substrates. From the experimental data the standard partial molar properties of the inclusion complex were calculated and compared to those of the sodium n-alkanoates + HP-beta-CD we reported elsewhere (Langmuir 1998, 14, 6045). The larger hydrophobicity of the CF2 group with respect to the CH2 one is reflected in the properties of the inclusion complex. Studies extended to HP-alpha-CD and HP-gamma-CD indicate that the size cavity of the cyclodextrin influences the thermodynamics of the inclusion complex formation more than that for the hydrogenated substrates. The presence of cyclodextrin affects the property of the surfactant in the aqueous phase while it scarcely does that in the micellar phase.