Volumetric Study of Modified β-Cyclodextrin/Hydrocarbon and /Fluorocarbon Surfactant Inclusion Complexes in Aqueous Solutions

The apparent molar volumes ( V φ,S ) of homologous series of hydrocarbon (hc) [C x H 2 x +1 CO 2 Na, x = 5, 7, 9, 11, 13] and perfluorocarbon (fc) [C x F 2 x +1 CO 2 Na, x = 3, 6−9] surfactants (S) have been determined in water and in binary solvent (H 2 O + modified β-cyclodextrin (R-β-CD)) systems at 25 °C. The apparent molar volumes ( V φ,R- β -CD ) of 2,6-di- O -methyl-β-CD (DM-β-CD) and 6-(2-hydroxypropyl)-β-CD (HP-β-CD) in water and in binary (H 2 O + S) systems containing hc and fc surfactants have also been obtained. The magnitudes of V φ,S and V φ,R- β -CD are greater in ternary solutions than in the binary aqueous systems. The apparent molar volumes of the modified cyclodextrins and the surfactants at infinite dilution ( ) in ternary solutions are found to depend on the following factors: (i) the magnitude of the binding constant ( K i ), (ii) the alkyl chain length of the surfactant, (iii) the mole ratio of the host to guest species, (iv) the nature of the host/guest stoichiometry, and (v) the physicochemical properties of the CD and/or the surfactant. The volumetric properties of the ternary systems have been analyzed in terms of the additive contributions of the complexed and uncomplexed species. R-β-CD/surfactant complexes having 1:1 and 1:1 plus 1:2 stoichiometries were successfully modeled using two-site and three-site equilibrium models, respectively. The binding affinities of hc and fc surfactants with DM-β-CD and HP-β-CD show different behavior.