Volumetric study of modified β-cyclodextrin/hydrocarbon and /fluorocarbon surfactant inclusion complexes in aqueous solutions

The apparent molar volumes (V-phi,V-S) of homologous series of hydrocarbon (hc) [C,H2x+1CO2Na, x = 5, 7, 9, 11, 13] and perfluorocarbon (fc) [CxF2x+1CO2Na, x = 3, 6-9] surfactants (S) have been determined in water and in binary solvent (H2O + modified beta-cyclodextrin (R-beta-CD)) systems at 25 degrees C. The apparent molar volumes (V-phi,V-R-beta-CD) of 2,6-di-O-methyl-beta-CD (DM-beta-CD) and 6-(2-hydroxypropyl)-beta-CD (HP-beta-CD) in water and in binary (H2O + S) systems containing he and fe surfactants have also been obtained. The magnitudes of V-phi,V-S and V-phi,V-R-beta-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 (V degrees(phi)) 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-beta-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-beta-CD and HP-beta-CD show different behavior.