A volumetric study of beta-cyclodextrin/hydrocarbon and beta-cyclodextrin/fluorocarbon surfactant inclusion complexes in aqueous solutions

The apparent molar volumes (V-phi,V-S) of a homologous series of hydrocarbon (hc) (CxH2+1CO2Na, x = 2, 5-9, 11, 13) and perfluorocarbon (fc) (CxF2x+1CO2Na, x = 1, 3, 4, 6-9) surfactants (S) have been determined in water and in binary solvent (H2O + beta-cyclodextrin (beta-CD)) systems at 25 degrees C. The apparent molar volumes of beta-CD (V-phi,V-CD) in water and in binary (H2O + S) systems containing he and fc surfactants have also been obtained. The results show that the magnitudes of V-phi,V-S and V-phi,V-CD are greater in ternary solutions than in the binary aqueous systems. The apparent molar volumes at infinite dilution (V(phi)degrees) of beta-CD and of the surfactants in ternary solutions are observed 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 host/guest stoichiometry, and (v) the physicochemical properties of the surfactant. The volumetric properties of the ternary systems have been analyzed in terms of the complexed and uncomplexed species by application of Young's rule. The formation of beta-CD/surfactant complexes having 1:1 and 1:1 plus 2:1 stoichiometries were successfully modeled using two-site and three-site models, respectively.