Conductivity studies of the molecular encapsulation of sodium perfluoroctanoate by beta-cyclodextrin derivatives

The molecular encapsulation of sodium perfluoroctanoate (SPFO) by hydroxypropyl-beta-cyclodextrin (HP-beta-CD) or 2,6-di-O-methyl-beta-cyclodextrin (DM-beta-CD) has been analyzed by measuring the conductivity in solution of the ternary systems formed by CD + SPFO + H2O. The studies were carried out at 25 degrees C using a fully computerized electrical conductivity technique. The measurements were made as a function of CD concentration at various non-micellar concentrations of SPFO, and as a function of CD and SPFO concentrations with [CD]/[SPFO] constant at stoichiometric ratio. The inclusion complexes, HP-beta-CD . SPFO and DM-beta-CD . SPFO, were characterized through the stoichiometry, which has been found to be 1 : 1 in both cases, and the binding constants, which have been evaluated from the conductivity data with a model proposed by us considering the variation of the ionic molar conductivities with the concentration and the association of the surfactant counterion to the inclusion complex. The resulting K values indicate that the interaction between the CD cavity and the monomeric SPFO is strong and similar in both cases.