Inclusion complex of n-octyl β-D-glucopyranoside and α-cyclodextrin in aqueous solutions:: Thermodynamic and structural characterization

Complex formation between octyl beta-D-glucopyranoside (OG) and alpha-cyclodextrin (alpha CD) was investigated on the basis of three highly accurate and appropriate experimental techniques. First, surface tension measurements showed that alpha CD directly acts on the surfactant monomers in the aqueous phase, leading to progressive depletion of the air-water interface with increasing cyclodextrin contents. Significant shift of OG critical micelle concentration (cmc) was consequently observed: the higher alpha CD concentration, the higher the cmc value. Experiments performed at surfactant and cyclodextrin concentrations in the Gibbs regime of surface tension versus OG content were performed on one hand to establish Job's plot that showed 1:1 stoichiometry of the OG-alpha CD complex and on the other hand to calculate the association constant found equal to (1.85 +/- 0.35) x 10(3) L mol(-1). An inclusion process of the surfactant alkyl residue within the cyclodextrin cavity was confirmed by one-dimensional H-1 NMR, and the structure of the mixed assembly was extensively characterized by two-dimensional NOESY H-1 NMR. OG penetrates alpha CD so that its hydrocarbon chain is embedded inside the cyclodextrin cavity, and its polar head as well as the a-methylene group emerges outside the alpha CD secondary face. Solubility behavior of the OG-alpha CD complex in a wide range of host-guest ratios and concentrations was finally examined by turbidity recording and optical microscopy. At very low free cyclodextrin levels in the solution, the complex presented high solubility behavior up to more than 70 mM. By increasing nonassociated alpha CD in the mixture, propensity of the cyclodextrin molecules to crystallize was observed at concentrations far below the 100 mM aqueous solubility of the pure cyclodextrin. The hexagonal shape of the crystals seen in the optical microscopy images suggested they were, partially at least, composed of the solid complex.