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