How humidity affects the solid-state inclusion of 2-phenoxyethanol in β-cyclodextrin:: a comparison with β-cyclodextrin

The solid-state inclusion compound ({betaCD.PhE}) of 2-phenoxyethanol (PhE) in beta-cyclodextrin (betaCD) was prepared from aqueous solution and studied by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), FT-Raman and C-13 CP MAS NMR, at ambient humidity and several defined relative humidites (RHs). It is shown that {betaCD.PhE} is a true inclusion microcrystalline compound whose PXRD pattern best matches that of an isostructural channel compound. The studies at defined RHs show that its crystalline structure is preserved for RHs equal or above 20% and that its hydration water is not as strongly bound as in betaCD. Relative intensities of betaCD Raman bands generally ascribed to C-O stretching and CH2 bending vibrations are found to be influenced by the presence of the guest in the betaCD cavity or the increase of ambient humidity, or by both of these factors. Comparison of the C-13 CP MAS NMR spectra for {betaCD.PhE} with those for betaCD, at RHs equal or higher than 20%, reveals a decrease in the multiplicities of the carbon atoms resonances and in the dispersions of chemical shift values of the various types of carbon atoms, thus pointing to an improved symmetrization of the betaCD macrocycle in the channel structure of {betaCD.PhE}.