Thermal and structural characterization of cyclomaltononaose (δ-cyclodextrin), cyclomaltodecaose (ε-cyclodextrin) and cyclomaltotetradecaose (ι-cyclodextrin)

Thermal behavior of cyclomaltononaose (δ-cyclodextrin, δ-CD), cyclomaltodecaose (ε-cyclodextrin, ε-CD) and cyclomaltotetradecaose (ι-cyclodextrin, ι-CD) and the solid-state phase transitions associated with the respective dehydration and rehydration processes were investigated. δ-CD, ε-CD and ι-CD in their amorphous and crystalline states were tested by differential scanning calorimetry (DSC), thermogravimetry (TG) and hot stage microscopy (HSM). X-ray powder diffraction (XRD) patterns both experimental and computer generated from single crystal data were used to support thermal data and to follow the solid-state phase transitions associated with thermal dehydration and rehydration of each cyclodextrin (CD). Thermal events associated with dehydration of δ-CD, ε-CD and ι-CD were evaluated and tentatively interpreted on the basis of water arrangements observed in the crystal lattice. Dehydrated δ-CD reverted to the starting 13.75H2O hydrate under high relative humidity (RH) conditions (≈100% RH) at room temperature (RT), while transformed into a 7H2O hydrate at 30–35% RH. Physicochemical characterization based on thermal behavior, which can be related to water arrangements observed in the respective crystals, permits to distinguish between δ-CD, ε-CD and ι-CD, as well as between the amorphous and crystalline states of each CD. The results can be useful for demonstrating the formation of a true inclusion complex of δ-CD, which occurs in two crystalline hydration states, with a guest drug.