Thermal decomposition of cyclodextrins (α-, β-, γ-, and modified β-CyD) and of metal—(β-CyD) complexes in the solid phase

The thermal decomposition of solid cyclodextrins (CyDs) (natural CyDs are α-, β-, and γ-CyD; modified β-CyDs are triacetyl- (TA), triphenylcarbamoyl- (TC), and dimethyl—β-CyD (DMβ-CyD)) and of metal—β-CyD complexed (metal = Ca, Zn, Cd) has been studied by thermal analysis under the same conditions (heating rate = 10 deg min−1; in He). Most of the water molecules are released below 100°C in natural CyDs. Other strongly bound (included) water molecules were found in both α-CyD and γ-CyD, from which the water molecules were liberated at 260–270°C. The decomposition temperature of the CyD ring is lowered in β-CyD because this ring is of lowest symmetry among the three natural CyDs. Although no difference could be found between the 50%-decomposition temperatures of the CyD rings, the final decomposition rate (up to 550°C) distinguished between α-CyD and γ-CyD (both 98%), and β-CyD (86%). The water contents of the modified β-CyDs are much smaller than the natural β-CyD, and TCβ-CyD has only included water molecules. The decomposition temperature of the CyD ring after dehydration, both in TA and DMβ-CyD is higher by 20–60°C than that of the parent β-CyD. The thermal stability of modified β-CyDs increased in the order TC < DM < TAβ-CyD. Metal—β-CyD complexes have been isolated. All complexes contain water molecules, but these are not tightly bound. Ethanol, which was used as a precipitation reagent, was included only in the Cdβ-CyD complex. Complexation of a heavy metal ion to β-CyD leads to a depression of the thermal degradation of the β-CyD ring.