THERMAL-DECOMPOSITION OF CYCLODEXTRINS (ALPHA-CYD, BETA-CYD, GAMMA-CYD AND MODIFIED BETA-CYD) AND OF METAL (BETA-CYD) COMPLEXES IN THE SOLID-PHASE

The thermal decomposition of solid cyclodextrins (CyDs) (natural CyDs are alpha-, beta-, and gamma-CyD; modified beta-CyDs are triacetyl- (TA-), triphenylcarbamoyl- (TC-), and dimethyl-beta-CyD (DM-beta-CyD)) and of metal-beta-CyD complexes (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-degrees-C in natural CyDs. Other strongly bound (included) water molecules were found in both alpha-CyD and gamma-CyD, from which the water molecules were liberated at 260-270-degrees-C. The decomposition temperature of the CyD ring is lowered in beta-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-degrees-C) distinguished between alpha-CyD and gamma-CyD (both 98%), and beta-CyD (86%). The water contents of the modified beta-CyDs are much smaller than the natural beta-CyD, and TC-beta-CyD has only included water molecules. The decomposition temperature of the CyD ring after dehydration, both in TA- and DM-beta-CyD is higher by 20-60-degrees-C than that of the parent beta-CyD. The thermal stability of modified beta-CyDs increased in the order TC- < DM- < TA-beta -CyD. Metal-beta-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-beta-CyD complex. Complexation of a heavy metal ion to beta-CyD leads to a depression of the thermal degradation of the beta-CyD ring.