Rehydration and phase changes of potassium acetate-intercalated halloysite at 298 K

The kinetics of rehydration of potassium acetate-intercalated halloysite are slow as determined by X-ray diffraction. Compared with the rehydration of potassium acetate-intercalated kaolinite where rehydration takes 20 min, the rehydration of the halloysite-intercalation complex takes 4 h. X-ray diffraction shows the halloysite with an initial d(001) spacing of 7.13 Å expands to 13.97 Å with the insertion of the potassium acetate through intercalation. Upon heating to 250°C, two expanded phases with d(001) spacings of 11.84 and 9.80 Å are observed which upon cooling to 25°C become 11.72 and 8.98 Å. Upon exposure to air, multiple expanded phases are observed. The molecular structure of the potassium acetate-intercalated halloysite during cooling from 250°C and upon rehydration has also been studied using Raman microscopy. Raman spectroscopy shows that although the halloysites have been completely expanded, intercalation is incomplete as evidenced by the intensity of the inner surface hydroxyl band at 3695 cm. Thermal treatment shifts the band attributed to the inner hydroxyl at 3620 cm to 3632 cm at 250°C, which upon cooling returns to the starting frequency. The interaction between the acetate and the inner surface hydroxyls is observed by the hydroxyl stretching frequency of 3605 cm. Upon dehydration the band shifts to 3599 cm. The observation of multiple expanded phases of halloysite during rehydration is strongly supported by the changes in the bands associated with the inserting acetate ion. Two C-C and two C=O stretching vibrations are observed. It is proposed that the acetate inserts into the halloysite layers with two different molecular structures. (C) 2000 Academic Press