Thermal Dehydration of
Magnesium Acetate Tetrahydrate:
Formation and in Situ Crystallization of Anhydrous Glass
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Abstract
The kinetics and mechanism of the thermal dehydration
of magnesium
acetate tetrahydrate were investigated as a typical example of the
glass formation process via the thermal decomposition of solids. Formation
of an intermediate fluid phase was identified as the characteristic
phenomenon responsible for the formation of anhydrous glass. Thermal
dehydration from the surface fluid layer regulates the zero-order-like
rate behavior of the mass-loss process with an apparent activation
energy <i>E</i><sub>a</sub> ≈ 70–80 kJ mol<sup>–1</sup>. Because of variations in the mechanism of release
of the water vapor with changes in the reaction temperature range,
the mass-loss behavior is largely dependent on the particle size of
the sample and heating conditions. The formation of hollow anhydrous
glass is the novel finding of the present study. The mechanism of
formation is discussed in terms of complementary interpretations of
the morphological changes and kinetic behavior of the thermal dehydration.
On further heating, the as-produced anhydrous glass exhibits a glass
transition phenomenon at approximately 470 K with an <i>E</i><sub>a</sub> ≈ 550–560 kJ mol<sup>–1</sup>,
and subsequently crystallizes via the three-dimensional growth of
nuclei controlled by diffusion. The crystallization process is characterized
by an <i>E</i><sub>a</sub> ≈ 280 kJ mol<sup>–1</sup> and an enthalpy change Δ<i>H</i> = −13.3
kJ mol<sup>–1</sup>, resulting in the formation of smaller,
rounded particles of crystalline anhydrate