Effect of water vapor pressure on thermal dehydration of lithium sulfate monohydrate

The effect of the atmospheric water vapor pressure on the rate of thermal dehyration of lithium sulfate monohydrate was studied by means of isothermal gravimetry under various water vapor pressures, ranging from 0 to 2666 Pa. The kinetics of dehydration was described by the two-dimensional Arvada–Erofe’ev type equation, A2(α). An unusual dependence of the rate of dehydration on the atmospheric water vapor pressure was observed as follows. At high temperature, the rate constant increased with increasing water vapor pressure, reached a maximum and decreased gradually to a constant value. At low temperatures, 90 and 100 °C, the rate constants decreased at first, passed through minimum, increased and then decreased. These unusual phenomena are known as the Smith–Topley effects. The mechanism of the phenomena can be discussed on the basis of the diffusion process of the dehydrated water molecules through the product phase.