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Heat-of-Sorption

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Temperature plays an important role in water vapor sorption as the interaction between a material and moisture changes with temperature.

In general, moisture sorption decreases at higher temperatures. Such behavior is observed in materials where the adsorption process is associated with an exothermic reaction, meaning heat is released. As the surrounding temperature rises, the system attempts to adjust by adsorbing less moisture to maintain equilibrium. The physical phenomenon is described by the principle of “Le Chatelier”.

Furthermore, an increase in temperature leads to an increase in thermal motion, resulting in higher kinetic energy for water vapor molecules. This leads to increased molecular mobility and wider intermolecular distances. Consequently, the probability of molecules binding to the material via attractive forces, such as van der Waals or hydrogen bonds, decreases. Additionally, at elevated temperatures, these bonds become more susceptible to rupture, causing water molecules to revert to their gaseous state.

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Due to the factors mentioned above, moisture sorption isotherms recorded at different temperatures are typically shifted downward as the temperature rises. Therefore, at a constant aw value, the moisture content of the sample increases as the temperature decreases. Conversely, at a constant moisture content, the water activity of the sample increases with temperature.

The term “Heat-of-Sorption” refers to the heat either released during adsorption or the energy required for the desorption of water vapor molecules. The stronger the interaction and binding force between water vapor molecules and a material’s surface, the more energy is released or required for desorption.