Three samples of silicon dioxide were syhthesized and their surface areas were measured. A thermo-chemical cycle was designed
to calculate the molar formation enthalpy. The molar formation enthalpy, ΔfHmΦ, for three amorphous silica with the Langmuir surface area 198.0854, 25.1108 and 11.9821 m2 g−1 gave −895.52, −910.86 and −915.67 kJ mol−1, respectively. With the increasing surface area, the values of ΔfHmΦ increased accordingly.
The results suggest that the silica with larger surface area is more unstable. The wetting heat was also measured by adding
the silica powder into water. With the rehydration of the more SiOH groups on the surface, the larger surface areas of silica
lead to the more wetting heat. A smaller particle has the more unstable hydroxyl groups and surface energy.