Enthalpy of water adsorption and surface enthalpy of goethite (α-FeOOH) and hematite (α-Fe₂O₃)

Goethite, α-FeOOH, and hematite, α-Fe₂O₃, have high affinity for water, especially when particle size is small. To determine the enthalpy of different types of adsorbed water, we performed water adsorption calorimetry on goethite and hematite with surface areas of 60-270 and 2-150 m²/g, respectively, using a new calorimetric technique combining a microcalorimeter and an automated gas dosing system. Several types of strongly bound water can be distinguished on hematite, depending on heating temperature and surface area. These have enthalpies of adsorption relative to liquid water (ΔH_ads) equal to -67.1 ± 4.9, -48.6 ± 1.8, and -25.5 ± 4.4 kJ/mol. The last value corresponds to water adsorption on very fine grained hematite and is very close to the water adsorption enthalpy for goethite, ΔH_ads = -19.4 ± 4.2 kJ/mol. Surface enthalpies for anhydrous surfaces of goethite (0.91 ± 0.09 J/m²) and hematite (1.9 ± 0.3 J/m²) determined experimentally are reported for the first time. The significant difference in surface enthalpies of goethite and hematite creates an energy crossover and makes fine-grained hematite metastable relative to goethite.