Enthalpy of water adsorption and surface enthalpy of lepidocrocite (γ-FeOOH)

Lepidocrocite (γ-FeOOH) appears to be thermodynamically metastable with respect to goethite (α-FeOOH) and yet the former phase forms and persists both in nature and laboratory. Here we show that the thermodynamic factors relevant to these observations cannot be dismissed, although kinetics undoubtedly plays a significant role in the formation and preservation of metastable phases. To understand the relationships of the FeOOH polymorphs in the bulk and nanoscale, we investigated the energetics of lepidocrocite nanoparticles. We measured enthalpy of water adsorption and enthalpy of formation of lepidocrocite samples with surface area of 42-103 m²/g. Having both quantities measured allowed us to calculate the surface enthalpy for a water-free surface of this phase as 0.62 ± 0.14 J/m² and the energy of a relaxed (hydrated) surface as 0.40 ± 0.16 J/m². Our measurements show that a portion of the adsorbed water (∼40% under laboratory conditions) is chemisorbed (strongly bound) with enthalpy of adsorption of -65.8 ± 2.6 kJ/mol of H₂O relative to vapor (or -21.8 ± 2.6 kJ/mol relative to liquid water). The standard enthalpy of formation from elements for a hypothetical lepidocrocite with nominal composition FeOOH and zero surface area is -552.0 ± 1.6 kJ/mol. Our results demonstrate that when considering the thermodynamic properties of iron oxides in the environment, a conclusive statement about their stability cannot be made without specifying the particle size of individual phases.