title: Energetic Insight into the Formation of Solids from Aluminum Polyoxocations.
abstract: The ε-Keggin [AlO4Al12(OH)24(H2O)12](7+) ion (AlAl12(7+)) is a metastable precursor in the formation of aluminum oxyhydroxide solids. It also serves as a useful model for the chemistry of aluminous mineral surfaces. Herein we calculate the enthalpies of formation for this aqueous ion and its heterometal-substituted forms, GaAl12(7+) and GeAl12(8+), using solution calorimetry. Rather than measuring the enthalpies of the MAl12(7/8+) ions directly from solution hydrolysis, we measured the metathesis reaction of the crystallized forms with barium chloride creating an aqueous aluminum solution monospecific in MAl12(7/8+). Then, the contributions to the heat of formation from the crystallized forms were subtracted using referenced states. When comparing the aqueous AlAl12(7+) ion to solid aluminum (oxy)-hydroxide phases, we found that this ion lies closer in energy to solid phases than to aqueous aluminum monomers, thus explaining its role as a precursor to amorphous aluminum hydroxide phases.
authors.affiliation: Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California Davis, One Shields Ave., Davis, CA 95616 (USA).
authors.forename: Dana
authors.initials: D
authors.lastname: Reusser
authors.affiliation: Department of Chemistry and Department of Earth and Planetary Sciences, University of California Davis (USA).
authors.forename: William H
authors.initials: WH
authors.lastname: Casey
authors.affiliation: Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California Davis, One Shields Ave., Davis, CA 95616 (USA). anavrotsky@ucdavis.edu.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Navrotsky
keywords: Al13; Keggin ions; aluminum hydroxide; calorimetry; thermodynamics