Thermochemistry of Hydrotalcite-like Phases Intercalated with CO₃^2-, NO₃^-, Cl^-, I^-, and ReO₄^-

The enthalpies of formation of hydrotalcite-like phases containing Mg and Al and intercalated with NO 3 -, Cl -, I -, ReO 4 -, or CO 3 2- were determined using high-temperature oxide melt and room-temperature acid solution calorimetry. The relative stability of phases bearing the various anions was gauged by comparing the enthalpy of formation from the single-cation components (Δ f H scc ). Trends relating Δ f H scc to the nature of intercalating anions (halides, NO 3 -, and CO 3 2- ) show small stabilization from the mechanical mixtures of single-cation components. The aim of this study was to relate the enthalpy of formation to the nature of interlayer bonding in hydrotalcite-like compounds (HTLCs) bearing various anions, to uncover trends in the relative aqueous solubilities of these phases. The entropy of formation of these compounds was estimated using an approximation based on third-law entropy measurements for the compound Mg 0.74 Al 0.26 (OH) 2 (CO 3 ) 0.13 ·0.39H 2 O which were performed in a previous study. This approximation for the third-law entropy was combined with the enthalpy data from our calorimetric measurements performed in this work in order to calculate the standard-state free energy of formation for the HTLCs. The solubility products for the compounds investigated in this study were calculated from these free energies of formation and were used in geochemical calculations. The results of these calculations support our previous hypothesis that carbonate-intercalated HTLCs are less soluble than phases bearing other anions such as nitrates and halides. We suspect that the solubilities of HTLCs bearing anions other than carbonate may correspond to the solubilities of single-cation phases bearing the same anions.