Room-temperature acid solution calorimetry, high-temperature
oxide
melt solution calorimetry, and low-temperature heat capacity measurements
were employed to calculate the thermodynamic stabilities of the [Zn–Al–X]
layered double hydroxides (LDH) containing different anions (X = Cl–, CO3
2–, and SO4
2–). Cryogenic heat capacity measurements demonstrated
a Schottky-type anomaly in the heat capacity of all three LDHs below
11 K. This anomaly is attributed to the tunneling of protons between
adjacent oxygen atoms in the LDH interlayer as this creates an energy
system similar to a two-level system modeled with a Schottky term.
These heat capacity measurements were also used to determine vibrational
entropies which, when combined with configurational entropies, provide
standard entropies of these LDHs. Enthalpies of formation of LDHs
from binary components were determined and combined with the entropies
of formation to calculate Gibbs free energies. Based on these values,
the order of stability is [Zn–Al–SO4] >
[Zn–Al–CO3] > [Zn–Al–Cl].
This trend results from a combination
of the interlayer spacing, amount of water in the interlayer, interactions
among the interlayer species, and interactions between the metal hydroxide
layer and the interlayer.