Molecular dynamics simulation of the energetics and structure of layered double hydroxides intercalated with carboxylic acids

Molecular dynamics (MD) simulation of the Mg/Al (3: 1) layered double hydroxide (LDH), hydrotalcite (HT), containing the monocarboxylic acids formate, acetate, and propanoate as the charge balancing interlayer anions provides new molecular-scale insight into the interlayer structure, hydrogen bonding, and energetics of hydration and consequent swelling of LDH compounds containing organic molecules and biomolecules with carboxylate functional groups. As for citrate-HT (Kumar, P. P.; Kalinichev, A. G.; Kirkpatrick, R. J. J. Phys. Chem. B 2006, 110, 3841), the hydration energy of these systems as a function of water content has no distinct minima, indicating the absence of energetically well-defined structural states with specific water contents. The hydration energies, however, approach the energy of bulk liquid water at lower water contents than for citrate-HT, suggesting that synthesis strategies involving delamination of the hydroxide layers in water are likely to be more successful using starting compounds containing larger molecules with multiple carboxylate groups. This result is consistent with recent experimental observations of the delamination of lactate-HT (Hibino, T.; Kobayashi, W. J. Mater. Chem. 2005, 15, 653. Jaubertie, C.; Holgado, M. J.; San Roman, M. S.; Rives, V. Chem. Mater. 2006, 18, 3114). The structural behavior of carboxylate anions in LDH interlayers is directly related to the energetic relationships, with electrostatic interactions and the H-bonding between the -COO(-)sites of the anions, the M-OH sites of the metal hydroxide sheets, and the interlayer H2O molecules playing dominant roles. The hydrogen bonds donated to the anions from water molecules are energetically preferable to those from M-OH sites, resulting in decreasing occurrence of the anions in inner-sphere coordination environments to the hydroxide sheets with increasing water content.