Molecular dynamics computer simulations of the effects of hydrogen bonding on the properties of layered double hydroxides intercalated with organic acids

Anion exchange capabilities of layered double hydroxides (LDHs) make them uniquely suitable for the creation of bio-inorganic nanocomposites, with amino acids and DNA fragments occurring as negatively charged species at most pH values. To better understand molecular-level structural, thermodynamic and kinetic aspects of their interactions with LDHs, we have performed molecular dynamics (MD) computer simulations of glutamate(1-) and glutamate(2-) intercalated in Mg-Al LDH. The results are compared with previous simulations of the hydration and swelling behaviour of similar LDHs intercalated with other organic anions (formate, acetate, propanoate and citrate). The MD simulations provide important insight into the interpretation of NMR and X-ray diffraction data for the same systems. The organic species interact with the LDH layers principally via electrostatic and van der Waal's forces, and the hydrated interlayer galleries are stabilised via the development of an integrated hydrogen-bonding network among the anions, water molecules and OH-groups of the LDH layers. Deprotonated carboxylate groups are the primary strong H-bond acceptors, whereas, for glutamate(2-), the amino groups are additional H-bond acceptors from the LDH surface. On the other hand, the protonated amine groups of glutamate(1-) serve as additional strong H-bond donors in the interlayer, responsible for up to 1/6 of all H-bonds formed by each carboxylic group in the interlayer. The organic species preferably accept H-bonds from H2O molecules rather than from surface OH-groups due to structural restrictions on the development of tetrahedrally coordinated H-bonding environments for the carboxylate groups at the surface.