Molecular modeling of the structure and energetics of hydrotalcite hydration

Molecular dynamics computer simulations of Mg/Al hydrotalcite with interlayer Cl- were performed to better understand the structure of layered double hydroxides and their hydration behavior. A set of models with variable numbers of interlayer water molecules was investigated, with the assumption of no constraints on the movements of any atoms or on the geometry of the simulation supercells. Crystallographic parameters and two components of the hydration energy were calculated. One of these components is related to the interaction of water molecules with the rest of the structure and is controlled primarily by formation of a hydrogen-bonding network in the interlayer. The other is related to expansion of the host structure itself and reflects decreasing electrostatic interactions as the c-axis expands upon swelling. The dependence of these two energy components on the degree of hydration provides useful insight into the nature of hydrotalcite swelling behavior. There are two stable hydration states with c-axis dimensions of 23.9 and 21.7 Angstrom, corresponding to hydrotalcite with 2 water molecules per each chloride in the interlayer, and dehydrated hydrotalcite, respectively. The first state is observed experimentally under ambient atmospheric conditions. The simulations also reveal a distorted octahedral structure of the hydroxide layer similar to that of hydrocalumite, the related Ca/Al phase.