Molecular dynamics simulation on structure, hydrogen-bond and hydration properties of diflunisal intercalated layered double hydroxides

Abstract

The supramolecular structure of diflunisal intercalated layered double hydroxides (DIF/LDHs) was modeled by molecular dynamics (MD) methods. Hydrogen bonding, hydration and swelling properties of DIF/LDHs were investigated. The interlayer spacing dc was found to be constant (ca 1.80 nm) when Nw(the ratio of the numbers of water molecule to DIF) ≤3. The interlayer spacing dc gradually increases as Nw≥4 and this increase follows the linear equation dc=1.261L/Vw+13.63. The hydration energy gradually increases as the water content increases. LDHs/DIF hydrates when Nw≤16 because hydration energy ΔU H -41.84 kJ •mol-1. Swelling of LDHs/DIF is thus limited in an aqueous environment. The interlayer of DIF/LDHs contains a complex hydrogen bonding network. The hydration of DIF/LDHs occurs as follows: water molecules initially form hydrogen bond with layers and anions. While the anions gradually reach a saturation state and water molecules continue to form hydrogen bonds with the hydroxyls of the layers. The L-W type hydrogen bond gradually substitutes the L-A type hydrogen bond and the anions move to the center of an interlayer and then separate with the layers. Last, a well-ordered structural water layer is formed on the surface hydroxyls of DIF/LDHs.