Interlayer arrangement of hydrated MgAl layered double hydroxides containing guest terephthalate anions: Comparison of simulation and measurement

The hydration of MgAl layered double hydroxides (LDHs) with interlayer terephthalate anions and controlled layer charge is studied. A combination of powder X-ray diffraction (PXRD), thermal gravimmetry (TG), and computer simulation is used to study in detail the effect of layer charge density and interlayer water content on the interlayer arrangement of the organo-LDH. For high water content and layer charge, an interlayer separation of approximately 14.0 Angstrom is favored, which corresponds to a vertical orientation of the terephthalate anion with respect to the hydroxide layers. For low water content and layer charge, an interlayer separation of approximately 8.4 Angstrom is favored, which corresponds to a horizontal terephthalate orientation. During cycles of dehydration-rehydration, PXRD indicates that the 14.0 and 8.4 Angstrom units coexist in varying proportions depending on the layer charge and water content; in certain cases, a 22.4 Angstrom interstratified phase consisting of a regular alternation of the 14.0 and 8.4 Angstrom component interlayers can be identified. Molecular dynamics simulations predict a gradual expansion of the interlayer accompanied by a change in the orientation of the terephthalate anion from almost horizontal to vertical as the number of interlayer water molecules included in the simulation is increased. As the layer charge is increased, fewer water molecules are required to ensure a vertical terephthalate orientation, presumably as a consequence of the increased layer-layer Coulombic repulsion and interlayer packing density. There is, therefore, general agreement between the computer simulations and experimental measurements. The results demonstrate how the disposition of organic anions within the interlayer of organo-LDHs may be controlled.