Molecular modeling of the structure and dynamics of the interlayer and surface species of mixed-metal layered hydroxides: Chloride and water in hydrocalumite (Friedel's salt)

The dynamical behavior of Cl- and H2O molecules in the interlayer and on th e (001) surface of the Ca-aluminate hydrate hydrocalumite (Friedel's salt) over a range of temperatures from -100 to 300 degrees C was studied using i sothermal-isobaric molecular dynamics computer simulations. This phase is c urrently the best available model compound for other, typically more disord ered, mixed-metal layered hydroxides. The computed crystallographic paramet ers and density are in good agreement with available X-ray diffraction data and the force field developed for these simulations preserves the structur e and density to within less than 2% of their measured values. In contrast to the highly ordered arrangement of the interlayer water molecules interpr eted from the X-ray data, the simulations reveal significant dynamic disord er in water orientations. At all simulated temperatures, the interlayer wat er molecules undergo rapid librations (hindered hopping rotations) around a n axis essentially perpendicular to the layers. This results in breaking an d reformation of hydrogen bonds with the neighboring Cl- anions and in a ti me-averaged nearly uniaxial symmetry at Cl-, in good agreement with recent Cl-35 NMR measurements. Power spectra of translational, librational, and vi brational motions of interlayer and surface Cl- and H2O were calculated as Fourier transforms of the atomic velocity autocorrelation functions and com pared with the corresponding spectra and dynamics for a bulk aqueous soluti on. The ordered interlayer space has significant effects on the motions. St rong electrostatic attraction between interlayer water molecules and Ca ato ms in the principal layer makes the Ca ... OH2 bond direction the preferred axis for interlayer water librations. The calculated diffusion coefficient of Cl- as an outer-sphere surface complex is almost three times that of in ner-sphere Cl-, but is still about an order of magnitude less than that of Cl- in bulk aqueous solution at the same temperature.