Ab Initio Molecular Dynamics Study of the Hydration of a Sodium Smectite Clay

Ab Initio molecular dynamics simulations have been performed to study the hydration of a sodium smectite clay. Water molecules are intercalated as a double layer in the interlayer spacing of this swelling clay mineral. The solvent structure was found to be very similar to predictions by a previous Monte Carlo study based on classical pair potentials (Boek, E. S.; Coveney, P. V.; Skipper, N. T. J. Am. Chem. Soc. 1995, 117, 12608). This lends support to our observations regarding the dynamics of the water molecules. First, the water shows a strong preference to form a hydrogen-bonded network between the solvent molecules. The hydrogen bonds to the aluminosilicate surface are only weak and short-lived. Second, the solvent displays liquidlike relaxation dynamics. Our results suggest that, because of the two-surface geometry in clays, intercalated water is prevented from freezing and should be considered in terms of a confined liquid. This is consistent with recent QENS experiments. Finally, we have calculated the power spectra for both hydrated and dehydrated clay from a Fourier transform of the MD trajectories. The calculated spectra are compared with experimental FTIR spectra.