Structure and dynamics of interlayer species in a hydrated Zn-vermiculite. A molecular dynamics study

Abstract

The structure and dynamics of the interlayer species in hydrated Zn-vermiculite clay at 300 K were studied by means of molecular dynamics calculations. In a water-free structure, the Zn²⁺ ions adsorb on the surface of the clay layers. In the presence of H₂O molecules in the interlayer space Zn(H₂O)₆²⁺ complexes are built under migration of the ions to the midplane of the interlayer space. The complexes are oriented in the interlayer space so that at least four water molecules interact via their H atoms with the O atoms of the clay surfaces. The calculations show that in the interlamellar space the Zn–water complexes have the same structure and internal dynamics as in aqueous solution. This dynamics was characterized in details on the basis of the calculations. The rotational motion of both the “bound” and “free” molecules proceeds mostly via a reorientation of the HH vector of the molecules. No exchange between the solvating water molecules and the “free” water of the interlayer space was observed in the time-scale of the calculations (2.4 ns). The residence time of the H₂O molecules in the second hydration sphere of the cations was computed to be approximately four times longer than in aqueous solution. This increase of the residence time corresponds to the decrease of the diffusion coefficient of the interlayer water, as compared to the molecules in the liquid. The single-particle dynamics of the non-solvating water molecules was studied by the analysis of the intermediate scattering functions and by calculation of the quasi-elastic neutron scattering spectra. The diffusion coefficients D = (0.91 ± 0.11) × 10⁻⁹ m² s⁻¹ was obtained to be very close to that of the H₂O molecules in the uncharged clay.