Structure and dynamics of the interlayer water in an uncharged 2 ∶ 1 clay

The structure and dynamics of interlayer water at T = 300 K in a 2 ∶ 1 uncharged model clay structure were studied by means of molecular dynamics simulation. The model is based on the structure of vermiculite with the interlayer spacing d001 = 14.85 A. The water molecules form an adsorbed layer in the proximity of the clay surface; the local structure of the interlayer water is similar to that of the molecules in the liquid. The calculations do not reveal the formation of stable hydrogen bonds between the molecules and the clay surface and, as a consequence, the confined molecules have 30% fewer H-bonds per molecule in comparison to the molecules in liquid water. The lack of the hydrogen bonding manifests itself in the centre-of-mass power spectrum with the polarization perpendicular to the surface and in the spectrum of the O–H stretching vibrations. The diffusional motion of the interlayer water was studied by analysis of the moments of propagator and by the calculation of the intermediate scattering functions (ISCF). The diffusion mechanism that emerges from the analysis depends on the approach used. Thus, the results obtained by the analysis of propagator and by calculation of the relaxation times of the ISCFs indicate the applicability of the continuous diffusion model. On the other hand, the use of the functions for modelling the quasi-elastic neutron scattering spectra (QENS) shows that the width of the QENS peak follows the Q-dependence predicted by the Gaussian jump-diffusion model. The values of the diffusion coefficient calculated by either approach are in a fair agreement with each other and the obtained diffusion coefficients of the interlamellar water D = 0.92 × 10−9–0.99 × 10−9 m2 s−1 are about three times less than that for the molecules in bulk water.