Combination of Inelastic Neutron Scattering Experiments and ab Initio Quantum Calculations for the Study of the Hydration Properties of Oriented Saponites

Swelling clay minerals are formed with two tetrahedral sheets (silica) sandwiching an octahedral sheet forming the 2:1 layer. Due to isomorphic substitutions by less charged cations in either the tetrahedral or octahedral sheet, the solid layer bears a net negative charge that is compensated by interlayer cations that can control the hydration properties. In this paper we study the hydration of synthetic saponite clay samples by combining inelastic neutron scattering (INS) experiments and ab initio quantum calculations, that include normal modes analysis and ab initio molecular dynamics simulations (AIMD). The first part of the paper focuses on the dry clay sample. The model used for the calculations was validated by a thorough comparison between INS results and ab initio calculations. Normal mode analysis provided a clear assignment of the different vibrational bands of dry saponite. Furthermore, as oriented samples were used in the INS experiment, it was possible to observe clear anisotropy effects thus confirming the two-dimensional (2D) nature of the system. The second part of the paper analyzes the hydration properties of both low- and high-charge saponite samples by combining INS experiments and AIMD calculations. The shift to higher energies of the librational bands of interlayer water upon increasing hydration is thus revealing a stronger H-bond network of interlayer water. The agreement between calculated vibrational density profile, AIMD, and grand canonical Monte Carlo simulations shows that a combined INS/density functional theory (DFT) study allows performing studies that avoid some of the common drawbacks linked to the use of empirical force fields.