Neutron diffraction studies of ion coordination and interlayer water structure in smectite clays: lanthanide(III)-exchanged Wyoming montmorillonite

We describe the application of neutron diffraction with isotopic substitution to the study of the interlayer region of smectite clays and present new results for lanthanide(III)-exchanged Wyoming montmorillonite. Use of samples hydrated with H2O and D2O allows us to obtain a composite radial distribution function centred on the interlayer water hydrogens. This shows that the interlayer water adopts an extensively hydrogen-bonded liquid structure. Isotopic substitution of the interlayer lanthanide(III) ion combined with H2O/D2O substitution allows us to obtain the partial pair distribution function, gLnH(r) and a composite radial distribution function gLnX(r) where Ln=Nd, Yb. These functions define the cation coordination and show that the lanthanide(III) ions are only partially hydrated and that they bind to the clay; this is thought to be a result of mild heat treatment at 100 °C during sample preparation.