Abstract
Experimental techniques as neutron diffraction (ND) with isotopic substitution (NDIS), extended Xโray absorption fine structure spectroscopy (EXAFS), and quasielastic neutron scattering (QENS), in combination with molecular dynamics (MD) simulations, are usually applied to the study of nonโcrystalline solids, but they are also very useful for the study of complex systems, where the short range order provides an insight of its structure and dynamics. Here, they are used in the study of the coordination of the Sm^3+^ in the interlayer of hydrated synthetic montmorillonite and hectorite. The ND results indicate that not all oxygen atoms in the first coordination shell of the Sm^3+^ belong to water molecules, supporting the formation of the Sm^3+^ innerโsphere complex. On the other hand, the other techniques suggest that the adsorbed Sm^3+^ cations form outerโsphere complexes with the clay surface. The hypothesis making compatible all results is that there are different Sm species adsorbed in the clay interlayer: a part of Sm is in the Sm^3+^ cationic form, forming outerโsphere adsorption complexes, another part is hydrolyzed and present in the interlayer space as Sm(OH)^2+^, ${\rm Sm(OH)}_{2}^{ + } $, or ${\rm Sm(OH)}_{3}^{0} $ species. The latter are more hydrophobic than Sm^3+^ cations and can be dehydrated and are able to stick to the clay surface.