Hydration of Na⁺, Ni²⁺, and Sm³⁺ in the Interlayer of Hectorite: A Quasielastic Neutron Scattering Study

Quasielastic neutron scattering experiments were performed with Na−hectorite, Ni−hectorite, and Sm−hectorite samples in order to find out whether Sm³⁺ is present in the clay interlayer as a fully hydrated cation (outer-sphere complex), or, as it follows from neutron diffraction data analysis, it is dehydrated and bound to the clay surface (inner-sphere complex). The results obtained for the Sm−hectorite were compared with other interlayer cations: strongly hydrated Ni²⁺ and relatively weakly hydrated Na⁺. It was found that water mobility in the Sm−hectorite sample is very close to the water mobility in Ni−hectorite. This is only possible if the Sm³⁺ ion is fully hydrated. It was shown that water molecules hydrating Ni²⁺ and Sm³⁺ exhibit diffusion mobility measurable with backscattering spectrometers. The diffusion coefficients of the exchangeable cations were found using the slow exchange approximations D _Ni = (0.05 − 0.14) × 10⁻⁹ m²/s and D _Sm = (0.04 − 0.18) × 10⁻⁹ m²/s.