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  Sobolev, O.; Le Forestier, L.; González, M.A.; Russina, M.; Kemner, E.; Cuello, G.J.; Charlet, L.: Hydration of Na⁺, Ni²⁺, and Sm³⁺ in the interlayer of hectorite: a quasielastic neutron scattering study. The Journal of Physical Chemistry C 113 (2009), p. 13801-13812

10.1021/jp903179g

Abstract:
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 (outersphere 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 DNi ) (0.05 - 0.14) × 10-9 m2/s and DSm ) (0.04 - 0.18) × 10-9 m2/s.