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

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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 (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 Ni 2+ 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.

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