The structural environments of cations adsorbed onto clays: 133Cs variable-temperature MAS NMR spectroscopic study of hectorite

Abstract

133Cs Variable-Temperature Magic-Angle-Spinning Nuclear Magnetic Resonance (VT-MAS NMR) spectroscopy shows that Cs on the clay mineral hectorite occurs in several distinctly different chemical environments, and that motional averaging of Cs between some of these sites occurs above ~ −40°C if water is present in the interlayer. At temperatures above ~ −10°C, spectra for slurries of hectorite in CsCl solutions yield two peaks, one due to Cs in solution, and the other due to Cs motionally averaged on the clay. At temperatures below ~ −60°C, motional averaging of the adsorbed Cs slows sufficiently to allow resolution of two peaks representing different Cs-environments on the clay. We use the Stern-Gouy model to explain these peaks, and assign one to Cs in the Stern layer (relatively tightly bound to the basal oxygens) and the other to Cs in the Gouy diffuse layer. A sample hydrated at 100% relative humidity yields similar results, except that there is no peak for Cs in the solution. Cs-exchanged hectorite dehydrated at 500°C yields peaks for two different sites on the clay, which we interpret to be due to a highly coordinated site (probably 12) and a less coordinated site (possibly 9), both in the interlayer. A sample partially dehydrated at 100°C yields similar peaks, but there also appears to be motional averaging of Cs over these two different Cs-environments in the interlayer.