A study on hydration behaviors of interlayer cations in montmorillonite by solid state NMR

Solid state 23Na NMR spectra of Na-montmorillonite under dry and hydrated conditions have been measured to study hydration structure of Na+ in interlayer spaces. The 23Na triple quantum (3Q) MAS NMR experiments have been performed to determine the quadrupole coupling constants (CQ) and the isotropic chemical shifts (δiso). The CQ values were found to remarkably depend on water content, i.e. the CQ values increase with changing from dry condition to hydrated one (at around 12.5 wt% water content) and gradually decrease with an increase in the water content. Since the CQ value is a sensitive parameter to the local structure of the nucleus measured, the changes in CQ values with water content should correspond to those in the hydration structure of NaC in the interlayer spaces. Thus, it is considered that the increment of the CQ value in the water content of 12.5 wt% is due to the formation of hydrated Na+ with planner structure, and that the decrease in CQ values with the water content is attributed to the formation of hydrated Na+ with spherical symmetric structure.