First-principles study of ammonium ions and their hydration in montmorillonites

Density functional theory calculations were performed to investigate the adsorption and hydration of an ammonium ion (NH4 (+)) confined in the interlayer space of montmorillonites (MMT). NH4 (+) is trapped in the six-oxygen-ring on the internal surface and forms a strong binding with the surface O atoms. The hydration of NH4 (+) is affected significantly by the surface. Water molecules prefer the surface sites, and do not bind with the NH4 (+) unless enough water molecules are supplied. Moreover, the water molecules involved in NH4 (+) hydration tend to bind with the surface simultaneously. The hydration energy increases with the intercalated water molecules, in contrast to that in gas phase. In addition, the hydration leads to the extension of MMT basal spacing.