A density functional theory study of the hydration of calcium ions confined in the interlayer space of montmorillonites

The structures of Ca2+ hydrates in the interlayer space of montmorillonites (MMT) were studied by periodic density functional theory (DFT) calculations under the GGA/PBE approximation. Affected by the internal surfaces, which are rich of negative charge, the Ca2+ hydration exhibits different behaviors from that in gas phase. The Ca2+ is located at the six-oxygen-ring (SOR) on the internal surface in dry MMT, while the incoming water molecules bind with the Ca2+, the O atoms on surface, and/or with each other. The water molecules have a tendency of forming a hydrogen bond (HB) network that connects the upper and lower surfaces. Attracted by surrounding water molecules, the Ca2+ gradually moves outward with increasing number of water molecules. Moreover, the hydration energy (EH) of Ca2+ is determined not only by the interaction between Ca2+ and H2O, but also by that among Ca2+, H2O and the surfaces. As a result, the EH has only small changes for additional incoming water molecules, in contrast to the great and monotonic decrease in gas phase.