Swelling of K+, Na+ and Ca2+-montmorillonites and hydration of interlayer cations: a molecular dynamics simulation

This paper performs molecular dynamics simulations to investigate the role of the monovalent cations K, Na and the divalent cation Ca on the stability and swelling of montmorillonite. The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer. The simulations reproduced the swelling pattern of these montmorillonites, suggesting a mechanism of their hydration different (K^{+ <} Na ^{+ <} Ca^{2 + )} from that of K ⁺ -, Na ⁺ -, and Ca^{2 +} -montmorillonites. In particular, these results indicate that the valence of the cations has the larger impact on the behaviour of clay--water systems. It also finds that the differences in size and hydration energy of K ⁺ , Na ⁺ and Ca^{2 +} ions have strong implications for the structure of interlayer. This leads to the differences in the layer spacings of the simulated K ⁺ -, Na ⁺ -, and Ca^{2 +} -montmorillonites. Furthermore, these simulations show that the K cations interact strongly with the clay sheets for the dehydrated clay sheets, but for the hydrated clays the Ca cations interact clearly strongly with the clay sheets.