A 50 ps portion of the Cs-montmorillonite, 2/3 water mono-layer MD simulation animation (750–800 ps). Interlayer species are represented as described in Plate 1. The four Cs+ toward the right of the animation are bound together by extended water sharing arrangements into a structure that is stable over the timespan of the simulation. The two Cs+ toward the left of the simulation cell (red and brown) display continuous diffusional motion and participate in an exchange reaction, with first one and then the other taking up a position near an octahedral charge site in the clay layer. During the time that each cation is associated with this exchange site, a pale color is assigned to the relevant sphere. See Additional file 4 to access the animation.