Cation hydration by confined water and framework-atoms have crucial role on thermodynamics of clay swelling.

The swelling capacity and stability of clay play a crucial role in various areas ranging from cosmetics to oil extraction; hence change in their swelling behaviour after cation exchange with the surrounding medium is important for their efficient utilisation. Here we focus on understanding the role of different hydration properties of cation on the thermodynamics of clay swelling by water adsorption. We have used mica as the reference clay, Na[Formula: see text], Li[Formula: see text], and H[Formula: see text] ions as the interstitial cations, and performed grand canonical Monte Carlo simulations of water adsorption in mica pores (of widths [Formula: see text] Å). The disjoining pressure ([Formula: see text]), swelling free energy ([Formula: see text]), and several structural properties of confined water and ions were calculated to perform a thermodynamic analysis of the system. We expected higher water density in H-mica pores ([Formula: see text]) due to the smaller size of [Formula: see text] ions having higher hydration energy. However, the counter-intuitive trend of [Formula: see text] (bulk density) [Formula: see text] was observed due to adsorption energy, where the interaction of water with mica framework atoms was also found to be significant. All three mica systems exhibited oscillatory behaviour in the [Formula: see text] and [Formula: see text] profiles, diminishing to zero for [Formula: see text] Å. The [Formula: see text] for Na-mica is characterised by global minima at [Formula: see text]   corresponding to crystalline swelling with significant and multiple barriers for crystalline swelling to osmotic swelling ([Formula: see text] Å). A shift in the location of global minima of [Formula: see text] towards the higher d values and [Formula: see text] becoming more repulsive is observed in the increasing order of hydration energy of [Formula: see text], [Formula: see text], and [Formula: see text] ions. The [Formula: see text] for all d in the H-mica system thus favours osmotic swelling. We found that the Na[Formula: see text] ions hydrate more surface oxygens, act as anchors, and hold the mica pore together (at smaller d), by sharing hydrating water with ions of the opposite side, forming an electrostatically connected mica-Na-water-Na-mica bridge. The Li[Formula: see text] ions do hydrate surface oxygen atoms, albeit in lesser numbers, and sharing of hydration shell with nearby Li[Formula: see text] ions is also minimum. Hydration by surface atoms and water sharing, both, are minimum in the H[Formula: see text] ion case, as they are mostly present in the center of the pore as diffusive ions, thus exerting a consistent osmotic pressure on the mica frameworks, favouring swelling.