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

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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^+, Li^+, and H^+ ions as the interstitial cations, and performed grand canonical Monte Carlo simulations of water adsorption in mica pores (of widths d = 4-40 Å). The disjoining pressure (Pi), swelling free energy (Delta Omega ^{ex}), 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 (rho_{ hbox{H}}) due to the smaller size of hbox {H}^+ ions having higher hydration energy. However, the counter-intuitive trend of rho _{hbox{Li}}> rho _{hbox{Na}} > rho_b (bulk density) > rho_{hbox{H}} 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 Pi and Delta Omega ^{ex} profiles, diminishing to zero for d ge 11 Å. The Delta Omega ^{ex} for Na-mica is characterised by global minima at d=6 {hbox {AA}} corresponding to crystalline swelling with significant and multiple barriers for crystalline swelling to osmotic swelling (d > 12 Å). A shift in the location of global minima of Delta Omega ^{ex} towards the higher d values and Delta Omega ^{ex} becoming more repulsive is observed in the increasing order of hydration energy of hbox {Na}^+, hbox {Li}^+, and hbox {H}^+ ions. The Delta Omega ^{ex} > 0 for all d in the H-mica system thus favours osmotic swelling. We found that the Na^+ 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^+ ions do hydrate surface oxygen atoms, albeit in lesser numbers, and sharing of hydration shell with nearby Li^+ ions is also minimum. Hydration by surface atoms and water sharing, both, are minimum in the H^+ 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.