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

Abstract

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 $$d = 4-40$$ d = 4 - 40 Å). The disjoining pressure ($$\Pi$$ Π ), swelling free energy ($$\Delta \Omega ^{ex}$$ Δ Ω 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 ($$\rho_{ \hbox{H}}$$ ρ H ) due to the smaller size of $$\hbox {H}^+$$ H + ions having higher hydration energy. However, the counter-intuitive trend of $$\rho _{\hbox{Li}}> \rho _{\hbox{Na}} > \rho_b$$ ρ Li > ρ Na > ρ b (bulk density) $$> \rho_{\hbox{H}}$$ > ρ 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 $$\Pi$$ Π and $$\Delta \Omega ^{ex}$$ Δ Ω ex profiles, diminishing to zero for $$d \ge 11$$ d ≥ 11 Å. The $$\Delta \Omega ^{ex}$$ Δ Ω ex for Na-mica is characterised by global minima at $$d=6 {\hbox {\AA}}$$ d = 6 Å corresponding to crystalline swelling with significant and multiple barriers for crystalline swelling to osmotic swelling ($$d > 12$$ d > 12 Å). A shift in the location of global minima of $$\Delta \Omega ^{ex}$$ Δ Ω ex towards the higher d values and $$\Delta \Omega ^{ex}$$ Δ Ω ex becoming more repulsive is observed in the increasing order of hydration energy of $$\hbox {Na}^+$$ Na + , $$\hbox {Li}^+$$ Li + , and $$\hbox {H}^+$$ H + ions. The $$\Delta \Omega ^{ex} > 0$$ Δ Ω 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.

Strontium and Cesium Adsorption on Exopolysaccharide-Modified Clay Minerals

“…By contract, the interlayer spacing changing was not significant with various concentrations of Cs addition (Figure b), and only one water layer was formed. A similar phenomenon was also reported due to the collapsing of Cs + -exchanged MMT. , …”

“…This confirmed the interlayer reaction of Sr with EPS . Conversely, the characteristic peak began to weaken after adsorbing Cs, almost disappearing after absorbing 1000 mg/L Cs, demonstrating a disordered feature formation or the collapsing of interlayers. ,, On the one hand, there should be the presence of substances with an amorphous structure and/or the formation of a disordered pillared structure, which was confirmed by SEM characterization (discussed below, Figure a,f), and was consistent with the basal spacing. On the other hand, the inner-sphere complexation may occur between fully or partially dehydrated Cs and the more durable siloxane groups where Cs irreversibly bound between the upper and lower tetrahedral sheets, resulting in collapsing interlayers ,, It was also shown in AFM (Figure f) that the distinct columnar, as well as the uniform, rounded, and thick structures appeared.…”

“…A similar phenomenon was also reported due to the collapsing of Cs + -exchanged MMT. 40,41 Notably, after adsorbing Sr, the characteristic peak became sharper and more obvious with an increasing loading, suggesting a more ordered feature of the composite. 34 This confirmed the interlayer reaction of Sr with EPS.…”

Comparative Study of Different Ion-exchange Membrane Types in Diffusion Dialysis for the Separation of Sulfuric Acid and Nickel Sulfate

“…for Ni 2+ [25], [26], [27], [28]. The osmotic flux was calculated as a difference between total water flux and drag water flux using the formula (4).…”

Diffusion Dynamics of Water and Ethanol in Graphene Oxide

“…The study of water confined in a matrix is of great importance in biology, 1 oil extraction, 2 and water purification, among others. Graphene oxide (GO) as a confining material provides an interesting and important case study because it is based on a hydrophobic material, graphene, which can be functionalized to alter its chemical surface properties.…”