Influence of Sub-angstrom Change in Interstitial Ion Size on Clay Swelling

Clay minerals in contact with aqueous bulk reservoir undergo a geological transformation of swelling or shrinking by exchanging interstitial cations. For geological applications, it is crucial to understand the stability of these layered materials. Here, we demonstrate that a sub-angstrom change in the interstitial cation size with similar hydration characteristics is enough to destabilize the optimum spacing of layered materials. We used molecular simulations to investigate the stability of water layers in the K-, Rb-, and Cs-mica pores. We find that ±0.1 Å, change in the size of interstitial cation - from Rb⁺ to K⁺ or Cs⁺ ion - leads to -15 to 5 % change in equilibrium loading of adsorbed water and 2 to 35 % change in interlayer spacing. Our thermodynamic analysis reveals an intricate interplay between enthalpic and entropic contributions caused by the structural change of water in the pores due to the hydration of interstitial cations. The understanding from this work has direct implications in designing clay swelling inhibitors in the oil/gas recovery using fracking and sealing materials for radioactive waste.