High-pressure intrusion of double salt aqueous solution in pure silica chabazite: searching for cation selectivity

Heterogeneous lyophobic systems (HLSs), i.e. systems composed of a nanoporous solid and a non-wetting liquid, have attracted much attention as promising candidates for innovative mechanical energy storage and dissipation devices. In this work, a new HLS based on a pure silica chabazite (Si-CHA) and a ternary electrolyte solution (KCl + CaCl₂) is studied from porosimetric and crystallographic points of view. The combined approach of this study has been fundamental in unravelling the properties of the system. The porosimetric experiments allowed the determination of the energetic behaviour, while high-pressure in situ crystallographic analyses helped elucidate the mechanism of intrusion. The results are compared with those obtained for systems involving the same zeolite but intruded with solutions containing only single salts (CaCl₂ or KCl). The porosimetric results of the three Si-CHA systems intruded by simple and complex electrolyte solutions (KCl 2 M, CaCl₂ 2 M and the mixture KCl 1 M + CaCl₂ 1 M) suggest that the intrusion pressure is mainly influenced by the nature of the cations. The CaCl₂ 2 M solution shows the highest intrusion pressure and KCl 2 M the lowest, whereas the mixture KCl 1 M + CaCl₂ 1 M is almost in the middle. These differences are probably related to the higher hydration enthalpy and Gibbs energy of Ca²⁺ compared with those of K⁺. It has been demonstrated that partial ion desolvation is needed to promote the penetration of the species, and a higher solvation energy requires higher pressure. The `intermediate' value of intrusion pressure shown by the complex electrolyte solution arises from the fact that, statistically, the second/third solvation cation shells can be assumed to be partially shared between K⁺ and Ca²⁺. The stronger interaction of Ca²⁺ with H₂O molecules thus also influences the desolvation of K⁺, increasing the pressure needed to activate the process compared with the pure KCl 2 M solution. This is confirmed by the structural investigation, which shows that at the beginning of intrusion only K⁺, Cl⁻ and H₂O penetrate the pores, whereas the intrusion of Ca²⁺ requires higher pressure, in agreement with the hydration enthalpies of the two cations.