Influence of cation nature on high pressure intrusion of aqueous salt solutions in pure silica MFI-type zeolite

High pressure intrusion-extrusion of water and aqueous salt solutions in hydrophobic porous solids such as pure silica zeolites (zeosils) is a promising way to absorb and store mechanical energy. The intrusion pressure has already been shown to be increased with salt concentration, but the influence of cation nature is not yet fully understood. The intrusion-extrusion experiments of aqueous chloride solutions of alkali, alkaline-earth and transition metals (MCl_<i>n</i>, where M^<i>n</i>+ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Ca²⁺, Mg²⁺, Sr²⁺, Al³⁺, Mn²⁺, Ni²⁺ Co²⁺, Cu²⁺, Zn²⁺, Cd²⁺) in MFI-type zeosil (silicalite-1) have been performed to study this effect. In contrast to the anions, the reported results indicate that the cation nature does not have a significant influence on intrusion behavior. All the systems demonstrate a spring behavior with fully reversible intrusion except the ones with highly concentrated ZnCl₂ and CsCl solutions, where a small part of the liquid remains trapped in the pores in the first intrusion-extrusion cycle. At fixed H₂O/salt molar ratio, a strong influence of cation nature on intrusion pressure is observed. For alkali, alkaline-earth metal and aluminium cations, the intrusion pressure rises with cation charge, whereas the increase of cation size leads to a pressure decrease. These trends are also in agreement with cation hydration enthalpy value for alkali and alkaline-earth metal cations, but no correlation is observed for the most part of the transition metal ones except Zn and Cd.