In the past years, a new field of application in energy storage was discovered for hydrophobic nanoporous materials, where the non-wetting liquid penetrates the pores under high pressure, resulting in conversion of mechanical energy into fluid-solid interface energy. When the pressure is released, the mechanical energy can be restored upon release of the fluid. Historically, intrusion of water in pure-silica zeolites (also called "zeosils") has been first reported in 2001 and represents the starting point of subsequent intrusion and extrusion experiments performed on various zeosils. Later, Tzanis and co-workers discovered that replacing pure water by electrolyte solutions is an effective way to increase the mechanical energy stored in zeosils, due to a shift of intrusion pressure towards higher values. For instance, it was shown that intrusion of a highly concentrated LiCl solution (∼ 20 M) in silicalite-1 increases intrusion pressure from 96 MPa (for pure water) to 280 MPa, resulting in an increase of stored energy from 10 J/g to 31 J/g. Since this pioneering work, the influence of the nature of the electrolyte and its concentration on the energetic properties of zeosils has been widely investigated. In a previous study, we have reported a computational investigation of the effect of the electrolyte solution (nature and concentration) on their intrusion pressure in defect-free pure silica zeolites. We found that due to poorer ion solvation in the pores than in the bulk, intrusion of electrolyte solutions starts with only intrusion of pure water (and exclusion of the ions). Moreovre, we showed that the shift of intrusion pressure is more complex than a simple osmotic effect, because electrolytes are non-ideal solutions. Taking into account the water activity, which we calculated by the Pitzer model for each electrolyte over a wide range of concentration, can explain the experimental data available.