We performed Monte Carlo simulations in the osmotic ensemble in three representative hypothetical pure-silica zeolites that are silicalite-1, chabazite, and faujasite for which the adsorption isotherm of water has been calculated from grand canonical Monte Carlo simulations. Monte Carlo moves in the osmotic ensemble to insert electrolyte reveal that the ions do not penetrate the zeosils at water intrusion pressure because they are better solvated in bulk for low-diameter pore zeosils, due to lower ion solvation in the nanopores. Then, the chemical potential of water in electrolyte solutions has been calculated and it highlights the same dependency in pressure as the chemical potential of pure water. From these conclusions, we propose a thermodynamic model that revisits the law of osmotic pressure in order to predict the intrusion pressure in zeosils taking into account the nature of electrolytes in solution.