If the input pressure P in Equation is below P vap water , water remains in its gaseous phase so that ϕ = 1 and the pressure is indeed equal to fugacity. On the contrary, if the pressure is above P vap water , the fugacity coefficient drops (ϕ < 1) and the fugacity is thus below to the real pressure P. The fugacity f is the pressure that the hypothetical perfect fluid would have for its chemical potential to be the same as the chemical potential of the real fluid at pressure P . Let us introduce the two following systems: the first one consists of a given zeosil in equilibrium with a reservoir of pure water while the second system deals with the same zeosil, but the reservoir is replaced with an electrolyte solution. Then, a certain intrusion pressure P water is required, resulting in an intrusion of a certain amount of water inside the zeosil of the first system, in equilibrium with the reservoir of water. Since we hypothesize that only water molecules are intruded/extruded in the zeosils in the presence of electrolyte solutions, we want to determine the intrusion pressure P CnAm water that must be applied to the zeosil of the second system surrounding by the electrolyte solution, to obtain the same amount of intruded water molecules at equilibrium as for the first system. In order to find exactly the same amount of water molecules in both zeosils, the probabilities in Equation 5 must be the same for the two systems. In other words, it requires the fugacity of pure water f water and water in the electrolyte solution f CnAm water to be equal: