The surface tension of the air/water interface is a phenomenon of particular interest in the water-unsaturated zone of porous media because it influences the contact angle and consequently the capillary water volume. A mechanistic model based on the modified Poisson-Boltzmann equation and the Pitzer theory is described and used to predict, under isothermal and isobaric conditions, the surface tension of 1:1 electrolytes at high salinity. These theories enable the determination of the electrical potential at the air/water interface and the activity coefficient of the ionic species in the bulk pore water, respectively. Hydration free energies of the structure-making and structure-breaking ions that influence the surface tension at high salinity are taken into account. Structure-making ions flee the air/water surface because they can better organize the water dipoles in bulk water than at the interface. Structure-breaking ions are positively adsorbed at the air/water interface because the bulk water can better organize their hydrogen bonding network without these ions. The resulting surface tension increases and decreases, respectively, compared to the surface tension of pure water. The model predictions are in good agreement with the surface tension data for 1:1 electrolytes (NaCl, KCl, HCl, NaNO3, KNO3, HNO3 electrolytes) and the optimized parameters depend on the effective electrostatic diameters of cations and on the hydration free energies of the ions at the interface.