An improved drainage model for foams involving ionic surfactants is presented which accounts for the effect of surfactant and salt concentrations. The effect of surfactant and salt concentrations on collapse is due mainly to their influence on the maximum disjoining pressure in the liquid films, The disjoining pressure is computed as a sum of the san der Waals force, the electrical double-layer force, and a short range repulsive force. The dissociated surfactant molecules on the inner surface of a film give rise to the electrical charge, while the undissociated molecules behave as dipoles. Since the surface charge determines the electrical double-layer force and the dipoles give rise to the short range repulsive force, the degree of dissociation of adsorbed surfactant molecules plays an important role in determining the characteristics of the disjoining pressure isotherm. Conditions are identified under which various kinds of disjoining pressure isotherms arise, It is shown that, under certain conditions, a high salt concentration, instead of causing collapse as predicted by the conventional DLVO theory, increases the number of surface dipoles due to counterion binding and gives rise to a strong short range repulsive force. Thus, under certain conditions, high salt concentrations result in the formation of very stable Newton black films. Simulations for foams are carried out taking into account, in addition, the effect of surfactant and salt concentrations on the surface viscosity and surface tension. Results indicate that the surfactant and salt concentrations play a crucial role in determining whether a foam will collapse. They also significantly affect the drainage through the Plateau border channels via their effect on the surface viscosity.