Insoluble surfactant (surface active material) monolayers at the water/air interface are called Langmuir Monolayers and mimic many properties of biological membranes. As function of lateral pressure and temperature, the monolayer undergoes several phase transitions. We use mean-field theory to investigate the phase behavior of such monolayers, in the case where the surfactants are electrically charged, and are in contact with a semi-dilute solution of oppositely charged polyelectrolytes. This situation resembles the interaction of a biological cell membrane with various charged macromolecules. We calculate the effect of the chains on the phase diagram on the of the monolayers as well as the role the surfactant monolayer has on the adsorption of the polyelectrolytes as function of the long-range electrostatic interactions and short-range interactions between the polyelectrolytes and the surface.