Due to intermolecular interactions, the surface composition of mixed self-assembled monolayers often differs markedly from the solution composition. In the case of charged surfactants, large deviations from the ideality have been reported. The effect of the electrostatic interactions between charged compounds on the surface composition is examined using a simple gas lattice model, in the Bragg-Williams approximation. The interaction potential is obtained from the Debye-Huckel treatment of electrolytic solutions. The model is able to explain and reproduce the deviations observed experimentally, both qualitatively and quantitatively. The key role played by the ionic strength on the surface composition is emphasized.