  A model for a monolayer of two types of particles spontaneously forming
ordered patterns is studied by a mesoscopic theory and by MC simulations. We
assume hard-cores of the same size for both components, short-range attraction
long-range repulsion between particles of the same species, and the
cross-interaction of opposite sign. The model is inspired by oppositely charged
hydrophilic and hydrophobic particles or macromolecules with preferential
solubility in different components of a solvent that is close to a miscibility
critical point. We determine the phase diagram in the chemical potentials and
in the concentration - density planes for a few fixed temperatures in the
mean-field one-shell approximation. We find that the presence of the second
component significantly enlarges the temperature range of stability of the
ordered phases. We obtain three stable phases with periodic concentration: the
lamellar L phase with alternating stripes of the two components for similar
chemical potentials, and a hexagonal arrangement of the clusters of the
minority component in the liquid of the majority component. The latter two
phases, however, are stable only at relatively high temperatures. At lower
temperatures, the L phase coexists with a disordered one-component fluid or
with very dilute gas with mixed components. At still lower temperatures, the
one-component phase coexisting with the L phase can be disordered or ordered,
depending on the chemical potentials. The theoretical results are confirmed by
MC simulations for selected thermodynamic states.
