Adsorption et condensation en couches mixtes: I. proprie´te´s des isothermes

sotherms for the mixed adsorbtion on two levels of a substance a on a isomorphe substrate b, proposed previously by one of the authors, are discussed for different values of the adsorption energy (ψ_ab), sublimation energy of the adsorbate (ψ_aa) and sublimation energy of the substrate (ψ_bb) per bond. The variation of ψ_bb, both other energies remaining the same, can modify the level of the adsorption, and the order of the two-dimensional phase transition. In the special case ψ_ab = 1/2ψ_bb the coverage θ is the same on both levels (inside and outside the top lattice plane of the substrate), until condensation takes place. The two dimensional condensation phenomena provoke always a demixing of the substances a and b between the two levels.