Interplay of wetting and adsorption at mixed self-assembled monolayers

Mixed monolayers of CH₃/OH-terminated alkanethiolates chemisorbed on gold exhibit anomalous variations of wettabilities, possibly because of adsorption of a minority species from the bulk. Starting from a lattice-gas model with proper quenched averaging over the chemically heterogeneous surface, we derive a mean-field theory appropriate for such systems by generalizing Cahn's wetting model. To study interplay of wetting and adsorption in the presence of impurity adsorbates, we introduce a second, coupled field representing the minority species concentration. Adsorption of the second species is allowed by assuming a step-function profile for its concentration. The interplay of wetting and adsorption is studied as a function of the chemical heterogeneity, the temperature, and the chemical potential of the minority species. We conclude with a brief discussion of the adequacy and limitations of this model and its possible generalizations for other systems exhibiting similar phenomena involving interplay of wetting, adsorption, and ordering at chemically heterogeneous surfaces.