Formation of 2‑D Crystalline Intermixed Domains at the Molecular Level in Binary Self-Assembled Monolayers from a Lyotropic Mixture

The organization and intermolecular interactions of mixed self-assembled monolayers (SAMs) of 1-decanethiol (DT) and 11-mercapto­undecanoic acid (MUA) formed on Au(111) from a lyotropic medium have been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and infrared reflection–absorption spectroscopy (IRRAS) as a function of their surface composition. The presence of a single reductive desorption peak in the voltammograms and thiol surface coverages close to 7.6 × 10–10 mol/cm2 reveal the formation of homogeneous mixtures of both molecules in an upright configuration in the mixed SAMs. Despite this fact, two different distributions of the individual components have been found in these MUA-enriched SAMs according to their different electron transfer blocking behavior and conformational order shown by EIS and IRRAS results. At short modification times, MUA–MUA and DT–DT homomolecular interactions prevail during the assembly process, and nanodomains of like molecules are probably formed by kinetically trapped metastable states. On the other hand, MUA–DT heteromolecular interactions are predominant in the layers formed at the higher immersion time, leading to a crystalline-like conformational order of molecularly intermixed domains. A discussion based on the Bragg–Williams thermodynamic approach and on the role of the spatial distribution and charge state of the carboxyl terminal groups in the SAMs’ organization is performed.