Structure of and competitive adsorption in alkyl dicarbamate two-dimensional crystals

The potential for relatively minor structural changes to dramatically impact materials properties is one of the primary obstacles to achieving the rational design of functional materials. For example, having an odd versus an even number of carbons between functional groups in polymers can cause large variation in melting point and mechanical properties. This odd-even effect is especially pronounced in hydrogen-bonded polymers and oligomers. To shed light on the structural basis of this phenomenon, physisorbed monolayers and single crystals of alkyl dicarbamates were investigated by scanning tunneling microscopy and X-ray diffraction, respectively. The related two- and three-dimensional crystal structures both demonstrated a clear odd-even effect in packing geometry. The differing accommodation of intermolecular interactions between odd and even packing motifs was directly related to the melting point trends and further dissected through computation. In addition, these oligomers displayed unusual competitive adsorption behavior; the relative preference for adsorption of a smaller species from a binary solution was increased compared to alkanes. These results were explained in the context of hydrogen bond density effects that arise due to competition for a limited substrate surface area. This study provides a model for understanding oligourethane surface coatings and demonstrates the importance of molecular structure and hydrogen bonding in determining adsorption behavior.