A simple model for dense phases of two-dimensional hard rods and its application to mono- and bidisperse alkanes physisorbed on graphite

Based on a simple mean field model, we study the positional ordering in dense systems of mono- and bidisperse parallel hard rods in two dimensions. We obtain phase diagrams exhibiting nematic, columnar, and crystalline order as a function of the packing density, the lengths, and the mixing ratio of the particles. Even though our theory neglects the details of the molecular interactions, it proves to be applicable to physisorption of large anisotropic molecules on smooth surfaces. This is demonstrated on the basis of scanning tunneling microscopy experiments on dense monolayers of highly ordered alkane chains physisorbed on graphite. Crystalline as well as columnar orderings are observed as a function of molecular weight for monodisperse adsorbates and, in addition, in terms of the mixing ratio for bidisperse adsorbates, in accord with the predictions of the model.