MODELS AND SIMULATION OF ORIENTATIONAL PHENOMENA IN DISCOTIC MONOLAYERS

A simple site-site model of discotic molecules in the presence of a planar external field is used to investigate the competition between standing-up states (condensed-liquid) and lying-down states (expanded-liquid) in adsorbed monolayers of insoluble disks. Computer simulation data are collected to map out this orientational phase behavior in a three-dimensional phase space defined by the set of thermodynamic fields (T, p(1), h(1)), denoting temperature, lateral pressure (favoring standing-up states), and surface field (favoring lying-down states), respectively. The results show fairly abrupt changes in orientational order, in quantitative agreement with the position of a first-order phase transition surface predicted by simple density functional theory. Mean-field lattice models also yield a similar transition, which can be interpreted as applying to our monolayer system. Despite a strong bimodal character to the orientational order, the identification of cluster formation in the molecular simulation data is not straightforward. In snapshots in the region of greatest fluctuations, one observes chains of standing-up disks wandering through the system, weakening island formation of incipient two-phase coexistence.