Boundary effects in the orientational ordering of adsorbed nitrogen

Monte Carlo calculations of the herring-bone ordering phase transition of N₂ adsorbed on graphite have been performed for a variety of boundary conditions. The model consists of a rigid lattice of two-dimensional quadrupoles and for a small infinitely periodic system (PBC), a first-order phase transition is found, in agreement with previous work. For weak boundary conditions (WBC) the transition temperature is reduced and the transition remains first order. For strong boundary conditions (SBC) the transition is continuous. There is little residual ordering above the phase transition for the SBC and WBC transitions. The presence of weak and strong boundaries causes a film of adsorbed phase of thickness theta against the one-dimensional wall. ln theta is linear with temperature. A simple mean-field theory accounts for the qualitative behaviour of the boundary effects.