Phase transitions in systems of grafted rods

Mobile rigid rods grafted on an interface are treated both by Onsager-type and Monte Carlo approaches. Symmetry arguments are presented which show that, unlike the case of hard rods in bulk, no orientational "standing-up" transition occurs: Attractive forces between rods are necessary for a discontinuous change in alignment. These conclusions are confirmed by Monte Carlo simulations and are discussed in the context of recent controversies concerning the "expanded" to "condensed" phase transition in adsorbed surfactant monolayers.