Dipole-induced, first-order phase transitions of nano-rod monolayers

The isotropic-nematic transition of nano-rod monolayers with fore-aft symmetry is second order, in stark contrast to the first-order phase transition explained by Onsager [L. Onsager, Ann. (N.Y.) Acad. Sci. 51 (1949) 627] for rods in three dimensions. Here we show that the coupling of a dipole potential to excluded volume is sufficient to re-instate a first-order phase transition of rods confined to two dimensions.