Phys. Rev. Lett. 85, 3460-3463 (2000)
Configurational-bias Monte Carlo simulations in the grand canonical ensemble with histogram reweighting were used to obtain the vapor-liquid coexistence curves of three hexane isomers physisorbed on a flat gold substrate. Examination of the critical ordering operator distributions confirms that these systems exhibit critical behavior consistent with the 2D Ising universality class. The critical temperatures for 2,2-dimethylbutane, 2,3-dimethylbutane, and n-hexane were determined as Tc2D 195.0, 161.2, and 152.4 K, respectively. This is qualitatively different from the behavior observed for bulk (3D) fluids, where branched alkanes typically exhibit lower critical temperatures than their linear counterparts.