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 T_c^2D  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.