Effect of branching on the fluid phase behavior of alkane monolayers

The vapor-liquid coexistence curves and critical points have been determined for 2,2-dimethylbutane, 2,3-dimethylbutane, and n-hexane physisorbed on a flat gold substrate. The critical density distributions were shown to map directly to the universal fixed point form for the 2D Ising universality class, confirming that these systems exhibit 2D Ising critical behavior. The effect of branching on the location of the vapor-liquid coexistence curves and critical points was found to be significant. Microscopic analysis of the physisorbed molecules shows that a high degree of branching causes a smaller footprint that leads to an increase in the saturated liquid density and the critical temperature - a striking contrast to bulk fluids, where branching tends to lower the saturated liquid density and critical temperature.