In contrast to alignment at the solid interface, which depends on the fluid-fluid and fluid-substrate interactions, the energetic driver for orientation at the vapour interface is simply expected to be related to the relative strength of the interactions for a face-on and side-on fluid particle with other fluid particles in the interfacial layer. Face-face interactions are maximised in the side-on orientation, so increasing the strength of these interactions is expected to promote the side-on orientation. Compared with the solid interface, the vapour interface does not show as clear universal scaling of the molecular orientation with the interface free energy parameter in eqn (3), but a strong trend with the free energy is still observed (Fig. ). Where κ < 1, the orientation at this interface converges towards isotropic at the vapour interface as the free energy approaches zero, and becomes more aligned as the free energy magnitude increases. At high values of κ (low interaction anisotropy), the energetic component (Fig. ) shifts towards very slightly favouring a face-on orientation, qualitatively consistent with previous predictions of ordering of bulk nematic fluids at the free interface. Several systems were also examined with κ > 1, corresponding to stronger side-side than face-face interactions, as it is expected to promote the face-on orientation at the vapour interface. respectively is shown in (a). The energetic component is coloured by the interaction anisotropy parameter, the entropy component by the shape anisotropy parameter, and the overall free energy by the ratio of the two parameters. Filled symbols correspond to simulations with the graphenestructure substrate (graphene or "strong") and unlled symbols to the silicon substrate. Shaded grey regions highlight where the points should fall if obeying the predicted scaling with the free energy or free energy component.