From classical molecular dynamics simulations, we identify a simple and general predictor of molecular orientation at solid and vapour interfaces of isotropic fluids of disk-like anisotropic particles based on their shape and interaction anisotropy. For a wide variety of inter-particle interactions, temperatures, and substrate types within the range of typical organic semiconductors and their processing conditions, we find remarkable universal scaling of the orientation at the interface with the free energy calculated from pair interactions between close-packed nearest neighbours and an empirically derived universal relationship between the entropy and the shape anisotropy and bulk volume fraction of the fluid particles. The face-on orientation of fluid particles at the solid interface is generally predicted to be the equilibrium structure, although the alignment can be controlled by tuning the particle shape and substrate type, while changing the strength of fluid--fluid interactions is likely to play a less effective role. At the vapour interface, only the side-on structure is predicted, and conditions for which the face-on structure may be preferred, such as low temperature, low interaction anisotropy, or low shape anisotropy, are likely to result in little orientation preference (due to the low anisotropy) or be associated with a phase transition to an anisotropic bulk phase for systems with interactions in the range of typical organic semiconductors. Based on these results, we propose a set of guidelines for the rational design and processing of organic semiconductors to achieve a target orientation at a solid or vapour interface.
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