To give comparable steepness for the repulsive potential at the vapour interface to the interaction with the solid substrate, the value of ε w was determined by fitting eqn (1) to the repulsive part of the potential energy versus z of a single fluid particle representative of perylene (κ = 0.36, κ = 0.19) interacting with the solid substrate in the face-on orientation for the region where the potential energy was < 2k B T at T = 0.62ε/k B . This gave a value of ε w = 17.8ε for the graphene substrate and ε w = 8.9ε for the silicon substrate. The same values for each substrate were used for both the attractive and repulsive surfaces, and the graphene value also used for the "strong" substrate, as the strength of the wall potential did not significantly affect the behaviour at the vapour interface (ESI Fig. ).