The alignment of particles with attractive interactions at the solid interface is complicated by attractive interactions of the fluid with the substrate and within the fluid itself. Qualitatively, the faceon orientation should be energetically favoured when the facesubstrate interactions are strong (although this will also depend on the relative strength of the fluid face-face, side-side, and sidesubstrate interactions), and the side-on orientation should be energetically favoured when the fluid particle's face-face interactions are stronger than both their side-side and face-substrate in-teractions. Assuming that the fluid is isotropic everywhere except for the layer adjacent to the interface, and that a fully face-on particle interacts with six nearest neighbours through side-side interactions (hcp packing) while a side-on particle has two faceface interactions and two side-side interactions (cubic packing with inequivalent lattice spacing) with its nearest neighbours (as found in simulations in which the interface layer was close to fully aligned, as shown in ESI Fig. ), the difference in energy per interfacial fluid particle between a completely face-on orientated interfacial fluid layer and a completely side-on oriented one can be estimated to be