OFMs are known to form vertically-orientated SAMs on iron oxide surfaces. Therefore, the surfactant headgroup size will mostly determine the hard-disk area of the adsorbed molecule. Two approaches were used to estimate the hard-disk area occupied by the OFMs on the hematite surface. In the first approach, the hard-disk radius was estimated using the ABF method to calculate the PMF for the lateral interactions between OFMs. Two OFMs were adsorbed on the hematite surface and the projection onto the x-y plane of the distance between the headgroups was used as colvar. Subsequently, the cross-sectional area occupied by each OFM on the hematite surface was estimated using the position of the PMF minimum, following the approach used by Jaishankar et al. for SA at the n-heptane-iron oxide interface and Sresht et al. for polyethylene glycol at the air-water interface. The results obtained using this first method can be found in the Supporting Information. In the second approach, annealing simulations of preformed OFM monolayers were performed. A similar approach to that described by Zhang et al. was used here. Different numbers of OFM molecules were placed close to the surface to form monolayers with a range of initial surface coverages. A subsequent equilibration of 2 ns allowed the molecules to adsorb onto the surface. Then the temperature was increased instantaneously from 300 K to 400 K and the system was equilibrated for 16 ns, which allowed the unbound molecules to desorb and escape the surfactant monolayer. Subsequently the temperature was reduced back to 300 K for another 2 ns. The final coverage was then estimated by determining the number of adsorbed and desorbed molecules, based on the distances of the hydroxyl oxygen atom of the headgroup to the hematite surface. The threshold distance (4.0 Å) was chosen such that for low concentrations, the number of adsorbed molecules at the end was equal to the number of initially placed molecules. Varying the number of the initially placed molecules allowed us to determine the maximum coverage and thus the cross-sectional area of each molecule. No base oil molecules were included in the annealing simulations since tests showed that their presence did not significantly affect the results. The same observation was made in previous MD simulations of SA monolayer stability on iron oxide surfaces.