All calculated adsorption PMFs (Figs. 4a, 4b, 4c) exhibit at least one minimum close to the surface. The PMF profiles also show energy barriers that correspond to the interaction of the tail with the layering of the solvent molecules close to the surface. There are only minor differences between PMFs from n-hexadecane and PAO4. The simulation time required to reach convergence of the PMF profile is much greater for PAO4 than n-hexadecane, due to its longer relaxation times. The results for SA are in reasonable agreement with the ones from Jaishankar et al. for the adsorption of SA from n-hexadecane onto Fe 3 O 4 (111). As the SA molecule approaches the α-Fe 2 O 3 (010) surface, the free energy oscillates with a period (∼0.5 Å) that corresponds to the thickness of the solvent layer (Fig. ). At OFM-surface distances of around 2 Å, 1.5 Å, 1.0 Å, and 0.5 Å, there are free energy barriers that correspond to the OFM moving through the solvent layers. The tailgroup is preferentially located in the maxima of the solvent mass density profile, and a small energy barrier has to be crossed for the tailgroup to move between these maxima. The height of these free energy barriers increases as the OFM gets closer to the surface, because the solvent layering is more pronounced.