The PMF is calculated by integrating the negative of the mean force and taking its value in the bulk region as reference (i.e. zero). The bulk solvent is defined as the region where the interactions of both the OFM and the solvent with the surface are negligible (3-4 nm from the surface). For the molecules considered here, the mean force F (ξ) close to the surface and thus the changes of the PMF in that region are mainly determined by the interaction of the headgroup with the surface, whereas the mean force in the transition from the bulk to the surface proximity is mainly determined by the tailgroup-solvent interactions, since the interactions of the polar headgroups with the surface are still weak. As we have chosen as reference the free energy within the bulk to be zero, the depth of the minimum will be determined by the interactions of the OFM molecule with both the surface and the solvent.