combinations. For SA, the barrier height is larger for PAO4 than n-hexadecane, despite the stronger layering for the latter (Fig. ). This implies that the solvent layers are more difficult for SA to penetrate for branched alkanes solvents than linear ones. In practical terms, this means that SA adsorption will be slower from PAO4 than n-hexadecane at 300 K. The improved ability for the saturated linear OFM molecule to penetrate the n-alkane of similar chain length could be the molecular mechanism of the chain matching phenomena observed experimentally. For GMO, the free energy barrier heights are similar for both solvents. In between these barriers, there are local minima in the free energy profiles corresponding to outer-sphere complexes. These have also been observed in previous MD simulations of oxalic acid adsorption on rutile from water and napthenic acid adsorption on calcite from water. For SA, the headgroup was defined as the carboxyl group -COOH. Only one innersphere adsorption mode was observed in the PMF at 2.3 Å above the surface, see Fig. kJ mol -1 ) from n-hexadecane, because the tailgroup is only able to lie flat to the surface in the latter case. This value is in good agreement with that obtained by Jaishankar et al. for SA adsorption from n-hexadecane onto Fe 3 O 4 (111) (29.1 kJ mol -1 ).