The PMFs for surfactant adsorption on circumovalene from pure iso-octane (gasoline) and n-hexadecane (diesel) are shown in Figure and Figure , respectively. Both base fluids share the same non-polar chemistry and so the choice of iso-octane or n-hexadecane is not expected to have a large effect on the adsorption of either DCA type. On the other hand, there are major differences in the polar head-groups of the PIBSI and zwitterionic surfactants, which are expected to give significant differences in the adsorption PMFs. The non-polar DCA tail-groups are not expected to play a significant role in adsorption because they will only have relatively weak CH-π interactions 100 with the deposit, which will be of similar strength to those from the fuel molecules. From Figure and Figure , it is clear that the adsorption of the zwitterionic surfactant onto circumovalene is much stronger than for PIBSI for both fuel surrogates. In all cases, the PMF shows a negative minimum, indicating that adsorption is exergonic due to interactions between the DCA head-groups and the PAH molecules. For PIBSI, the adsorption free energy on circumovalene is approximately -1.8 kcal mol -1 in iso-octane and -1.0 kcal mol -1 in n-hexadecane, whereas for the zwitterionic surfactant is approximately -5.8 kcal mol -1 and -5.1 kcal mol -1 , respectively. These result are consistent with previous experimental measurements that showed enhanced adsorption strength for molecules with additional functional groups that can interact with the deposit through multiple noncovalent interactions besides the standard polyamine groups in PIBSI. The adsorption energies are comparable to previous MD studies of the adsorption of small molecules on graphene from water (-4 kcal mol -1 to -8 kcal mol -1 ). The somewhat weaker adsorption in our MD simulations can be attributed to the stronger interaction of the fuel solvent with the aromatic surface compared to water, the use of finite PAHs rather than infinite graphene sheets, and penalising entropic effects due to the many adsorption conformations of the DCA head-groups.