Pure Graphene Acts as an "Entropic Surfactant" at the Octanol-Water Interface

Graphene oxide (GO) has proved itself as a nanomaterial capable of acting as a surfactant by lowering the interfacial tension of the oil-water interface due to its polar oxygen groups. However, the surfactant behavior of the pure graphene sheet─since prevention of edge oxidation in experimental setups is nontrivial─is still an unresolved issue in graphene research despite significant progress in the field in recent years. Here, we conduct both atomistic and coarse-grained simulations to demonstrate that─surprisingly─even pristine graphene, which only consists of hydrophobic carbon atoms, is attracted to the octanol-water interface and consequently reduces its surface tension by 2.3 k_BT/nm² or about 10 mN/m. Interestingly, the location of the free energy minimum is not precisely at the oil-water interface itself but is rather buried about two octanol layers into the octanol phase, being about 0.9 nm from the water phase. We demonstrate that the observed surfactant behavior is purely entropically driven and can be attributed to the unfavorable lipid-like structuring of octanol molecules at the free octanol-water interface. In essence, graphene enhances the inherent lipid-like behavior of octanol at the water interface rather than directly acting as a surfactant. Importantly, graphene does not display surfactant-like behavior in corresponding Martini coarse-grained simulations of the octanol-water system since the free liquid-liquid interface loses essential structure at the lower coarse-grained resolution. However, a similar surfactant behavior is recovered in coarse-grained simulations of longer alcohols such as dodecan-1-ol and hexadecan-1-ol. The observed discrepancies at different model resolutions enable us to construct a comprehensive model of the surfactant behavior of graphene at the octanol-water interface. The here-gained insights may facilitate the broader utilization of graphene in numerous domains of nanotechnology. Furthermore, since a drug's octanol-water partition coefficient is a crucial physicochemical parameter in rational drug discovery, we also believe that the universality of the here-illustrated entropic surfactant behavior of planar molecules deserves special attention in the drug design and development field.