Molecular dynamics simulation studies of fullerene, fullerene derivatives and their clusters in aqueous solution and at the interface.

摘要

The property studies of C60 fullerene, C60 fullerene derivative with poly (ethylene oxide) (PEO), and their clusters in water and at the air-water interface were performed using molecular dynamics (MD) simulations. First, the vibrational cooling of C60 fullerenes in water was an inefficient process due to little overlap of density of states between the fullerene and water. Attachment of short PEO oligomers dramatically increased the interfacial conductance and decreased the vibrational relaxation time of fullerenes. Energy was transferred primarily from harmonic modes of the fullerene to harmonic modes of similar frequency associated with attached PEO segments. This energy was then transferred to the surrounding water through the anharmonic PEO modes and less efficiently transferred to the higher frequency harmonic modes of the attached PEO segments and harmonic modes associated with PEO segments further along the chain(s). Secondly, a coarse-grained implicit solvent (CGIS) potential, given by the potential of mean force (PMF) for the fullerene pair in water, was found to describe well the free energy of formation of the linear cluster, indicating that many body effects, i.e., the influence of neighboring fullerenes on the water-induced interaction between a fullerene pair, were negligible for the one-dimensional geometry. For the two-dimensional and particularly the three-dimensional geometry, however, many-body effects were found to strongly influence hydration. This strong influence of geometry on hydration translated into water-induced interactions that were not well described by the two-body CGIS potential, particularly for the three-dimensional geometry. Finally, single C60 fullerene and fullerene derivative with hydrophilic PEO were more favorable for settling on the air-water interface rather than staying in the water. The probability for end positions of attached PEO chains of settled fullerene derivatives revealed that the half of PEO chains were located on the interface and the rest of them were stretched to the water. Thirteen fullerenes formed a cluster with a close-packed hexagonal structure at the interface, while 13 fullerene derivatives showed a chain-like structure. However, it was difficult to decide the formation of monolayer for fullerenes or fullerene derivatives due to the roughness of water surface and the concentration dependence.