A molecular-dynamics simulation study of solvent-induced repulsion between C 60 fullerenes in water

Molecular-dynamics simulations of a single C 60 fullerene and pairs of C 60 fullerenes in aqueous solution have been performed for the purpose of obtaining improved understanding of the nature of solvent-induced interactions between C 60 fullerenes in water. Our simulations reveal repulsive solvent-induced interactions between two C 60 fullerenes in aqueous solution in contrast to the associative effects observed for conventional nonpolar solutes. A decomposition of the solvent-induced potential of mean force between fullerenes into entropy and energy (enthalpy) contributions reveals that the water-induced repulsion between fullerenes is energetic in origin, contrasting strongly to entropy-driven association observed for conventional nonpolar solutes. The dominance of energy in the solvent-induced interactions between C 60 fullerenes arises primarily from the high atomic density of the C 60 molecule, resulting in strong C 60 -water van der Waals attraction that is reduced upon association of the fullerenes. The water-induced repulsion is found to decrease with increasing temperature due largely to an increasing contribution from a relatively weak entropy-driven association.