How Hydrophobic Buckminsterfullerene Affects Surrounding Water Structure

Abstract

The hydrophobic hydration of fullerenes in water is of significant interest as the most common Buckminsterfullerene (C₆₀) is a mesoscale sphere; C₆₀ also has potential in pharmaceutical and nanomaterial applications. We use an all-atom molecular dynamics simulation lasting hundreds of nanoseconds to determine the behavior of a single molecule of C₆₀ in a periodic box of water, and compare this to methane. A C₆₀ molecule does not induce drying at the surface; however, unlike a hard sphere methane, a hard sphere C₆₀ solute does. This is due to a larger number of attractive Lennard-Jones interactions between the carbon atom centers in C₆₀ and the surrounding waters. In these simulations, water is not uniformly arranged but rather adopts a range of orientations in the first hydration shell despite the spherical symmetry of both solutes. There is a clear effect of solute size on the orientation of the first hydration shell waters. There is a large increase in hydrogen-bonding contacts between waters in the C₆₀ first hydration shell. There is also a disruption of hydrogen bonds between waters in the first and second hydration shells. Water molecules in the first hydration shell preferentially create triangular structures that minimize the net water dipole near the surface near both the methane and C₆₀ surface, reducing the total energy of the system. Additionally, in the first and second hydration shells, the water dipoles are ordered to a distance of 8 Å from the solute surface. We conclude that, with a diameter of approximately 1 nm, C₆₀ behaves as a large hydrophobic solute.