Dynamics of water in solvation shells and intersolute regions of C60:: A molecular dynamics simulation study

We present dynamical characteristics of the solvent in various solvation shells of a C-60 molecule as well as intersolute region between two fullerene molecules. Results are obtained from extensive atomistic molecular dynamics simulations of aqueous solutions of C-60 in the isothermal-isobaric ensemble. Information about translational mobilities of water in and around fullerene molecules are obtained from velocity auto correlation function and mean square displacements as calculated from simulation data. Reorientational behavior also is investigated by analyzing first and second rank dipole-dipole correlation functions as calculated from simulation trajectories. Both the translational and rotational mobilities of water in the first solvation shell around a C-60 molecule are found to be much slower than the same in the second and third solvation shells. Slower decay of the occupation time correlation function for the water in the first hydration shell indicates slower exchange of these water molecules with those from the bulk. Bimodal nature of the relaxations of both the reorientational and occupation time correlation functions is observed. Dynamics of water in the intersolute region between two C-60 molecules also is investigated. Translational and orientational dynamics of water in the intersolute region are found to be even slower as compared to the same in the first solvation shell. Occupation time correlation function for intersolute water, however, decays faster than that of the first solvation shell water. In general, dynamics of water in the vicinity of a nonpolar molecule like C-60 is qualitatively the same as that found near a biornacromolecule or micelle.