  Dipolar susceptibility of interfacial water and the corresponding interface
dielectric constant were calculated from numerical molecular dynamics
simulations for neutral and charged states of buckminsterfullerene C$_{60}$.
Dielectric constants in the range 10--22, depending on temperature and solute
charge, were found. The hydration water undergoes a structural crossover as a
function of the solute charge. Its main signatures include the release of
dangling O-H bonds pointing toward the solute and the change in the
preferential orientations of hydration water from those characterizing
hydrophobic to charged substrates. The interface dielectric constant marks the
structural transition with a spike. The computational formalism adopted here
provides direct access to interface susceptibility from configurations produced
by computer simulations. The required property is the cross-correlation between
the radial projection of the dipole moment of the solvation shell and the
electrostatic potential of the solvent inside the solute.
