We have performed a careful study of the adsorption of C-60 molecules on a Au(111) surface by using scanning tunneling microscopy and spectroscopy at room temperature. In coincidence with results from other techniques, differential conductance spectra give a value of 2.3 eV for the HOMO-LUMO gap of a monomolecular layer, with the LUMO level located at 0.6 eV above the Fermi level as a consequence of electronic charge transfer from the substrate into the molecule. Small differences in position (and shape) of the LUMO-derived resonance, in the order of 0.1 eV, are found on molecules adsorbed at step edges. We consider the Smoluchowski effect, i.e., the interaction of the molecules with a charge-depleted region, to explain the observed differences in their bonding nature. On some molecules forming part of bidimensional fullerene islands, similar differences were also detected with spatially resolved scanning tunneling spectroscopy, giving rise to a 2x2 commensurate structure of the molecular adlayer with respect to the substrate. This finding is attributed to different electronic properties of the adsorption site, indicating that molecules adsorbed on the top position are less charged than those on bridge sites.