The interparticle distance between nanoparticles (NPs) dispersed on on SiO2 was shown to be controlled by PEGylation. Ferritins with nanoparticle cores were prepared and PEGylated with poly(ethylene glycol)s (PEGs) of two different molecular weights. It was shown that the thickness of the PEG layer on the ferritin surface determines the interparticle distance under short Debye lengths. Under conditions where the Debye length was greater than the PEG layer thickness, distance between ferritins increased due to the electrostatic repulsive force. Results suggest that the PEG layer accommodated a small amount of counterions insufficient to cancel the ferritin outer surface charges. Simulation showed that ferritins adsorbed randomly and interparticle distance can be predicted theoretically. We demonstrate that PEGylated ferritins, that is, NP cores, can be dispersed on a surface with interval distances between particles determined by the combination of the ionic strength of the solution and the molecular weight of the PEG.