As I understand salt-aging gradually decreases the associated zeta potential of the gold nanoparticles such that you can adsorb a negatively charged molecule such as DNA, I guess you are trying to attach the PEG molecules through their thiol group forming a dative bond. Increasing salt concentration favors the hydrophobic interactions among PEG molecules, actually if you add too much salt you will see two phases. To increase the amount of adsorbed PEG molecules you need to have a lot of initial PEG molecules with respect to the coverable surface.