If the nanoparticles are only stabilized electrostatically, you can sonicate all you want. They wont redisperse. Moreover if you are adding dried material it is even worse.
I have already dispersed Stober silica in high salinity brines (not at the same concentration level). However, my strategy is to first dissolve the particles in water and only then to dissolve the salts. In your case is this change of strategy possible?