The hydronium concentration is generally much smaller than the concentration of cations from dissolved salt and added base, the main effect of increasing the pH is to increase (in magnitude) the nanoparticle charge. This requires a larger number of base cations inside the cell to preserve the overall charge neutrality. The higher the packing fraction, the larger the concentration of dissolved cations, resulting in higher osmotic pressures. This explains why the pressures increase much stronger at larger η when the pH values are higher (see Fig. ) in comparison to the case of smaller pH (see Fig. ). This trend is weaker in the case of larger salt concentrations (see bottom panels), when the dominant population of cations arises from the dissociated salt, instead of the added base.