Stabilization of polymer colloid dispersions with pH-sensitive poly-acrylic acid brushes

Polyelectrolyte brushes are widely used for surface modification of nano-and colloidal particles because of their ability to dramatically change their conformation, hydrophobicity, polarity, charge, etc., as a response to smooth variations in environmental conditions. In this work, we have studied experimentally the stability behavior of polymer colloids with grafted poly-acrylic acid (PAA) surface brushes. We have measured the Fuchs stability ratio (W) as a function of electrolyte concentrations at different pH. It is observed that at pH < 3, the W values with 1 % and 2 % PAA brushes do not differ significantly from those without PAA, indicating that in their protonated state, the carboxylic groups do not contribute significantly to the colloidal stability. At the intermediate pH ~ 5, the PAA brushes are partially deprotonated, and their contribution to the colloidal stability is substantial and increases as the length of the PAA brushes increases. Under alkaline conditions (pH > 8), since most of the carboxylic groups are ionized, the colloidal stability is much higher than that at pH ~ 5. However, the W values are basically the same with 1 % and 2 % PAA, implying that the contribution of the ionized AA in the two cases is practically the same. This experimental evidence indicates that only the ionized AA groups in the outer region of long brushes contribute to colloidal stability, thus supporting the hypothesis of local electroneutrality in the inner region of long brushes (LEA).