ADSORPTION OF POLY(METHACRYLIC ACID) AND ITS EFFECT ON THE STABILITY AND ELECTROSURFACE PROPERTIES OF GRAPHITE DISPERSIONS - 3 - THE EFFECTOF PH OF A MEDIUM

The effect of pH on the adsorption of poly(methacrylic acid) (PMA) fro m aqueous solutions on graphite at constant ionic strength of a backgr ound electrolyte (5 x 10(-3) mol/dm(3) KCl) and on the character of th e changes of the surface charge, stability, and the zeta-potential of polymer-containing graphite dispersions was studied. The nonmonotonic character of PMA adsorption in the pH range varying from 1 to 5 is att ributed to the change in the value and the sign of the adsorbent and a dsorbate charge. As the pH value increases above 5.0 (IEP), the PMA ad sorption on graphite decreases down to zero due to an increase in the similar negative charge of the surface and macromolecules, as well as the prevalence of Coulombic forces of repulsion over the forces of the adsorbate-adsorbent specific interaction. Based on the combined analy sis of the pH dependences of adsorption and the conformational state o f the PMA macromolecules as well as the pattern of the variation of su rface charge and the zeta-potential of graphite at various pH values, the structure of polymer adsorption layer on graphite was determined, which is consistent with the extreme pattern of the dependence of the stability of polymer-containing dispersions on pH of a medium. There i s no simple correlation between the value of PMA adsorption, stability , and the zeta-potential of graphite dispersions. The data obtained in dicate the key contribution the macromolecule conformations and electr ostatic interactions make towards the stabilization of the systems stu died. The stabilization is attributed to the complex combination of el ectrostatic and sterical mechanisms of colloidal protection.