INTERPARTICLE FORCES ARISING FROM ADSORBED POLYELECTROLYTES IN COLLOIDAL SUSPENSIONS

The effects of molecular weight (M(w)) and concentration of polyacrylic acid (PAA) on the yield stress tau(y) of ZrO2 suspensions were evaluated. The M(w) range studied was from 1656 to 750 000. The effect of adsorbed polyelectrolyte on the forces between particles was determined by focussing on the pH at which zero zeta-potential(zeta=0) is observed in the presence and absence of polymer. At this zeta=0 condition a maximum in yield stress (tau(ymax)) is observed. Low molecular weight PAA imposes a steric (repulsive) interaction energy which is gradually replaced by a bridging (attractive) energy as the molecular weight is increased. The shift in tau(ymax) and the pH at which zeta=0 both show systematic changes with M(w) as electrostatic, steric, van der Waals and bridging forces change in relative importance.