Silicon nitride colloidal probe measurements: Interparticle forces and therole of surface-segment interactions in poly(acrylic acid) adsorption fromaqueous solution

Direct measurements of forces between silicon nitride surfaces in the prese nce or poly(acrylic acid) (PAA) are presented. The force-distance curves we re obtained at pH > pH(iep) with an atomic force microscopy (AFM) colloidal -probe technique using a novel spherical silicon nitride probe attached to the AFM cantilever. We found that PAA adsorbs onto the negatively charged s ilicon nitride surface, which results in an increased repulsive surface pot ential. The steric contribution to the interparticle repulsion is small and the layer conformation remains flat even at high surface potentials or hig h ionic strength. The general features of the stabilization of ceramic powd ers with PAA are discussed; we suggest that PAA adsorbs onto silicon nitrid e by sequential adsorption of neighboring segments ("zipping"), which resul ts in a flat conformation. In contrast, the long-range steric force found i n the ZrO2/PAA system at pH > pH(iep) arises because the stretched equilibr ium bulk conformation of the highly charged polymer is preserved via the fo rmation of strong, irreversible surface-segment bonds on adsorption.