The effect of adsorbing naphthalene sulfonate formaldehyde condensates upon the interactions between metal oxides

The effects of naphthalene sulfonate formaldehyde condensate (NSF) adsorption on the colloid and interfacial chemistry of titania particles have been investigated using acoustophoresis, rheology and atomic force microscopy. The NSFs were observed to specifically adsorb at the solid-aqueous solution interface with the isoelectric point of the titania particles shifting to lower pH values. The magnitude of the shift in the isoelectric point was found to be a function of NSF concentration. For fixed NSF concentration and at pH values greater than the isoelectric point of the titania, the yield stress of the dispersion (and, hence, the interaction force between particles) was seen to vary with the square of the zeta potential of the particles. This behaviour was attributed to variation of solely electrostatic forces over the pH range in question, in line with the DLVO theory of colloid stability. However, an increase in the magnitude of the yield stress was observed at pH values below the isoelectric point of the titania. This was ascribed to the presence of attractive forces between the particles due to bridging of the positively-charged titania surfaces by the oppositely-charged NSF molecules. The rheological data gathered for the concentrated dispersion exhibited qualitative agreement with atomic force microscopy measurements performed in (very) dilute solution.