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

The effects of naphthalene sulfonate formaldehyde condensate (NSF) adsorpti on on the colloid and interfacial chemistry of titania particles have been investigated using acoustophoresis, theology and atomic force microscopy. T he NSFs were observed to specifically adsorb at the solid-aqueous solution interface with the isoelectric point of the titania particles shifting to l ower pH values. The magnitude of the shift in the isoelectric point was fou nd to be a function of NSF concentration. For fixed NSF concentration and a t pH values greater than the isoelectric point of the titania, the yield st ress 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 o ver the pH range in question, in line with the DLVO theory of colloid stabi lity. However, an increase in the magnitude of the yield stress was observe d at pH values below the isoelectric point of the titania. This was ascribe d to the presence of attractive forces between the particles due to bridgin g of the positively-charged titania surfaces by the oppositely-charged NSF molecules. The theological data gathered for the concentrated dispersion ex hibited qualitative agreement with atomic force microscopy measurements per formed in (very) dilute solution. (C) 2001 Elsevier Science B.V. All rights reserved.