Controlled heteroflocculation of non-aqueous silica dispersions

Abstract

The controlled flocculation of silica dispersions in toluene has been investigated using two sets of silica particles. One set (whose flocculation is to be brought about) contained a copolymer with “acidic” (carboxylic) groups, whereas the second set of smaller particles (i.e. the added “bridging” particles) contained a copolymer with “basic” (tertiary amine) groups. The adsorption isotherms for the individual copolymers were of the high-affinity type, showing no desorption. Turbidity studies indicated that the addition of small particles (with a copolymer of opposite polarity) to large particles could lead to aggregation of the dispersion over certain concentration ranges of added small particles (particle “bridging”). The addition of a strong base (n-propylamine) or strong acid ( p-toluene sulphonic acid) could lead to deflocculation of the mixed dispersions. Particle adsorption isotherms were of the high-affinity type indicating strong interaction between the “acidic” copolymer-coated particles and the “basic” copolymer-coated particles. Sediment volume studies showed a maximum at a volume fraction of small particles, ø s, corresponding to half coverage of the small particles onto large particles. Scanning electron micrographs indicated “bridging” of the large particles by small ones. Rheological studies also showed a maximum in the yield value, τ β, at half coverage by the small particles. The yield value at this maximum corresponded to large energies of attraction between the small and large particles (of the order of tens of kT units).