Interfacial behaviour of binary polymer mixtures II. Competitive polymer adsorption and its effect on the stability of colloidal dispersions

Abstract

This paper describes the adsorption of methylcellulose (MC), polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) and of their binary mixtures on arsenic trisulphide sol and polystyrene latex, respectively, and the effect of these polymers and polymer mixtures on the stability of the dispersions. The conditions for binary polymer mixtures resulting in a synergistic effect and the relation of this phenomenon to competitive adsorption of the polymers were studied. Each polymer itself shows high-affinity type adsorption isotherm on both solid surfaces. Upon simultaneous adsorption from binary mixtures, the order of preferential adsorption for the polymers is as follows: PVP>MC>PVA on the arsenic trisulphide sol, and MC>;PVP>PVA on the polystyrene latex. At high surface coverage, the weakly adsorbed polymers can be displaced from either interface by the preferentially adsorbed polymers. The effectiveness of the polymer mixtures, both as flocculants and stabilizers, closely correlates with the order of preferential adsorption for the polymers. Synergism in the flocculating effect of MC-PVP mixtures with components of different molecular mass, on arsenic trisulphide sol was found. At the onset of the stabilizing region, some increase in the effectiveness of MC-PVP and PVA-PVP mixtures on arsenic trisulphide sol was also observed. These phenomena are attributed to irregularly extended adsorption layers formed by the competitive adsorption of the polymers. Synergism can be observed when considerable difference exists in the segment affinity of the competing polymers for the particle surfaces and when the weakly adsorbed polymer itself is a more effective flocculant or stabilizer than the preferentially adsorbed polymer.