The role of polyelectrolytes in the stabilisation and destabilisation of colloids

The aim of this study was to investigate the adsorption of polyelectrolytes on charged colloids with regard to their different modes of action in the destabilisation or stabilisation of colloidal suspensions. Polyvinyl pyridine was employed as polyelectrolyte and polystyrene latexes bearing sulfonate or carboxylate groups as colloids. tion of the polyelectrolyte was followed using a radiolabelling technique which allowed determination of (i) the concentration of polymer remaining in the supernatant liquid phase and (ii) the rate of establishment of the interfacial layer at very low polymer concentrations. In fact, this situation generally led to destabilisation of the colloidal suspension. Changing the rate of polymer supply to the adsorbent enabled characterisation of the interfacial reconformation of the adsorbed polymer molecules. Slow supply rates limited the amount of polymer instantaneously adsorbed, while fast rates induced a desorption of the macromolecules in excess. ilisation and stabilisation processes were investigated using a particle counting method (Coulter Counter TA 11, Coultronics). This technique provides a histogram which is analysed to give the size (mass) frequency curve of the aggregates c(n,t) vs. n and the weight S(t) and number N(t) average sizes of the aggregates. The experimental data were interpreted with the aid of results from numerical and theoretical studies. Self-similarity of the aggregate and fragment size distributions throughout aggregation and fragmentation could be demonstrated, while modifications of the reduced size distribution revealed changes in the rate limiting phenomena. Dynamic scaling laws were derived to describe the temporal variations of the aggregation and fragmentation processes. According to the relative values of the dynamic exponents z and w and the exponent of the power-law cluster size distribution τ it was possible to determine the aggregation mechanism. In fragmentation however the situation appeared to be more complex. No clear correlation could be established between the dynamic exponents λ and μ of the rates of variation of S(t) and N(t) and the exponents of the reduced size distribution λ′ and ν determined at large and small values of the fragment size n. Whereas λ and μ were functions of the interfacial and solution characteristics, λ′ and ν depended on the mode of aggregate formation.