Use of viscoelastic measurements for investigating the stability/flocculation of concentrated dispersions

Abstract

The use of viscoelastic measurements for studying stability/flocculation of concentrated dispersions is discussed. With electrostatically stabilised dispersions, the system becomes predominantly elastic when significant double-layer overlap occurs. This was demonstrated using polystyrene latex dispersions in 10–5 and 10–3 mol dm–3 NaCl. Plots of the elastic modulus G′versus surface-to-surface separation distance h showed a rapid increase when h was less than twice the double-layer thickness. The experimental G′ values were compared with theoretical values calculated from the second differential of the interaction energy vs. distance relationship. With sterically stabilised latex dispersion [containing grafted poly(ethylene oxide) chains], predominantly elastic response was also obtained when h became less than twice the adsorbed layer thickness (2δ). The G′vs. volume fraction curves were converted to G′vs. h and this was compared with the values of G′ calculated from direct force, F, vs. distance curves. In both cases there was a rapid increase in G′ with decrease in h when h < 2δ.Viscoelastic measurements could also be applied for flocculated dispersions. With weakly flocculated systems such as those obtained by addition of a free non-adsorbing polymer (depletion flocculation), the systems showed pronounced non-Newtonian behaviour above a critical volume fraction of free polymer (ϕ+P). The occurrence of this behaviour decreased with increasing molecular weight of the free polymer. The extrapolated yield stress was used to calculate the energy of separation between particles, Esep, in the flocculated dispersion. Esep was compared with the theoretical value of the free energy of depletion, Gdep, that was calculated using available theories. With strongly flocculated dispersions, scaling concepts could be applied and the power exponent in ϕ(G′∼ϕn) could be used as a measure of the strength of flocculation. From G′ and the critical strain γc, above which non-linear response is obtained, the cohesive energy of the flocculated structure, Ec, was calculated. log–log plots of Ecversus ϕ were used to obtain the power exponent in ϕ and this should give a measure of the strength of flocculation.