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, E_sep, in the flocculated dispersion. E_sep was compared with the theoretical value of the free energy of depletion, G_dep, 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 γ_c, above which non-linear response is obtained, the cohesive energy of the flocculated structure, E_c, was calculated. log–log plots of E_cversus ϕ were used to obtain the power exponent in ϕ and this should give a measure of the strength of flocculation.