Depletion flocculation of concentrated latex suspensions by surfactant micelles

The influence of addition of a high concentration of non-ionic surfactants on the rheology of a concentrated (volume fraction φ_p = 0.4) polystyrene latex suspension was investigated. Four different surfactants containing the same hydrophobic group (C₉H₁₉C₆H₅(CH₂CH(CH₃)O)₁₃) and increasingly large hydrophilic chains (poly(ethylene oxide) with 27, 48, 79 and 174 ethylene oxide (EO) units) were used. The rheology was studied using steady-state shear stress (τ)—shear rate (γ) measurements as well as oscillatory measurements. From the τ—γ curves, the extrapolated yield stress τ_β and plastic viscosity η_pl were obtained as a function of surfactant concentration. From oscillatory measurements, the complex modulus G*, the storage modulus G′ and loss modulus G″ were obtained as a function of frequency at different surfactant concentrations. All results showed a rapid increase in the rheological parameters with increase of surfactant concentration, once the latter reached a critical concentration. This rapid increase (non-Newtonian flow) was considered to be due to depletion flocculation. The latter was thought to occur at a surfactant concentration in excess of that required for maximum random packing of surfactant micelles, which were assumed to be spherical. A simple calculation (using a value of 0.63 for random packing) predicted the surfactant concentration at the onset of depletion flocculation, using the aggregation number of surfactant molecules in the micelle (obtained from surface-tension or light-scattering measurements).

The extrapolated yield value was used to calculate the energy of separation E_sep between contact points in a floc. E_sep was compared with the theoretical free energy of depletion attraction G_dep that was calculated using the Asakura and Oosawa theory. Agreement between the calculated G_dep and experimental E_sep values was fair.