INFLUENCE OF SURFACTANT ADSORPTION ON THE RHEOLOGY OF CONCENTRATED LATEX DISPERSIONS

The adsorption of a series of non-ionic surfactants on polystyrene latex particles was determined at room temperature. The surfactants consisted of the same hydrophobic part, namely nonyl phenyl with 13 moles of (poly)propylene oxide, but with various amounts of ethylene oxide (27-179 moles). The adsorption isotherms showed a systematic decrease in the plateau adsorption GAMMA(infinity) with increase in the ethylene oxide (EO) chain length. This showed that the area per molecule was governed by the radius of gyration of the poly(ethylene oxide) chain. Plots of GAMMA(infinity) versus n1/2 (where n is the average number of moles of EO units in the chain) were linear. The hydrodynamic diameter of the latex particles containing adsorbed layers of these surfactants was measured using photon correlation spectroscopy (PCS). The results showed a systematic increase in the adsorbed layer thickness with increase in EO chain length. Concentrated dispersions of latex were prepared using these surfactants and their viscoelasticity was measured as a function of the volume fraction of the dispersion. The results showed a change from a predominantly viscous to a predominantly elastic response above a critical volume fraction, phi(cr), which decreased with increase in the EO chain length. This reflected the steric repulsion which becomes longer in range as the EO chain length increases. The adsorbed layer thicknesses were calculated from phi(cr) and the values obtained were comparable to those obtained using PCS.