(75g) Probing Colloidal Interactions And Stability Of A Colloid-Surfactant Mixture By Means Of Static Light Scattering, Surface Tension And High Turbulent Shear

Zaccone, A., ETH Zurich
Wu, H., Institute for Chemical and Bioengineering
Lattuada, M., Swiss Federal Institute of Technology (ETH)
Morbidelli, M., Institute of Chemical and Bioengineering, ETH Zurich

For a colloidal system composed of poly-styrene-acrylonitrile particles and potassium stearate (KS) anionic surfactant molecules, its stability in terms of the Fuchs stability ratio, W, has been determined as a function of the surfactant concentration, by measuring the initial aggregation kinetics in the presence of a small amount (15mM) of MgSO4, using the small angle light scattering (SALS) technique. The structure of the particle surface is peculiar, being irregularly patterned, and thus represents a model system to investigate colloidal stability of non-smooth colloidal particles (the most recurrent case in nature and applications). The characteristic size of the asperities as well as the particles size distribution, have been evaluated by means of SEM and image data analysis. From the SALS kinetic experiments, it is found that the stability increases dramatically as the KS concentration increases until the saturation of the available surface occurs, at about 0.10 g/dm3. At concentrations higher than the saturation concentration, the W value decreases markedly with KS, as a consequence of attractive depletion forces induced by formation of micelles in the water phase. Adsorption isotherm determined based on the surface tension technique agrees with the W vs KS behaviour, with respect to the onset of saturation and surface-per-molecule, and it can be well described by the two-steps Langmuir isotherm. The first step corresponds to hydrophobic-driven single-molecule adsorption while the second step to associative adsorption of bunches of molecules connected by chain-chain van der Waals forces. The thickness of the KS layer was measured at various KS concentrations by static light scattering (SLS), by fitting the scattering spectrum with the Lorenz-Mie theory. The first adsorption step (at low KS concentration) does not contribute to the layer's growth, an observation which can substantiate speculations in the literature about the horizontal configuration of the adsorbed molecule at hydrophobic surfaces. Another important observation, is that the surfactant molecules adsorb preferentially in the depressions of the particle surface, producing the effect of evening the surface (i.e., decreasing the roughness) as evidenced by the SLS analysis. Stability measured under high fluid shear in a turbulent micro-channel (in the absence of any screening salt) fit well into the scenario described above and show an inter-relation between morphology of the adsorbed surfactant and stability under shear. Moreover, experiments under shear bring further evidence of significant steric short-range repulsion whilst depletion forces are shown to be non-cooperative with turbulent shear in the absence of screening electrolytes. In summary, a thorough and detailed study of the stability of a colloidal dispersion under both stagnant and turbulent flow conditions gives evidence for a dramatic influence of the surface features (irregular surface morphology as well as non-uniform surfactant patches) on the stability and on the aggregation properties of the dispersed particles. This knowledge can be exploited in applications aiming to improve the control and the tuning of stabilization/destabilization of dispersions.