The dispersion of Hematite particles and their ionic interaction adsorbed by surfactants were investigated by measuring zeta potential and sedimentation length in $FeCl_3$ solution, when cationic, anionic and nonionic surfactant were adsorbed on the particle surface.
For model particles, ferric oxide particles were prepared by forced hydrolysis. Depending on the concentration and species of additives, the different particle in composition, size and morphology were produced. The particles made with surfactant additives have the smaller spherical shape than without additives, while acids induced the asymmetrical growth in the lateral and axial directions.
As a model calculation of adsorbing materials at surface, the surface excess were evaluated assuming that adsorbing state of surface molecule on the solid particle depends on the nonideality of liquid phase, heterogeneity of adsorbent, different size of molecule, interaction between adsorbed molecule, and multilayer formation. The surface excess can be solved by knowing the concentration and entropy of adsorbed phase, and shown to have excellent agreement with experiments for variety of systems.
For adsorbing surfactant on the particle surface as the double layer, a model for potential distribution in solution, inner surfactant layer and outer surfactant layer was proposed using Poisson-Boltzmann equation incorporating the amount of surfactant adsorbed, size of surfactant, and concentration of electrolyte solution. The surface potential at surface depends on the layer thickness, surface concentration of surfactant and electrical valency of surfactants. In particular, it was found that the calculated surface potential vary not only by the amount of surfactant but also by the internal structure of surfactant layers.
When adsorbing anionic, cationic and nonionic surfactant, the zeta potential and sedimentation length were measured for determining the dispersion stability. In case of anionic surfactant, the zeta potential is positive and sedimentation length is long at the initial concentration which means the system is stable, but the concentration of surfactant is 0.4mM, the zeta potential is zero and sedimentation length is shorter than those of initial concentration which means the system is unstable. The zeta potential and sedimentation length give the information to describe the mechanism of particle dispersion, flocculation and redispersion with potential distribution curve, and to predict the dispersion-stability of particles which exists in aqueous solutions.
