Stabilization of High Ionic Strength Slurries Using Surfactant Mixtures: Molecular Factors That Determine Optimal Stability

Abstract

The successful formulation of particulate dispersions is often hindered by the presence of charged species dissolved in solution. If present at high enough concentrations, these species can reduce the screening length between particles to near zero, allowing rapid coagulation and destabilization. The addition of a mixture of ionic and nonionic surfactants has been shown in a previous publication (B. J. Palla and D. O. Shah, 2000, J. Colloid Interface Sci. 223, 102) to stabilize alumina particles in the presence of high concentrations of charged species. The mechanism has been explained as enhanced adsorption of a nonionic surfactant using a strongly adsorbing ionic surfactant as a binding agent. In this work, the various factors that influence this stabilization mechanism are examined in detail. The factors are then verified by showing the same results for two different chemical environments, both of which are model slurries for chemical mechanical polishing (CMP) of metal surfaces. The stabilizing ability of the surfactant system is found to increase with increasing hydrophobicity of the nonionic surfactant and increasing hydrophobicity of the ionic surfactant. The effect of surfactant concentration on stabilizing ability is shown to have an optimum concentration range for a number of surfactants. The effect of the ratio of ionic to nonionic surfactant is also investigated for both chemical environments and is shown to have significant effects on stabilizing ability. All of the results are then compiled to present a concise picture of the factors that influence the stabilizing ability of mixed surfactant systems for high-ionic-strength slurries.