Influence of the dielectric constant of the media on oxide stability in surfactant solutions

The influence of the dielectric constant of the suspension on the colloidal stability of alumina in the presence of surfactant was investigated. Colloid stability measurements were supplemented with electrokinetic, conductivity, and surfactant adsorption studies. Studies with physadsorbed sodium dodecyl sulfate were performed in the presence of different volume percents of short chain alcohols or dioxane in order to vary the dielectric constant of the suspension. It was observed that the surfactant adsorption decreased with the dielectric constant of the media. At 50 vol % of different alcohols, the addition of surfactant could not reverse the alumina charge and restabilize coagulated particles. To understand the influence of lower dielectric constant on surfactant adsorption at the surface or in the second layer, (hydrophilic head pointing toward the solution), alumina and silica particles were chemically precoated with dodecyl chains. The addition of anionic surfactant suspended such hydrophobic particles and increased the negative surface charge. Cationic surfactant also successfully dispersed dodecyl-coated alumina and increased the positive charge on the surface. When sufficient alcohol was added to the solution, no change in zeta potential was evident as the surfactant concentration was increased. This indicated the absence of surfactant adsorption in the second layer in low-dielectric-constant media. Conductivity measurements indicated that approximately 40% of the surfactant molecules were dissociated, even at 50 vol % of ethanol. Adsorption measurements indicated no surfactant adsorption. These results strongly suggested that, in suspensions with low dielectric constant, no surfactant molecules adsorb in the second layer with hydrophilic heads oriented toward the solution.