Forces between surfaces in the presence of a cationic polyelectrolyte and an anionic surfactant

An atomic force microscope has been used to investigate the interaction forces between a mica surface and a colloidal glass sphere in the presence of a high molecular weight cationic polyelectrolyte, and the anionic surfactant sodium dodecylbenzenesulfonate (SDbS). The effect of addition of SDbS on the interaction forces between the polyelectrolyte coated surfaces was investigated. At low concentrations of SDbS the electrostatic repulsion initially present was progressively neutralized as a function of increasing surfactant concentration. Neutralization resulted in a strong attraction between the surfaces. As the SDbS concentration was increased further, additional surfactant association took place and the surfaces were recharged. At these surfactant concentrations the surfaces became less attractive at small separations. At high SDbS concentrations the interaction profile became purely repulsive as the surfaces were recharged. Diffuse double layer potentials derived from the direct force measurement data correlated well with zeta-potential measurements of silica particles treated with the polyelectrolyte and surfactant. Particulate stability behaviour was also seen to compare well with the force measurement and electrostatic potential data. (C) 1999 Elsevier Science B.V. All rights reserved.