Adsorption of the Poly(oxyethylene) Nonionic Surfactant C₁₂E₅ to Silica: A Study Using Atomic Force Microscopy

The forces between a spherical silica particle of colloidal dimension and a smooth silica surface have been measured in aqueous solution as a function of the concentration of the nonionic surfactant pentakis-(oxyethylene) dodecyl ether. In the absence of surfactant the interaction between silica surfaces was purely repulsive, being composed of electrostatic and hydration forces, whereas at low surfactant concentrations (≈2 × 10⁻⁵ M, about one-third the critical micelle concentration (cmc)) the repulsion was replaced at short separations by an attractive force which pulled the surfaces into adhesive contact. At higher concentrations (≈4 × 10⁻⁵ M) the surfaces still experienced an attractive force at small separations, but the adhesion decreased markedly. At concentrations above the cmc repulsive steric forces are observed. A tentative scheme for the adsorption of poly(oxyethylene) surfactant to silica is presented.