Adsorption of non-ionic surfactants on hydrophobic silica particles and the stability of the corresponding aqueous dispersions

The temperature stability of aqueous dispersions of hydrophobic monodisperse silica particles stabilized with nonionic surfactants has been investigated. Adsorption isotherms in conjunction with surface tension measurements showed that the surfactant formed a monolayer on the surface of the particles, where the adsorbed amount depended on the molecular weight of the ethylene oxide headgroup. The temperature stability of these dispersions has been measured by a standard turbidimetric technique and visual observations in terms of their critical flocculation temperature (CFT). Parameters controlling the CFT of the individual dispersions stabilized with a monolayer of surfactant include the thickness of the steric layer, the particle size, and the volume fraction of the particles. Calculations show that the van der Waals attraction between the particles with adsorbed polymer layers increases as the temperature of the dispersion increases, and this largely accounts for the observed CFT behavior.