ABSTRACT
Small-angle neutron scattering (SANS) studies of aqueous dispersions of Stöber silica particles (which have been hydrophobised by having 1-octadecanol grafted to their surface), carrying an adsorbed layer of the nonionic surfactant C12E24, in water, have been performed as a function of temperature. Using mixtures of D2O and H2O, the composition of the continuous phase was adjusted to have the same scattering length density as the silica particles. Hence, only the scattering from the 1-octadecanol and C12E24 layers was detected. The data have been analyzed using both a surface Guinier analysis and a two-layer structure model. It has been found that a step profile best describes the inner combined adsorbed layer (1-octadecanol grafted chains, plus the penetrating alkyl chains from the surfactant) and a semi-Gaussian profile the extended poly(ethylene oxide) outer layer. Both analyses demonstrated that the combined surface layer contracted with increasing temperature.
ABSTRACT
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.
