Interactions between large colloids and surfactants

Abstract

The interfacial adsorption of an anionic SDS and a nonionic surfactant C₁₂E₅, above the cmc onto submicron-sized, negatively charged silica particles in aqueous solution has been investigated by using electrokinetics, conductometry and static light scattering. It was found that both surfactants are prone to being adsorbed onto the silica/water interface. Addition of C₁₂E₅ to the silica dispersion leads to a decrease in mobility. This reduced surface charge causes a decrease in the stability of the silica particles. Surprisingly, the addition of SDS brings about an increase in the negative electrophoretic mobility of the anionic silica particles, leading to a super-charging effect. Subsequent addition of C₁₂E₅ gives rise to even higher negative electrophoretic mobilities. This unexpected hyper-charging effect can only be understood as a cooperative effect based on mixed micelles of C₁₂E₅ and non-adsorbed SDS. Not so unexpectedly, the sequence of surfactant addition was found to be decisive, as quite different results are obtained when C₁₂E₅ is added before SDS.