Non-ionic surfactant interactions with hydrophobic nanoparticles: Impact on foam stability

The interactive behaviour of particles and surfactant at an air–water interface has been investigated, with particular reference to the effect of the interactions on the stability of air–water foams. For a system combining octyl grafted silica particles and Triton X-100, the effects of both individual particles and surfactants with the interface have been considered, along with particle–surfactant interactions. Because of the complexity of the system, the change in aqueous foam stability was inferred from a number of key parameters: namely, the bulk adsorption of surfactant onto the particles, the combined influence on system interfacial tension, the role of surfactant on particle aggregation and the role of interfacial elasticity. It was found that adsorption on the particles occurred in a two-stage process, with an initial Langmuir monolayer being augmented as interactions between surfactant aromatic moieties produced a perpendicular arrangement with head-groups facing into the solution, rendering the particles completely hydrophilic at high concentrations. At low-to-moderate concentrations of surfactant, the foam stability was increased by ∼20% in comparison to pure particle systems. The reasons for the improved stability were two-fold. Firstly, in low concentrations, surfactant caused bridging flocculation of the particles at the interface, producing enlarged sterically strong interfacial barriers. Secondly, at moderate concentrations, the surface elasticity was increased by the presence of the surfactant. The maximum interfacial elasticity was higher in composite systems, in comparison to pure surfactant solutions, probably a result of reduction in available interfacial area for adsorption of free surfactant to the air–water interface.