Silica nanoparticles cationic surfactants interaction in water-oil system

Abstract

The aim of this experimental study is to get insight into the combined interfacial and bulk properties of silica nanoparticles and CTAB cationic surfactant mixture in water/oil systems. The study is focused on low surfactant/nanoparticle ratios to avoid extra free surfactants in the bulk. Surfactant concentration was fixed at 9.0×10−2mM, (0.1 CMC) and the nanoparticle concentration varied between 0 and 5wt.%. Drop profile analysis tensiometry (PAT) was utilized to measure the equilibrium and dynamic interfacial tension at water/heptane interface. To comprehend the layer structure of nanoparticle-surfactant complex at the interface, the interfacial rheology was also studied using drop oscillation experiments. The formation of the surfactant-nanoparticle complex layer at the drop surface and its different structures were explained using surface pressure vs. drop surface area curves and the standard deviations of experimental drop profiles from Young-Laplace equation during large compression/expansion cycles. The results demonstrate significant changes in the standard deviation at the onset of closed-packed state of surfactant-nanoparticle complex collapse. This illustrates the formation of irregular adsorbed layer (multilayer formation). The number of surfactant-nanoparticle complexes at drop surface was calculated by accurately assessing the surface coverage at the maximum packing of particles at the interface. For the studied concentration range, surfactant-nanoparticle complexes demonstrate surface activity higher than similar surfactant concentration, however with slower dynamics of adsorption. Elasticity measurements during large compression/expansion path can also provide more details about the structure of mono-/multi-layer formation.