Association in Like-Charged Surfactant–Nanoparticle Systems: Interfacial and Bulk Effects

The association of similarly charged surfactant molecules and nanoparticles in an aqueous solution remains unresolved, and the understandings reported in the literature are conflicting. To address this issue, we undertake a fundamental study to investigate bulk and interfacial phenomena in binary mixtures of (i) positively charged nanoparticles and cationic surfactants and (ii) negatively charged nanoparticles and anionic surfactants. We find that the surfactant molecules adsorb on the surface of the nanoparticle despite similar charge, leading to supercharging of particles and simultaneously driving more surfactant molecules to the air–dispersion interface. Hence, the properties of the dispersed species, such as the size and zeta potential, and the interfacial properties, such as the surface tension and surface excess concentration, change significantly. This effect is more pronounced at a low surfactant concentration and is observed irrespective of the size of nanoparticles and surfactant–particle combination. Further, we elucidate the important role of electrostatic interactions in the surfactant–particle complexation process by varying the pH of the dispersions. Contrary to changes in the properties of the dispersed species and interface, the presence of particles does not appreciably change the bulk property, such as the critical micelle concentration.