Comparative effects of zinc oxide nanoparticles over the interfacial properties of low concentrations of ionic surfactants at interfaces

Nanoparticles (NPs) are well-known for their surface activities and recently their use in petroleum industry is on rise which call for better understanding of implications of interactions of NPs and surfactants. Furthermore, this study can provide additional benefits because zinc oxide nanoparticles (ZnO NPs) are widely used in the industrial products and are therefore of special importance. Surfactants, on the other hand are also industrial chemicals and almost an essential constituent of every other formulation with or without the presence of nanoparticles. The self-assembling properties of surfactants on the interface and remarkable surface activities of nanoparticles led to the studies of interactions between nanoparticles and surfactants in a hope to tune the interfacial properties. The surface of ZnO NPs is charged and therefore they can electrostatically interact with ionic surfactants molecules as well as their self-assembled structures such as monolayers at the interfaces and micelles in solutions. This work is set to find how and to what degree, the self-assembling properties of surfactants are affected by ZnO NPs interactions under the influence of change in pH, and ionic strength using sodium chloride which is commonly used among many other ions to simulate "low salinity environment". Surface tension (ST), dynamic light scattering technique with hydrodynamic size (Dh) and zeta potential (zeta) measurements were used to characterize the possible interactions of nanoparticles with surfactants. Commercially important sodium dodecyl sulfate (SDS) and cetrimonium bromide (CTAB) were chosen ionic surfactants for their interactions with ZnO NPs. The study concluded that the interactions of particles with surfactants, due to change in pH, and ionic strength have an appreciable impact on the activities of SDS and CTAB at the air-water (A/W) and n-hexane (oil)-water (O/W) interfaces. The effect of electrolytes was found more prominent in the case of CTAB+ZnO NPs system, while the effect of pH was more pronounced for SDS+ZnO NPs system i.e., in case of both A/W and O/W interfaces. The study will help to improve the understanding and use of ZnO NPs in combination with surfactants and similar systems in enhanced oil recovery and many other processes of industrial importance.