Nanoparticles can induce microemulsions with tunable properties

Emulsions are mixtures of two immiscible liquids, often oil and water, stabilized by a surfactant. Microemulsions are a subset of emulsions with such a high surfactant concentration that they are thermodynamically stable and form spontaneously, without the need for agitation. Typically, molecular surfactants have to be matched to particular emulsions, and sometimes lend the mixture undesirable properties.

Hasnain et al. used theory and experiment to show that nanoparticles which bind to the oil-water interface can also act as surfactants capable of inducing spontaneous emulsification to produce microemulsions. Because emulsion droplets inherit some properties of the nanoparticles, the group believes this method may have many applications.

“If the nanoparticles have catalytic, medicinal or useful optical properties, then the surfaces of the microemulsion droplets will have them as well,” said author Jaffar Hasnain.

The researchers used a thermodynamic approach to show spontaneous emulsification usually requires nanoparticle concentrations so high that they are almost unachievable experimentally. However, by exploiting size-dependent packing effects and nanoparticle-polymer interactions, the group reduced the concentration necessary for spontaneous emulsification to attainable levels.

“We found that the nanoparticles need to be small, less than 10 nm, and that they need to have a strong preference for the oil-water interface,” said Hasnain. “Based on these recommendations, the experimentalists in our group synthesized Noria, a tiny organic cage of atoms that can be altered so that it partitions to the oil-water interface.”

Though previous microemulsions could only be made using specific oil-water-surfactant combinations, the synthetic chemistry used in this research is generalizable.

“We expect that there is a whole host of nanoparticle microemulsifiers waiting to be discovered,” Hasnain said.

Source: “Spontaneous emulsification induced by nanoparticle surfactants,” by J. Hasnain, Y. Jiang, H. Hou, J. Yan, L. Athanasopoulou, J. Forth, P. D. Ashby, B. A. Helms, T. P. Russell, and P. L. Geissler, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0029016 .