“To me, the most exciting result of this study is that these particles defy the common rule of thumb that ‘like dissolves in like’ – a finding that deepens our understanding of interparticle interactions and nanoscale colloidal chemistry,” says team leader Nicholas Kotov.

Hydrophobic (water-repelling) particles aggregate, or clump together, in water and hydrophilic (water-loving) particles aggregate in oil. However, they can form colloidal dispersions if their surfaces are chemically “camouflaged” with surfactants, such as organic tethers, adsorbed polymers or other particles that have an affinity for the solvent in question or that increase the repulsion between the particles themselves.

Surface corrugation

Another way to control the interaction between a solid particle and a liquid is by “surface corrugation”, a technique that produces a special type of wetting. Kotov and colleagues have now used this technique to disperse micron-sized polymer particles in a wide range of solvents without having to use any camouflaging chemicals. Avoiding surfactants is a good thing as they are generally harmful for the environment.

The researchers corrugated the surface of carboxylated polystyrene microspheres by growing rigid zinc oxide (ZnO) nanoscale spikes or needles on them. They did this by first absorbing positively charged ZnO nanoparticles onto the negatively charged microspheres and then extending these spikes using nanochemical reactions. The length, diameter and, to some extent, the shape of the nanowires can be varied using different processing conditions.

Hydrophobic and hydrophilic

The resulting particles, which look like a hedgehog with its spines sticking out, are hydrophilic thanks to their polar ZnO surfaces, and disperse in water and other hydrophilic solvents. They can be made hydrophobic by silanizing the ZnO nanospikes with (7-octen-1-yl) trimethoxysilane (OTMS) or 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFTS). Surprisingly, highly corrugated OTMS-modified hydrophobic hedgehog particles also disperse in water, and hydrophilic hedgehog particles disperse in hydrophobic solvents such as heptane, hexane and toluene. This shows that surface topography can be used to modulate the interaction between micron-sized particles, say the researchers.

“We use surface corrugation to alter the balance of particle-particle attraction and repulsion,” explains Kotov. “It turns out that the presence of spikes drastically reduces the attractive component of these interactions and the particles do not thus aggregate but stay dispersed in virtually any type of solvent.”

Flexible and scalable technique

The good thing about the technique is its flexibility, say the researchers. It can be performed on virtually any type of particle, and we can vary the number and size of the spikes by adjusting the amount of time the particles sit in various solutions while the protrusions are growing. We can also make the spikes out of materials other than zinc oxide.

It is also scalable so the hedgehog particles can be made in large quantities, which is crucial if they are to be used in real-world applications.

Potential applications in drug delivery, VOC-free paints and catalysis

The Michigan team says that the particles are “quite special”. “As well as them being able to disperse in both hydrophobic and hydrophilic solvents, they do not aggregate,” Kotov tells nanotechweb.org. This is because the particles do not penetrate each other with their spikes, which drastically deceases the contact area between the particles and therefore the attractive forces between them. “They also strongly scatter light thanks to their needles. And that is not all: their amphiphilic nature makes them ideal for drug delivery applications in the body because many drugs are hydrophobic while biological tissue is predominately hydrophilic.”

The researchers say that the particles could help reduce the use of volatile organic compounds (VOCs) in the paintings and coatings industry too. "VOC solvents are toxic, flammable, expensive to handle and dispose of safely," Kotov says. "So if you can avoid using them, there's a significant cost savings in addition to environmental benefits.

Previously, so-called superhydrophobic coatings were made using special sprays or surface treatments based on organic solvents,” he explains. “Here we demonstrate that – surprisingly – superhydrophobic coatings that display the “lotus” (or self-cleaning) effect can be made without any organic solvents by using aqueous dispersions of hedgehog particles in water."

The research is detailed in Nature doi:10.1038/nature14092.