An unlikely attraction

The interface between air and water is negatively charged. Negatively charged nanoparticles suspended in water should therefore be repelled by the surface of the water. However, researchers from ETH Zurich and the Paul Scherrer Institute (PSI) observed the opposite effect: the larger the negative charge of the nanoparticles, the closer they approach the interface. “For this experiment, we used a water gun, which we are all familiar with as a toy,” explains group leader Matthew Brown from the Department of Materials at ETH. “However, the diameter of our jet is less than a hair’s width.” In order to examine what occurs at the jet surface, the researchers used a large facility, the Swiss Light Source (SLS) at the PSI in Villigen, which generates high-intensity X-ray beams.

In their experiment, the researchers mixed silicon dioxide nanoparticles with water. A pump transports the liquid to the nozzle, through which the jet, which is only a few micrometres wide, exits. The experimental assembly is installed in a vacuum measurement chamber at the synchroton in Villigen. The liquid must be sprayed at high speed from the nozzle. “Thus, we get a continuous, free-flowing jet of water,” says the material scientist. “If the water jet stops, it freezes immediately.” The difficulty then is keeping the microjet stable. When the X-rays come into contact with it, they separate electrons from the jet’s surface, allowing inferences to be drawn about the material. “Once we arrive at this point, our investigation is a normal, solid state physics experiment,” explains Brown.

International team

Using X-ray photoelectron spectroscopy, the researchers were able to determine the distribution of the nanoparticles at the interface between air and water. On the scale of only a few atom radii, they could detect small differences in the distribution at the interface, which could be traced back to differences in the charges of the nanoparticles. “Until now, we are the only group to have achieved this,” says Brown. The ETH researcher realised the degree of interest experts had in this research when he searched for theoreticians who could interpret the measured data. Colleagues in Sweden, the US and Canada were enthusiastic about the measurement results, and immediately agreed to participate in the project, explains Brown.

And how is the unlikely attraction between negatively charged nanoparticles and the negatively charged interface explained? “It is complicated, but at the same time completely logical,” the group leader says. The nanoparticles have a strong electrical field that triggers a complex redistribution of water ions, so that the charge at the air-water interface turns from negative to positive. “The electric potential of one particle can be half a volt,” adds Brown. “That really is a lot.” As a result, the particles can modify the structure of the air-water interface with little trouble.

Diverse applications

The transition from air to water is the largest interface on earth. The researchers therefore hope their findings will enable further fundamental knowledge. Their results are also applicable for interfaces between oil and water, or for bubbles in emulsions; for example, in cosmetics, yoghurt or dyes. In each case, specific particles stabilise the air in the fluid. “We are interested in the principles of this process,” says Brown. “What propels the particles to the interface and what keeps them there?” On the basis of this knowledge, customised particles may one day be used to identify particular material properties at the microscopic level.

With their new measurement method, the researchers now want to determine a physical quantity that, according to the textbooks, is not measurable: the electric surface potential of a particle in fluid. Until now, one had to dry a particle for this measurement. “Now we can examine it in its natural environment,” says Brown. Based on this, the researchers hope for other applications, including in medicine, more specifically vaccines, where the active ingredient is often bound to nanoparticle carriers. “In the body these particles are located in a fluid,” explains the material scientist. “It would be beneficial to know their structure in a liquid environment.” Structural knowledge about nanoparticles could also help in terms of energy storage, the desalination of seawater or the purification of ground water.