New light on emulsions

Emulsions are mysterious phenomena – at the molecular level, at least. Found in industrial products such as lubricants, foods such as milk and in each of our cells, emulsions are made up of tiny droplets of one liquid that are dispersed in another.

The MINE project used an innovative spectroscopic light-scattering based method to study the molecules at the interface of these two liquids. This provides direct information about the molecules’ composition, orientation and environment – information that is useful for industry and developing new medicines.

“Our research can help us understand emulsions better and find better, cheaper ways to produce products,” says the project’s principal investigator, Sylvie Roke, head of the Laboratory for fundamental BioPhotonics at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. She adds: “Companies from different industrial sectors are very interested in our work.”

Understanding the interface structure and the mechanics of its formation could boost drug development and understanding how drugs interact with the body’s cells. It could also reduce manufacturing's need for surfactants – substances that help oil and water blend in products such as perfumes.

Although scientists already know that the composition and structure of these interface molecules determines an emulsions’ properties (such as stability and reactivity), until now they had no way to study them directly. Emulsions are liquids that contain moving droplets, and existing molecular probes cannot reach or are not sensitive enough to focus on the one-molecule-thick interface layers around the droplets.

MINE’s method – developed by Roke – overcomes this barrier and has produced new information about how these molecules organise themselves.

Observation innovation

The method combines non-linear optics and light scattering. Non-linear optics uses infrared laser light that lets molecules vibrate at the surface of a droplet. The vibrating electrons in the molecules allow the emission of light particles (known as photons) of a specific colour – but only when the molecules are at the droplet/liquid interface.  

This light travels in a specific direction, measured by light scattering. It contains unique information about the composition of the interface – which molecules are present, how they are organised and whether they interact with water.

By analysing this information, Roke was able to update scientific views on liquids and especially emulsions. One fundamental question concerns charge: droplets or air bubbles in water move towards a positive electrode. This means they possess a negative charge. The common explanation is that hydroxide ions in the liquid are the source of this charge, yet Roke found this is incorrect, finding no such ions on droplet surfaces.

She also found that charged surfactants – that protect the fatty oil droplets from the water – form very different structures than traditionally expected. Rather than forming a film of densely packed molecules around the droplets, they form dilute films with only a surfactant molecule here and there.

These results are unexpected, says Roke. “But since we are not assuming anything, and these are direct measurements, they open up a new understanding of soft nanoscopic systems, which include emulsions. This will also be useful for other branches of nanotechnology, for example it will help to tailor food products that also have biological functions, for our health.”

MINE was fully funded with a starting grant from the European Research Council. “The funding was fantastic,” says Roke. “Building this technology was high risk/high gain and took a long time to get right. The ERC grant gave me time to answer fundamental questions.”

She and her team are now helping other labs to implement her techniques and models. She is also adapting her technique to study water droplets that are billionths of a metre across. “This will teach us more about how water behaves in small spaces, which is relevant for understanding the molecular architecture of our own bodies”