Scientists have identified the mechanism by which oil and water mix together. This research can be extended to other bonds of chemical compounds.

Scientists noted that water can only mix with oil in exceptional cases – for example, when it is dispersed in a liquid in the form of small droplets. This strange behavior of substances has not been studied, and the researchers did not have an explanation for this process. Now a group from the Ecole Polytechnique de lausanne (EPFL) has looked into the matter with new optical technology and discovered a mechanism by which they can mix.

“Water molecules have such strong interactions with each other that they don’t like to include molecules that don’t participate in them,” said Professor Sylvie Roque, lead author of the study.

For example, oil and water are separated from each other by simple mixing. However, with a sufficient inflow of energy in the form of ultrasound, oil droplets less than a micron in size are mixed with water. Moreover, when placed in an electric field, the droplets move towards the positive electrode. Thus, mixing neutral oil and water leads to the formation of negatively charged oil droplets.

Researchers determined that the answer to this riddle was in the border between oil and water droplets. Water molecules prefer to give and receive electrical charges from their neighbors through hydrogen bonding. However, when they approach the oil molecules on the surface of the droplet, they can no longer find enough water neighbors to bond. Instead, water molecules donate unbalanced electrical charges to oil molecules on the droplet surface. This study showed that the interaction of water and oil occurs through a so-called improper hydrogen bond.

To unravel this mechanism, the team used ultra-fast optical technology. At the molecular level, the interface between oil and water droplets is very similar to the boundaries involved in the folding of proteins or the formation of biological membranes.