Credit: EPFL.
Oil and water do not mix according to common experience. When oil is dispersed in water, they can mix. There is no explanation for this strange behavior. A team of EPFL and ICTP scientists have studied this question using novel optical technology and discovered the mechanism by which neutral and immiscible compounds can mix together and form emulsions. The electrical charge distribution at the interface is the answer.
For more than a hundred years, chemists have wondered how tiny oil droplets can exist in water. The lead investigator of the study says that the water molecule has strong and favored interactions with one another, so they don't like to incorporate molecules that don't participate in these interactions.
When mixed, oil and water separate. Droplets of oil with sizes less than 1 micron in pure water can be found for several weeks or months with sufficient input of energy. The droplets move towards the positive electrode when placed in an electric field. Oil droplets are negatively charged when neutral oil and neutral water are mixed. The source of the charge has been debated.
A group of scientists from the Laboratory of fundamental Bio Photonics are in the school of engineering at EPFL. The negative charge was found by studying both the charge and the structure of the droplets interfacial oil and water. Their results have been published.
Improper hydrogen bonds.
The answer to this longstanding puzzle lies at the interface between oil droplets and water. Water molecule prefer to donate and accept electrical charges from their neighbors via an interaction known as hydrogen bonding. They can't find enough water neighbors to bond with when they are close to the oil molecule. The water molecule donates their electrical charges to the oil molecule at the droplet surface. The improper hydrogen bond is what causes the water-oil interaction. Many of the bonds between oil and water will be stable.
The team used an ultrafast optical technique. There were two laser beams on a mixture of oil droplets and water. New particles are created and scattered from the droplet interface when we do this. The sum of the two incoming laser beams and the report on the motion of atoms at the interface are what these photons have. "This tells us about the structure and interactions between oil and water." The interface between oil droplets and water is similar to the one involved in folding or forming a biological Membrane. The structure of oil droplet/water interface has implications for understanding interactions throughout biology and chemistry.
Saranya Pullanchery and her colleagues studied how charge transfer across C–HO hydrogen bonds affects oil droplets in water. There is a science.abj3007
Science journal information.
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