History

Oil & Water Plus Detergent: Exploring the Importance of Oil/Water Phase Separation Materials as common as oil, water and detergents can be used to illustrate and illuminate a number of important phenomena. Every child knows what mayonnaise and salad dressings are, and many have also seen how old paint has separated into two layers, which on stirring form a single layer of usable paint. The principles operating in these systems are the same as operate in forming cells, folding proteins and in making advanced new materials for use in fuel cells and in turning sunlight into electricity (among many, many other applications). They also explain lots of things in the world around us; how dishwashing liquid, soap, shampoo and laundry detergent work, why soap bubbles are round, why dishwashing liquid is used to clean up oil from birds and animals caught in oil spills. The accompanying experiments were designed to start illustrating some of these important principles. We all know that oil and water will separate. We often discuss this by saying that the attraction between water molecules is much stronger than between water molecules and oil molecules and so the water molecules stick to themselves. The oil molecules are then forced to do the same. Another way to think of this is by considering the different surface tensions of the two liquids. Surface tension is a measure of how strongly the molecules in a liquid bind to each other. Strong binding means high surface tension. High surface tension (for example, water) is what lets insects walk on water. Oil and water have very different surface tensions and so where oil and water are forced to be in contact is a region of high surface tension. Systems that have low surface tension are more stable than those with high surface tension, and so any system will change to decrease the surface tension as much as possible. This is what happens when paints and salad dressings separate. In both cases they start by having lots of small...