Shampoo and Conditioner Science 76 hydrophobicity, while potentially replenishing components of the cell membrane complex. However, after a day or so, buildup of this substance begins to result in a greasy look and feel. Moreover, particulate dust and dirt adhere readily to this sebum layer. In modern cultures such sebum-soiled hair is deemed to be undesirable, and therefore, it should be removed on a regular basis by a facile process. This process is, of course, shampooing. Sebum cannot be removed by water alone due to the well-known fact that oil and water do not mix. Aqueous shampoos can remove oily soil from the hair surface because shampoos contain surface-active agents, commonly abbreviated as surfactants. The molecules of these surface-active agents self-assemble into micelles, which are the agents that solubilize oily soils. In order to understand how surfactants work, it is necessary to first consider the exact process that leads to oil and water being incompatible. There are two different possibilities for substances to be insoluble in water. In one case, substances have stronger intermolecular cohesion than water. This is why substances like sand, clay, and glass are insoluble in water the molecules of sand attract each other more strongly than the molecules of water and this attraction leads to the sand being insoluble. This reason for the insolubility is exactly opposite to the reasons for the insolubility of hydrophobic substances such as oils. The intermolecular forces between the oil molecules are weaker than the intermolecular bonds between water molecules and the oils are expelled from water. This expulsion arises largely from entropy and the effect has been coined hydrophobic interaction.1,2 From the time of the Phoenicians it has been known that oil spreads to calm troubled waters. This effect arises from the fact that the spread oil has a lower surface tension than the water. At this point it is appropriate to consider the effect known as surface tension. Molecules in the bulk of liquids are attracted on all sides by their neighboring molecules. However, molecules at the surface are subjected to imbalanced forces because they are attracted by the underlying liquid molecules, but there is essentially no interaction with the vapor/gas molecules