Surfactancy

In detail

Water and oil do not mix because water molecules are polar and attract each other strongly, while oils are non-polar. A surfactant molecule is built to sit at the boundary: one end is polar and comfortable in water, the other is a hydrocarbon chain comfortable in oil. Placed in water containing greasy soil, surfactant molecules arrange themselves at the oil surface with their tails in the oil and their heads in the water, which lowers the interfacial tension between the two and allows the oil to be broken into small droplets. Above a certain concentration, surfactant molecules also cluster into micelles — spherical arrangements with the tails inward — which can carry oily material suspended in water until it is rinsed away. This is why detergent cleaning is described as physical rather than chemical: nothing about the grease is changed, it is simply made transportable. It is also why rinsing is not optional, since soil suspended in water on the surface has not yet gone anywhere.

Why

• Detergent is explained by Surfactancy
• Neutral cleaner is explained by Surfactancy
• Wool detergent is explained by Surfactancy
• Solvent wiping is explained by Surfactancy

  By contrast rather than by application: a solvent dissolves the oil outright, where a surfactant only makes it transportable in water.

• Fingerprints is explained by Surfactancy
• Isopropyl alcohol is explained by Surfactancy

  By contrast: it dissolves oil directly rather than emulsifying it, which is why it leaves no residue and needs no rinse.