1. On the surface of a substance, a layer of electric dipoles, the axes of which have an average orientation normal to the surface. Double layers may appear on interfaces of solid and gas, liquid and gas, liquid and liquid, etc. They arise whenever media with different electron affinities (forces of attraction or work function) are contiguous, and dipoles are available. A net potential difference, the electrokinetic potential exists across a double layer. 2. See electric double layer.