Fig. 4: Mechanism of liquid–solid CE and formation of electric double-layer.

a The liquid contacts a virgin surface (before CE). b The water molecules and ions in the liquid impact the virgin surface and electron transfer between the water and the surface. c The surface is charged and the charge carriers are mainly electrons (WCA > 90°, pH = 7), some ions may be generated on the surface caused by the ionization reaction etc. d The opposite polarity ions are attracted to migrate toward the charged surface by the Coulomb force, electrically screening the first charged layer.