Surface charge
Surface charge is the electrical potential difference between the inner and outer surface of the dispersed phase in a colloid. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field. Surface charge causes a particle to emit an electric field, which causes particle repulsion and attraction, affecting many colloidal properties.
Surface charge practically always appears on the particle surface when it is placed into a fluid. Most fluids contain ions, positive (cations) and negative (anions). These ions interact with the object surface. This interaction might lead to the adsorption of some of them onto the surface. If the number of adsorbed cations exceeds the number of adsorbed anions, the surface would have a net positive electric charge.
Dissociation of the surface chemical group is another possible mechanism leading to surface charge.
Surface charge density is defined as the amount of electric charge, q, that is present on a surface of given area, A:
According to Gauss’s law, a conductor at equilibrium carrying an applied current has no charge on its interior. Instead, the entirety of the charge of the conductor resides on the surface, and can be expressed by the equation:
where E is the electric field caused by the charge on the conductor and is the permittivity of the free space. This equation is only strictly accurate for conductors with infinitely large area, but it provides a good approximation if E is measured at the surface of the conductor.
...
Wikipedia