Surface charge
Stern

The Otto Stern model of the double layer is essentially a combination of Helmholtz and Gouy-Chapman theories. His theory states that ions do have finite size, so cannot approach the surface closer than a few nanometers. Through a distance known as the Stern Layer, ions can be adsorbed onto the surface up to a point referred to as the slipping plane, where the ions adsorbed meet the bulk liquid. At the slipping plane the potential Ψ has decreased to what is known as the zeta potential. Although zeta potential is an intermediate value, it is sometimes considered to be more significant than surface potential as far as electrostatic repulsion is concerned.
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