When a charged particle is suspended in a liquid, ions of opposite charge will be attracted to the surface of the suspended particle. A negatively charged sample attracts positive ions from the liquid and conversely a positively charged sample attracts negative ions from the liquid. Ions close to the surface of the particle will be strongly bound while ions further away will be loosely bound forming a ‘diffuse’ layer. Within the diffuse layer there is a notional boundary and any ions within this boundary will move with the particle when it moves in the liquid; but any ions outside the boundary will stay where they are – this boundary is called the slipping plane.

A potential exists between the particle surface and the dispersing liquid which varies according to the distance from the particle surface – this potential at the slipping plane is called the zeta potential. The magnitude of the zeta potential gives an indication of the potential stability of the colloidal system.

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