Surface or zeta potential and electrophoretic experiments

An electric double layer travels together with the particle. The surface potential is approximately equal to 

The zeta potential can also be defined as the potential where the centre of the first layer of solvated ions moving relative to the surface is located. Goodwin (2009) states that this is about 0.5 nm or so from the surface. A mle of thumb often cited suggests that the absolute value of the zeta potential, for stability in many practical situations, should be larger than 30 mV. However, other parameters also play a role as we will see later. 

The zeta potential gives an indication of the extent to which ions from the solutions are adsorbed into the Stem layer. When the solid surface is able to ionize, e.g. oxides and proteins, the zeta potential is a measure of the extent of ionization. The Stem layer is indeed a few A large and it reflects the finite size of charged groups as well as that of ions associated 

The zeta potential is measured via the so-called electrophoresis experiment. What is measured is the velocity (in m s ) of a charged colloid particle, u, induced by an electric field with strength in a stationary liquid (typically given in V m or V cm ). Then, the electrophoretic mobility or electromobUity is calculated and finally the zeta potential is obtained. The values of electromobUity are of the order 10 m s for colloids. 

The quantity which is measured, the so-called electrophoretic mobility (in m s ), is defined as  

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