[+Zeta Potential Theory+]
Zeta potential is used to determine electrophoretic mobility of colloidial suspensions.
Surface Charge and Zeta Potential
A ‘colloid’ exists when one state of matter is finely dispersed within another. Within a colloid, Brownian motion refers to the random movement of particles in a suspension (think electric interactions). The particles’ attraction or repulsion from one another, and therefore stability of these particles in solution, can be measured with zeta potential.
A stable solution will tend to not aggregate and so have a large magnitude zeta potential. As the tendency to flocculate (insert link to other part of page) increases, stability decreases, and zeta potential magnitude decreases.
Each particle has an electrical double layer, akin to (but not the same as) the surface potential of the particle (See figure 1). The measure of the electrical double layer is dependent on the particulate bound to the surface since the extra surface bound material slows diffusion.
Figure 1: Electric double layer (in yellow) around a particle (in blue).
A large part of the mathematical theory upon which this is based was developed by a cunning man named Smoluchowski. In fact, when running tests with zeta potential we use the Smoluchowski setting (which bases calculations on his electrophoresis equation). Mu is the electrophoretic mobility, epsilon_r is the dielectric constant of the medium (solvent), epsilon_o is the permittivity of free space, eta is the dynamic viscosity, and zeta is the zeta potential.
Smoluchowski Electrophoresis Equation
It should be noted that this equation is valid for thin layers.
Interesting information on aggregation terminology
When particles in a colloid/suspension first aggregate they form a floc. This is called flocculation. If the aggregate becomes more dense it is said to coagulate. Generally, coagulation is irreversible. Flocculation can be reversed by deflocculation.
For more, detailed information see these wonderful references…
References:
“Zeta Potential: An Introduction in 30 Minutes.” Malvern Instruments. Accessed 20 April 2010. http://www.nbtc.cornell.edu/facilities/downloads/Zeta%20potential%20-%20An%20introduction%20in%2030%20minutes.pdf.
“Zeta Potential.” Wikipedia. Accessed 20 April 2010. <http://en.wikipedia.org/wiki/Zeta_potential>.
