Zeta potential

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1 Definition
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3 Explain
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[edit] Definition

Zeta potential refers to the electrostatic potential generated by the accumulation of ions at the surface of a (colloidal) particle that is organized into an electrical double-layer, consisting of the Stern layer and the diffuse layer.

[edit] Photo

[edit] Explain

Dissolving a particle in aqueous solution, a counter ion cloud, consisting of the Stern or Helmholtz layer and the diffuse Gouy-Chapman layer, will attach according to the particle’s surface charge to compensate the potential difference. A decrease of potential can be linear or exponential, depending on the spatial distribution of counter ions. The decrease is linear in case of a regular distribution of counter ions and exponential in case of irregular distribution, with a decreasing number of counter ions with increasing distance. The outer Helmholtz layer therefore involves a linear potential decrease, the Gouy-Chapman layer an exponential potential decrease.

Contrary to the ion fixed Stern layer, the diffuse layer consists of movable ions. During the diffusion movement of a particle, part of the diffuse layer is being removed. Because of the loss of counter ions, the particle is now charged towards the exterior. When a voltage is applied to a solution, the potential at this sliding surface is called the zeta potential.

The zeta potential of a particle can be calculated by Henry's Equation if the electrophoretic mobility of the sample is known:

$U_e=\frac{2 \varepsilon \zeta f(ka)}{3 \eta }$

Where $U e$ is the electrophoretic mobility, $\varepsilon$ is the dielectric constant of the sample, $ζ$ is the zeta potential, $f (k a)$ is Henry's Function (most often used are the Huckel and Smoluchowski approximations of 1 and 1.5, respectively), and η is the viscosity of the solvent.

The zeta potential of a sample of colloidal particles is easily quantified using an LDV, or Laser Doppler Velocimeter. The LDV applies an electrical field of known strength across the sample, through which a laser is then passed. The electrophoretic mobility of the colloid will dictate the velocity with which the charged particles move. This will then induce a frequency shift in the incident laser beam. Using either the Huckel or Smoluchowski approximation for Henry's Function, the dielectric constant of the sample, the viscosity of the solvent, and finally the measured electrophoretic mobility, the zeta potential of the particles within the colloid can be calculated.

The primary relevance of the zeta potential of a colloid is as a relative measure of the stability of a system. The DLVO theory for colloidal interactions dictates that a colloidal system will remain stable if and only if the Coloumbic repulsion, arising from the net charge on the surface of the particles in a colloid, is greater than the Van der Waals force between those same particles. When the reverse is true, the colloidal particles will cluster together and form flocculates and aggregates (depending on the strength of the Van der Waals attraction and the presence/absence of Steric effects). Since the higher the absolute zeta potential, the stronger the Coloumbic repulsion between the particles, and therefore the lesser the impact of the Van der Waals force on the colloid.