Charges in a Colloidal Suspension

On the currently-accepted theory, some of the cations are held close to the surface of the clay particle as a strongly-adsorbed mono-molecular layer (known as the Stern Layer) the remainder form a more diffuse layer in which the concentration of ions falls off exponentially with distance from the surface. 

Approximately, the system may be considered as a spherical condenser, with two concentric charged plates, and it can be shown from elementary electrostatics that, if a is the amount of electrical charge per unit area on the particle, d the effective distance apart of the positive and negative layers, and D the dielectric constant of the surrounding liquid (water), the electrical potential at the inner surface, f, is  

Although there is little doubt that the colloidal particle functions as a condenser, it will be clear that the distance d has no precise geometrical meaning but is regarded as an equivalent distance . 

In addition to ions, molecules of water, which act as electrical dipoles, are also attracted to the charged particle, and form a layer or sheath round it this layer of firmly-held water molecules is known as the lyosphere. The lyosphere is limited in thickness because the electrostatic forces holding it fall off with distance from the particle. The particle, with its lyosphere, can be regarded as a single entity, called the colloidal micelle. 

The use of the Greek letter C (zeta) to denote the effective potential of the system is now universally accepted, and it is usually spoken of as the zeta potential. The zeta potential is very important in determining the properties of colloidal suspensions, as we shall now see. 

---

Distribution of Charges in a Colloidal Suspension So long as the balancing cations are not disturbed by electrophoresis, they remain close to the negatively-charged colloidal particles, forming a layer or swarm around them (Figure 18). 

Figure 4. (a) The distribution of charges in a colloidal suspension higher charges at the particles surfaces can stabilize the system, (b) Zeta potential of raw AI2O3 and Si02 colloidal particles. 

---