Electrokinetic Properties

Cations and anions adsorbed by soil particles carrying surface charges are not present totally on the surface of the particles. Actually, in a soil-water system, a portion of adsorbed ions is distributed near the surface, forming an electric double layer at the interface between the solid particle and the liquid phase. When the two phases have a relative movement in an electrical field or are affected by other forces, the system can exhibit certain electrical properties, called electrokinetic properties. Electrokinetic properties of soils are the overall reflection of the distribution of various kinds of ions in the electric double layer of a soil-water system. They are related to both the characteristics of the soil and the nature of ions. For variable charge soils, because they adsorb anions as well as cations and during the adsorption both electrostatic force and specific force are involved, their electrokinetic properties frequently manifest themselves in a complex manner. As shall be seen in the present chapter, the electrokinetic properties of variable charge soils exhibit certain characteristics different from those of constant charge soils, and these characteristics are of significance for further distinguishing soil types among these soils. All the electrokinetic phenomena occurring in any colloid system result from the existence of the electric double layer. The same holds true for soils. Therefore, in this section the theory of the electric double layer along with its relation to various electrokinetic properties will be introduced first, and then the complexities in soil systems in this respect will be examined. When two phases are in contact, owing to the difference in properties, a redistribution of electric charge will occur at the interface between the two phases, leading to the formation of two layers with charges equal in quantity but opposite in sign between the two sides of the interface. This pair of charged layers is called electric double layer. It is a microscopically charged system present in the interfacial region between the two phases. The electric potential may vary at different positions within the system, but the system as a whole is electrically neutral.