ABSTRACT
Referring to Figure 6.1, a charged colloidal particle suspended in an electrolyte solution is surrounded by a diffuse cloud of ions containing a net amount of charge which is roughly equal but opposite in sign to that of the particle. The linear size of this ionic cloud, 1/k, known as the Debye length or the thickness of the electrical double layer surrounding the particle, depends mainly upon the bulk concentration of electrolytes. For instance, if the liquid phase contains N types of ionic species with bulk number concentration nj0, j = 1,2,…,N, then κ ε= ∑ =[ / ] ,j 1 2 1/2( )N j j Bn ez k T0 where zj, e, e, kB, and T are the valence of ionic species j, the elementary charge, the permittivity of the liquid phase, the Boltzmann constant, and the absolute temperature, respectively. When an external electric field is applied to the system shown in Figure 6.1, the positively charged particle migrates toward a cathode and the ions in the double layer toward an anode. The phenomenon that a particle is driven by an applied electrical field is called electrophoresis.
