Zeta-Potential and Interfacial Properties

When colloidal particles are dispersed in a liquid solvent, it is very likely that they acquire a surface charge (cf. Sect. 3.1.5) which has to be compensated for by a corresponding countercharge in the vicinity of the surface. Both regions, charged surface and counter-charged solvent, form the electric double layer (EDL). The charge separation between the two layers creates an electric potential between the particle surface and the bulk solvent, which is responsible for attractive or repulsive interactions between the colloidal particles. 

Important measurands for the characterisation of the EDL are the surface charge density and the electrokinetic potential or zeta-potential. The zeta-potential is the electric potential at a h3q)othetical shear plane, which separates the mobile solvent from solvent molecules that adhere to the particle surface. The zeta-potential can he probed by imposing a relative motion between bulk solvent and particle (Delgado et al. 2007). 

Such a relative motion can be induced by external electric fields or by pressure gradients or bulk forces (e.g. gravity). It is possible that particles move in a quiescent solvent or that the solvent flows through a fixed bed of particles. A detailed description on electrokinetic phenomena is e.g. given by Hunter (1988). Zeta-potential measurements on colloidal suspensions are fiequently conducted via electrophoresis or by means of electroacoustics. Besides this, there are recent techniques based on non-linear optics that are sensitive to interfacial changes. 

If an electric field is applied to a suspension of charged colloidal particles, the mobile ions within the double layer become spatially separated according to the 

The double layer thickness is decisive for the retarding impact of double layer polarisation on the electrophoretic motion. Equation (2.42) yields simple linear relationships only for the two limiting cases of very thin double layers (ka oo) and infinitely thick ones (kg 0). The correction function/(/ca) then simplifies to a constant value of 3/2 and 1, respectively (Fig. 2.21 von Smoluchowski 1903 Debye and Hiickel 1924 O Brien and White 1978). 

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