Surface charges from electrokinetic measurements

DETERMINING THE SURFACE CHARGE FROM ELECTROKINETIC MEASUREMENTS... 

For a charged surface in aqueous solution the surface potential decays with distance away from the surface to a reduced potential at the Stern plane boundary, to zero in bulk solution. The Stern plane potential is readily estimated from electrokinetic measurements. 

In the following sections, the relationship between surface charge and electrokinetic phenomena is expounded in terms of classical theory. First, a few possible mechanisms and models for the development of charge at a surface in contact with an aqueous solution are described in order to form a basis for the formation of an electrical double-layer at an interface. Secondly, the electrical double-layer is discussed in terms of an equilibrium charge distribution and electrostatic potential near the interface. With an adequate description of the interface, the discussion turns to explication of electrokinetic phenomena according to the charge distribution in the electrical double-layer and the Navier-Stokes equation. A section then follows which describes common methods and experimental requirements for the measurement of electrokinetic phenomena. The discussion closes with a few examples of the use of measurement of the pH dependence of electroosmosis as an analytical characterization technique from this present author s own experience. The intention is to provide... 

In the preceding examples, the measurement of elec-trokinetic phenomena has been shown to be useful in characterizing a surface in terms of source of charge and surface interfacial properties. The literature abounds with similar examples. Other than accounting for surface charge, the zeta potential derived from electrokinetic measurements have also been useful as the sole parameter for characterizing the electrostatic contribution to... 

In Eq. (12), cr is the specific surface-charge density of the diffuse layer (the diffuse-layer charge), I is the ionic strength in molar units, e is the dielectric constant of the medium (water), Eq is the permittivity of fi ee space, R is the gas constant, is the potential at the head end of the diffuse layer (the diffuse-layer potential, sometimes equated to the zeta potential obtained in some way from electrokinetic measurements), F is the Faraday constant, and T is absolute temperature. 

The concentration of potential-determining ions at which the zeta potential is zero (C = 0) is called the isoelectric point (iep). The isoelectric point is determined by electrokinetic measurements. We have to distinguish it from the point of zero charge (pzc). At the point of zero charge the surface charge is zero. The zeta potential refers to the hydrodynamic interface while the surface charge is defined for the solid-liquid interface. 

As already stated in Section 3.3.2, the precise values of the potential of the diffuse electric layer (po can be obtained by the method of equilibrium foam film . The results correlate well with the values of the electrokinetic potential, measured by the method of the rotating bubble [65], Table 8.1 presents the -potential values and the surface charge density Oo for foams from various surfactant kinds [65]. 

With smooth, nonporous surfaces the zero-point of charge and the isoelectric point usually do not differ much from each other. However, when porous particles, e.g., of activated carbons, are measured, the surface of the grains or particles may be acidic in character due to ageing while the internal surface is stiU basic. As mentioned before, aging in narrow pores is very slow due to diffusion restrictions. The electrokinetically measured lEP is determined by the -potential of the particle surface while the PZC is determined by the much larger interior surface of the particles [21]. 

Electrokinetic Phenomena. Electrokinetic motion occurs when the mobile part of the EDL is sheared away from the inner layer (charged surface). There are four types of electrokinetic measurements, electrophoresis, electroosmosis, streaming potential, and sedimentation potential, of which the first finds the most use in industrial practice. Good descriptions of practical experimental techniques in electrophoresis and their limitations can be found in references 18-20. 

Electrokinetic motion occurs when the mobile part of the EDL is sheared away from the inner layer (charged surface). There are several types of electrokinetic measurements ... 

Neither the surface charge density nor the surface potential at the solid/liquid interface can be measured directly. They are to be retrieved from pH potentiometric or electrokinetic data on the basis of certain theoretical concepts and models. 

There are four basic types of electrokinetic techniques that can be used to evaluate the zeta potential under a given set of conditions. The common feature of these electrokinetic measurements is the relative motion between a charged surface and its surrounding fluid, with an electric field either applied or generated from the movement. Perhaps the most familiar type and most widely used electrokinetic technique is electrophoresis, which involves the movement of charged particles under the influence of an applied electric field [28]. 

---