Surface charge electrokinetics

Kallay, N., Colic, M., Fuerstenau, D. W., Jang, H. M., and Matijevic, E. (1994). Lyotropic effect in surface charge, electrokinetics, and coagulation of a rutile dispersion. Colloid Polym. Sci. 272, 554-561. 

In particular, in polar solvents, the surface of a colloidal particle tends to be charged. As will be discussed in section C2.6.4.2, this has a large influence on particle interactions. A few key concepts are introduced here. For more details, see [32] (eh 13), [33] (eh 7), [36] (eh 4) and [34] (eh 12). The presence of these surface charges gives rise to a number of electrokinetic phenomena, in particular electrophoresis. 

During the formation of polycation-polyanion multilayer coatings on halloysite, we monitored the surface potential (electrokinetic zeta potential). Initially negative halloysite (—40 mV) was converted to a positive surface with polycation layer adsorption in the first step of the LbLassembly (figure 14.10). Adsorption of polyanions in the second step re-established the negative charge which was reversed... 

Zhang, H. Zhang, X.-N. (1992) Contribution of iron and aluminum oxides to electrokinetic characteristics of variable charge soils in relation to surface charge. Pedosphere 2 31-42 Zhang, J. Buffle, J. (1995) Kinetics of hematite aggregation by polyacrylic acid Importance of charge neutralization. J. Colloid Interface Sci. 174 500-509... 

DETERMINING THE SURFACE CHARGE FROM ELECTROKINETIC MEASUREMENTS... 

Surface charge density and surface potential are of primary interest. For insulating surfaces, charge can be determined by potentiometric or conductometric titration, though this is a tedious procedure. For metals, the relationship between surface charge and potential can be determined by measuring the capacitance. Finally we discuss electrokinetic effects. Elec-trokinetic experiments yield the potential at the outer Helmholtz plane. 

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. 

Biddiss, E., Erickson, D., Li, D., Heterogeneous surface charge enhanced micromixing for electrokinetic flows. Anal. Chem. 2004, 76, 3208-3213. 

Two important parameters describing the EDL of a mineral are the point of zero charge (PZC) and the isoelectric point (IP). Healy et al.18) define the PZC as the concentration of PDI with the surface charge of a mineral metal oxides, PZC is determined by the concentration of PDI H+ or OH", in sparingly soluble salts by the concentration of PDI of the lattice. When both mechanisms of surface charge formation operate simultaneously, both ion species and their reaction products determine the PZC16,31). The IP is defined18) as the concentration of PDI at which the electrokinetic potential = 0. 

Calculations of the deposition rate show a dramatic dependence on the mechanism by which the surface charge is generated, in addition to the dependence on the charge itself. This reflects the importance of surface chemistry in particle deposition. Characterization of the electrostatic interactions, involving a given surface, require electrokinetic measurements on that surface under a broad spectrum of electrolyte conditions so that the number density and dissociation constant of ionizable surface groups can be deduced a single electrokinetic measurement is not sufficient. 

The electrical double layer at the metal oxide/electrolyte solution interface can be described by characteristic parameters such as surface charge and electrokinetic potential. Metal oxide surface charge is created by the adsorption of electrolyte ions and potential determining ions (H+ and OH-).9 This phenomenon is described by ionization and complexation reactions of surface hydroxyl groups, and each of these reactions can be characterized by suitable constants such as pKa , pKa2, pKAn and pKct. The values of the point of zero charge (pHpzc), the isoelectric point (pH ep), and all surface reaction constants for the measured oxides are collected in Table 1. 

A very useful type of phenomenon in the study of colloidal particles is the electrokinetic phenomenon that results from the movement of a solid phase with surface charge relative to an electrolyte-containing liquid phase. An applied electric field induces movement or, conversely, movement induces an electric field. The phenomena can be divided into two types ... 

The surface charge density at the slipping plane or electrokinetic surface charge density may be calculated from the potential by means of the Gouy theory (17) which for a plane surface gives ... 

The data about the electrophoretic behaviour of bubbles in aqueous electrolytes, the first concerning electrophoretic mobilities and zeta-potential, can be regarded as a main direct source of information about surface charge at solution/air interface. As cited by many earlier authors, the electrokinetic behaviour of a gas bubble in aqueous solutions has been studied for over a century [e.g. 174-181]. However, the mechanism of creation of surface charge and the effect of inorganic salts, etc. are not completely clear. Recently Li and Somasunderan [182,183] and Kelsall et al. [184,185] have reported some new results in this field. 

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]. 

Values of lhe electrokinetic potential and surface charge density at the surfactant solution/gas interface... 

Figure 4.13 gives an experimental Illustration for a few systems for which both titration and electrokinetic data as a function of c and pH or pAg are available ). Data are plotted in terms of charges because the surface charge a° is experimentally accessible the electrokinetic charge 0 is obtained from 13.5.131 after replacing CT and y/ by cr and respectively. Electrokinetic... 

Figure 4.13. Observed electrokinetic charge as a function of the surface charge for a number of systems (same source as fig. 4.12).

Relatively complete elaborations for the cylinder model have been given by for instance, Anderson and Koh and Levine et al. K In these two theories the solution Is assumed to contain (1-1) electrolytes with =u. Both theories fail to account for conduction behind the slip plane, and both solve the electrokinetic equations, taking double layer overlap into account. Anderson and Koh assume this overlap to take place at fixed surface charge (which, because of the implicit rigid particle model of the cylinder wall, comes down to fixed tr =cT ), whereas Levine et al. do so for constant surface potential (essentially fixed Anderson and Koh also considered capUlaries of other... 

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