Electrokinetic experiments

The most familiar type of electrokinetic experiment consists of setting up a potential gradient in a solution containing charged particles and determining their rate of motion. If the particles are small molecular ions, the phenomenon is called ionic conductance, if they are larger units, such as protein molecules, or colloidal particles, it is called electrophoresis. 

The presence of surface conductance behind the slip plane alters the relationships between the various electrokinetic phenomena [83, 84] further complications arise in solvent mixtures [85]. Surface conductance can have a profound effect on the streaming current and electrophoretic mobility of polymer latices [86, 87]. In order to obtain an accurate interpretation of the electrostatic properties of a suspension, one must perform more than one type of electrokinetic experiment. One novel approach is to measure electrophoretic mobility and dielectric spectroscopy in a single instrument [88]. 

The charges present on the insulator surface in contact with the solution give rise to an accumulation of ions of opposite sign in the solution layer next to the surface, and thus formation of an electric double layer. Since straightforward electrochemical measurements are not possible at insulator surfaces, the only way in which this EDL can be characterized quantitatively is by measuring the values of the zeta potential in electrokinetic experiments (see Section 31.2). 

FIGURE 6.8.1 Schematic diagram of apparatus for carrying out electrokinetic experiments. Pressure is applied by moveable pistons P and P on liquids in compartments R and S. Electric fields are set up by condenser plates C and C. Solvent and positive ions can move through a membrane M separating the two compartments. Fluids can move through an inlet I and outlet 0 via fitted stopcocks, mounted on the pistons. 

Electrophoresis The most familiar electrokinetic experiment consists of setting up an electric field, E, in a solution containing charged particles and determining their velocity. The particle velocity, V, is measured by direct microscopic observation at the stagnation point (i.e., zero velocity point for electro-osmosis at the radius 0.707i c) in a capillaiy as shown in Figure 9.19. The zeta potential is then computed... 

Isoelectric points require electrokinetic experiments as a function of pAg. pH, etc. which will be discussed in sec. 4.4. Theoretical problems are all but absent because phenomena like surface conduction and relaxation retardation vanish as 0. However, experimental problems may arise because the systems become... 

In contrast to the results from electrokinetic experiments the curves of the potentiometric experiments do not show any approach to a plateau region observed in electrokinetic experiments at pH>9. The steady slope of the exponential function os=os (pH) appears from taking the inner surface of the silica particle into account while electrokinetic experiments reflect only the charge distribution in the shear plane which is situated outside of particle surface. 

Fig. 5 (a) Dependence of Ex (30) values of several PVFA-co-PVAm/silica hybrid particles on the amino content in the PVFA-co-PVAm copolymer ET (30) values were determined from solvatochromic experiments using the adsorption of dye 2 from 1,2-dichlor-oethane (open circles), and toluene (filled circles) (b) Ej (30) values compared with the IEP values of several PVFA-co-PVAm/silica hybrid particles. ET (30) values were determined from solvatochromic experiments using the adsorption of dye 2 from 1,2-dichlor-oethane (open circles), and toluene (filled circles). IEP data were determined by means of pH-dependent electrokinetic experiments in aqueous 10-3 mol L-1 KCl solution... 

Numerous publications report specific solid-to-liquid ratios used in electrokinetic experiments. These ratios are of limited significance when the dispersion is unstable, and the dispersion in the instrument cell has a different composition from that originally prepared. The range of solid-to-liquid ratios used in electrophoresis is illustrated in Tables 2.1 and 2.2. The volume fraction can be converted into mass fraction, and vice versa, when the specific densities of the components are known. The specific densities of most powders of interest are in the range of 2000-6000 kg/m and their mass fractions are about two to six times higher than their volume fractions, thus both quantities are of the same order of magnitude. 

Equation 5.378 connects the phenomenological coefficients appearing in electroosmosis (the left-hand side) with those in streaming potential experiments (the right-hand side). We must note that Equation 5.378 is valid even if the surface conductivity is important or when the double layers are not thin with respect to the capillary diameter. Eurthermore, this type of relationship is vahd even for electrokinetic experiments with porous plugs and membranes with pores of nonuniform size and shape. The respective counterparts of the other relations (Equation 5.376) are... 

The atrazine, molinate, and bentazone behavior in soils when submitted to an electric field is presented as a case study. The applicability of the electrokinetic process in these herbicides soil remediation is evaluated. Four polluted soils were used, respectively with and without herbicides residues, with the last ones being spiked. Eleven electrokinetic experiments were carried out at a laboratory scale. Determination of the herbicide residues were performed by different methods by ELISA for atrazine, by GC-FID for molinate, and by HPLC-UV for molinate and bentazone residues. GC hyphenated with mass spectrometry was used to confirm and identify molinate on the samples. 

The conventional point of zero charge (PZC) is the pH value of a soil solution when the total net particle charge vanishes. By Eq. 3.3a, this condition is met when c = 0. The PZC can be measured directly in electrokinetic experiments and in coIlbiHar sitaBility studied since both involve phenomena sensitive to the total net charge on suspended particles. 

AUo Icrtnod IBP, iNwIccirlc point, when meiiiiured by an electrokinetic experiment. 

This conclusion, which can be extended to the other electrokinetic phenomena considered in this section, illustrates the tenuous nature of any assumption concerning the exact location of the plane of shear. Any position of the plane aXx = d n Fig. 3.3 is, in principle, consistent with the results of an electrokinetic experiment. 

See, e.g., D. H. Everett, Manual of Symbols and Terminology for Physi-cochemical Quantities and Units. Appendix II Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Butterworths, London, 1972. When the PZC is measured by an electrokinetic experiment (Sec. 3.4), it is often termed tm isoelectric point (lEP). However, other definitions of the lEP are used in the soil chemistry literature. ... 

The limitations imposed on DDL theory as a molecular model by these four basic assumptions have been discussed frequently and remain the subject of current research.In Secs. 1.4 and 3.4 it is shown that DDL theory provides a useful framework in which to interpret negative adsorption and electrokinetic experiments on soil clay particles. This fact suggests that the several differences between DDL theory and an exact statistical mechanical description of the behavior of ion swarms near soil particle surfaces must compensate one another in some way, at least in certain applications. Evidence supporting this conclusion is considered at the end of the present section, whose principal objective is to trace out the broad implications of Eq. 5.1 as a theory of the interfacial region. The approach taken serves to develop an appreciation of the limitations of DDL theory that emerge from the mathematical structure of the Poisson-Boltzmann equation and from the requirement that its solutions be self-consistent in their physical interpretation. TTie limitations of DDL theory presented in this way lead naturally to the concept of surface complexation. 

The surface potential, yj/t, which we introduced above is essentially a model potential, and although estimates of this can be obtained from experiments such as interaction between mica hemicylinders these are rather sophisticated experiments and not readily available on a day-to-day laboratoiy- basis. However, electrokinetic experiments, particularly electrophoresis, can often be carried out quite readily even with simple equipment. For example, measurement of the electrophoretic mobility, u, of a spherical polymer colloid particle in a medium of... 

Zeta potential is defined as the electrical potential at the shear plane of the electric double layer. Measurement techniques are based on indirect readings obtained during electrokinetic experiments. Typically, the magnitude of the zeta potential varies between 0 and 200 mV where both negative and positive values are possible depending on the electrochemistry of the solid-liquid interface. 

Electrokinetic phenomena arise when the mobile layer of the EDL interacts with an externally applied electric field resulting in relative motion between the solid and liquid phases. There are three types of electrokinetic phenomena relevant to microfluidics electroosmotic flow, streaming potential, and electrophoresis. In aU of these cases, the zeta potential is a key parameter that defines either the fluid flow or particle motion. Since it is not possible to probe the zeta potential directly, measurements are based on indirect readings obtained from electrokinetic experiments. The following discussion focuses on modem methods of measuring the zeta potential using electroosmotic flow, electrophoresis, and streaming potential. 

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