Electrokinetic measurement

Protein adsorption has been studied with a variety of techniques such as ellipsome-try [107,108], ESCA [109], surface forces measurements [102], total internal reflection fluorescence (TIRE) [103,110], electron microscopy [111], and electrokinetic measurement of latex particles [112,113] and capillaries [114], The TIRE technique has recently been adapted to observe surface diffusion [106] and orientation [IIS] in adsorbed layers. These experiments point toward the significant influence of the protein-surface interaction on the adsorption characteristics [105,108,110]. A very important interaction is due to the hydrophobic interaction between parts of the protein and polymeric surfaces [18], although often electrostatic interactions are also influential [ 116]. Protein desorption can be affected by altering the pH [117] or by the introduction of a complexing agent [118]. 

Electrokinetic measurements consisted of measuring the viscosity with and without NaGl (Carlo Erba, Argentine) (Figures 3-a and 3-b), while the isoelectric point (figure 2) and zeta potential (figure 3-c) were measured at different pH (HCl Ciccarelli and NaOH Tetrahedrom, Argentine). 

Electrokinetic measurements, for polymer colloids, 20 383-384 Electrokinetics, 9 740-741 colloids, 7 284-286 in soil and ground water treatment, 25 843-844... 

Many processes involving carbonates - ubiquitous minerals in natural systems -are controlled by their surface properties. In particular, flotation studies on calcite have revealed the presence of a pH-variable charge and of a point of zero charge (Somasundaran and Agar, 1967). Furthermore, electrokinetic measurements have shown that Ca2+ is a charge (potential) determining cation of calcite. (Thompson and Pownall, 1989). 

Returning to our introductory remarks about the existence of various models for the oxide/solution interface, It may be appropriate to point out that the results of very relevant experiments based on electrokinetic measurements are often not used in conjunction with titration data. Granted that there may be additional difficulties in identifying the precise location the slipping plane and hence the significance of the electrokinetic c potential may be open to debate, both titration and electrokinetic data ought to be combined where possible to elucidate the behaviour of the oxide/solution Interface. 

Lyklema(18) considers that the slipping plane may be identified with the Stem plane so that x/ss — f. Thus, since the surface potential xj/o is inaccessible, zeta potentials find practical application in the calculation of Vr from equation 5.16. In practice, electrokinetic measurements must be carried out with considerable care if reliable estimates of f are to... 

Electrokinetic Measurements. Electrophoretic mobilities were measured with a flat-cell apparatus manufactured by Rank Brothers, Cambridge, England. In addition, several mobility values were checked for accuracy with a Zeta Meter, New York. Mobilities were determined with a small volume of the suspension (approximately 25 cc) that had been prepared for the adsorption experiments. The pH of the solution was measured prior to determining the electrophoretic mobilities, which involved measuring the velocities of five to ten particles in each direction. An average value of the mobilities was recorded. Samples containing the flocculated particles were dipped into an ultrasonic bath for approximately one second prior to making the pH and mobility measurements. 

The mechanism of interaction of amino acids at solid/ aqueous solution interfaces has been investigated through adsorption and electrokinetic measurements. Isotherms for the adsorption of glutamic acid, proline and lysine from aqueous solutions at the surface of rutile are quite different from those on hydroxyapatite. To delineate the role of the electrical double layer in adsorption behavior, electrophoretic mobilities were measured as a function of pH and amino acid concentrations. Mechanisms for interaction of these surfactants with rutile and hydroxyapatite are proposed, taking into consideration the structure of the amino acid ions, solution chemistry and the electrical aspects of adsorption. 

The reaction finished within 1 h at 26°C.. They used seed crystals of CdS to promote the uniformity of the final product, and analyzed the growth kinetics using Nielsen s chronomal. The isoelectric point in terms of pH was determined to be 3.7 by electrokinetic measurement. They also prepared zinc sulfide (ZnS polycrystalline spheres), whose isoelectric point in pH was 3.0 (2), lead sulfide (PbS monocrystalline cubic galena) (3), cadmium zinc sulfide (CdS/ZnS amorphous and crystalline spheres) (3), and cadmium lead sulfide (CdS/PbS crystalline polyhedra) (3), in a similar manner. 

On calcination of this prepared powder, particles having the composition ZrY(, k03.2 were obtained. The electrokinetic measurements with aqueous dispersions of the latter showed an isoelectric point at pH 6.8, characteristic of Y203. This example further substantiates the inhomogeneity within the particles, but also indicates that heating, as carried out in this case, did not produce internal uniformity. 

In each case the electrokinetic measurements can be interpreted to yield a quantity known as the zeta (f) potential. It is important to note that this is an experimentally determined potential measured in the double layer near the charged surface. Therefore it is the empirical equivalent... 

The definition of streaming potential was presented in the previous section. Here, we derive the relation between the streaming potential and the zeta potential and discuss some of the issues that must be considered in comparing zeta potentials obtained by different electrokinetic measurements. 

Two conditions must be met to justify comparisons between f values determined by different electrokinetic measurements (a) the effects of relaxation and surface conductivity must be either negligible or taken into account and (b) the surface of shear must divide comparable double layers in all cases being compared. This second limitation is really no problem when electroosmosis and streaming potential are compared since, in principle, the same capillary can be used for both experiments. However, obtaining a capillary and a migrating particle wiih identical surfaces may not be as readily accomplished. One means by which particles and capillaries may be compared is to coat both with a layer of adsorbed protein. It is an experimental fact that this procedure levels off differences between substrates The surface characteristics of each are totally determined by the adsorbed protein. This technique also permits the use of microelectrophoresis for proteins since adsorbed and dissolved proteins have been shown to have nearly identical mobilities. 

DETERMINING THE SURFACE CHARGE FROM ELECTROKINETIC MEASUREMENTS... 

This discussion shows that the evaluation of charge from electrokinetic measurements involves all the complications inherent in the evaluation of f plus the additional restrictions of low potentials and spherical particles. Additional relationships have been developed that permit these restrictions to be relaxed, but we do not discuss these here. 

Throughout most of this chapter the emphasis has been on the evaluation of zeta potentials from electrokinetic measurements. This emphasis is entirely fitting in view of the important role played by the potential in the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability. From a theoretical point of view, a fairly complete picture of the stability of dilute dispersions can be built up from a knowledge of potential, electrolyte content, Hamaker constants, and particle geometry, as we discuss in Chapter 13. From this perspective the fundamental importance of the f potential is evident. Below we present a brief list of some of the applications of electrokinetic measurements. 

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. 

Electrokinetic measurements at 25°C on silver iodide in 10 3 mol dm-3 aqueous potassium nitrate give d /d(pAg) = -35 mV at the zero point of charge. Assuming no specific adsorption of K+ or NO3 ions and no potential drop within the solid, estimate the capacity of the inner part of the electric double layer. Taking the thickness of the inner part of the double layer to be 0.4 nm, what value for the dielectric constant near to the interface does this imply Comment on the result. 

Microelectrophoresis is the most common technique for electrokinetic measurements in colloidal systems. Here individual particles can be observed, in their normal environment, under the microscope. Very dilute dispersions can be studied and very small particles, down to about 0.1 pm diameter, can be observed using the dark-field microscope (ultramicroscope). High magnifications allow minimization of observation times, and in polydisperse systems a given size range of particles can be studied to the exclusion of others. 

For an interpretation of the adsorption process it is important to know the so called zeta-potential ( ) that can be calculated from electrokinetic measurements. It may be defined as the potential difference at the shear plane (near to the outer Helmholtz plane) between the diffuse layer outside the slipping plane and the bulk phase, when the solid and liquid phases are moved tangentially to each other. The location of the slipping plane is not exactly known, but it can be assumed that the shear plane is only very little further... 

PZC determined from electrokinetic measurements at pH 8.2 agrees well with PZC values obtained from measurements of the minimum solubility and flotability of calcite53 . 

This was shown, for example, by electrokinetic measurements of Kavanagh and Quirk177) in the system Fe203-illite at pH 2.5. Electrophoretic and cation-exchange data indicated that the net charge of clay surface became strongly positive at low pH values as a result of adsorption of polycations Fe3+. Further examples of metal ion hydrolysis in oxide and silicate flotation systems were published by Fuerstenau178) and Stumm et al.62). 

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. 

Electrical double layer properties at the solid/electrolyte solution interface were analyzed by potentiometric titration and electrophoresis measurements. Potentiometric titration and electrokinetic measurements were performed for three different concentrations 1 x 10 3, 1 x 10 2, and 1 x 10 1 M of NaClCXt solutions. The initial concentrations of Cd(II) and oxalate or citrate ions were 1 x 10 6, 1 x 105, 1 x 10 4, and 1 x 10 3 M, respectively. Double distilled water was used to prepare all solutions. All reagents used for experiments were analytical grade. 

Electrokinetic measurements. In this study the capillary electrokinetic method was applied [16],... 

Electrokinetic measurements for the polyelectrolyte layers were carried out by means of an Electrokinetic Analyzer device (A. Paar KG, Austria). The glass substrate after polyelectrolyte deposition was several times rinsed with deionized water to ensure that unadsorbed polymers do not contribute to the f -potential measured. The values of -potential were calculated according to the formula ... 

Schwarz S, Buchhammer H-M, Lunkwitz K, Jacobasch HJ (1998) Polyelectrolyte adsorption on charged surfaces study by electrokinetic measurements. Colloids Surf A Physicochem Eng Aspects... 

Electrokinetic measurements Enzymatic treatment Surface properties Wool... 

Also, the enzymatic treatment causes a change in the isoelectric point (IEP) value of wool, as demonstrated by the electrokinetic measurements (Fig. 5). With an increase in enzyme concentration the IEP value shifts in the direction of more acidic pH (from 3.8 for untreated wool to 3.7 for wool after 1% enzymatic treatment, and 3.45 for wool... 

Some writers have urged that the results of electrokinetic measurements should be expressed in terms similar to those usual with electrolytes containing small ions, i.e. of mobilities and valencies, and not as potentials. One difficulty here is that, although the mobility is measured, it is very seldom that the actual magnitude or density of the charge on a colloidal particle or surface is known. The charge on, or valency of, a colloidal particle must depend on its size, and may also depend on its shape. The potential probably depends but little on either. 

Measurement of the potential by means other than electrokinetic measurement. ... 

A dsorption studies of ionic species at the solid-liquid or liquid-liquid interfaces can be carried out using data obtained from electrokinetic measurements (1, 6, 7, 12, 13, 14, 26, 27, 29, 30, 37, 41,44). In the case of solid-water most of the measurements have been obtained by using either the streaming potential technique or microelectrophoresis. Since the hydrocarbons investigated previously were liquid, the microelectrophoretic technique was used (7, 37, 41,44). It is not an easy task to obtain precise results on f potentials of oil droplets from mobility measurements unless a certain number of corrections are introduced (4, 5, 20, 21, 22). 

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