Electrophoretic mobility measurements

Surface active electrolytes produce charged micelles whose effective charge can be measured by electrophoretic mobility [117,156]. The net charge is lower than the degree of aggregation, however, since some of the counterions remain associated with the micelle, presumably as part of a Stem layer (see Section V-3) [157]. Combination of self-diffusion with electrophoretic mobility measurements indicates that a typical micelle of a univalent surfactant contains about 1(X) monomer units and carries a net charge of 50-70. Additional colloidal characterization techniques are applicable to micelles such as ultrafiltration [158]. 

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

Saarinen, TR Woodward, WS, Computer-Controlled Pulsed Magnetic Field Gradient NMR System for Electrophoretic Mobility Measurements, Reviews of Scientific Instruments 59, 761, 1988. 

In most experiments the smallest amount of electrolyte needed to coagulate the sols measured after 2 hours standing was chosen as the CCC. When using HC1, this point is the critical coagulation pH. A constant temperature water bath was used for temperature different than 23°C. The pH values were measured with a Beckman Model 96A pH meter and a Fisher combination electrode. The electrophoretic mobility measurements were made with a Laser Doppler Electrophoresis apparatus. These experiments were performed by Mr. J. Klein of the Chemistry Department, Syracuse University. 

Electrophoretic mobility measurements can be performed by laser Doppler anemometry (LDA). LDA is fast and capable of high resolution of particle velocities [144]. It measures particle velocity, which is measured in the stationary... 

Modifications of surface layers due to lattice substitution or adsorption of other ions present in solution may change the course of the reactions taking place at the solid/liquid interface even though the uptake may be undetectable by normal solution analytical techniques. Thus it has been shown by electrophoretic mobility measurements, (f>,7) that suspension of synthetic HAP in a solution saturated with respect to calcite displaces the isoelectric point almost 3 pH units to the value (pH = 10) found for calcite crystallites. In practice, therefore, the presence of "inert" ions may markedly influence the behavior of precipitated minerals with respect to their rates of crystallization, adsorption of foreign ions, and electrokinetic properties. 

Outer-sphere adsorption of Pb(ll)EDTA on goethite. Geochim. Cosmochim. Acta 63(19/20) 2957-2969 Bargar, J.R. Reitmeyer, R. Davis, J.A. (1999) Spectroscopic confirmation of uranium(Vl)-carbonato adsorption complexes on hematite. Environ. Sci. Techn. 33 2481-2483 Bargar, J.R. Reitmeyer, R. Lenhart, J.J. Davis, J.A. (2000) Characterization of U(Vl)-car-bonato ternary complexes on hematite EX-AFS and electrophoretic mobility measurements. Geochim. Cosmochim. Acta 64 ... 

Bargar, J. R., Reitmeyer, R Lenhart, J. J. Davis, J. A. 2000. Characterization of U(VI)-carbonato ternary complexes on hematite EXAFS and electrophoretic mobility measurements. Geochimica et Cosmochimica Acta, 64, 2737- 2749. 

In all the sections of this chapter until now we have focused attention on electrophoresis. We have seen that the potential at the surface of shear can be measured from electrophoretic mobility measurements, provided the system complies with the assumptions of a manageable model. One feature that has been conspicuously lacking from our discussions is any comparison between electrophoretically determined values of f and potential values determined by another method. The reasons for this are twofold ... 

The NMR measurements were made on sonicated phospholipid vesicles to obtain relatively narrow 31P-NMR signals while the electrophoretic mobility measurements were made on unsonicated vesicles. Since there are differences between the two systems (e.g., area per molecule (13,14)), we do not attempt to quantitatively compare the 31P-NMR data with the electrophoretic data, but rather use the 31P-NMR data as an independent demonstration of the difference between the binding of calcium and magnesium. We note, however, that the linewidth ratio for the outer monolayer of the sonicated vesicles were identical within experimental error (see Table II). This implies that the Ca++/Mg++ selectivity of the two monolayers is identical. We had expected the selectivity to be greater for the inner monolayer because the polar head groups of the lipids in this monolayer occupy a smaller area (13). 

The 31P-NMR technique, using cobalt as a probe, thus provides information about the electrostatic potential at the binding site of the divalent cations (presumably the phosphate group). In contrast, the electrophoretic mobility measurements give information about the potential at the plane of shear. 

As mentioned above, Edelhauser (7) showed that the concentration gradient across the dialysis membrane must exceed a critical value to make the dialysis proceed at a practical rate. In contrast, Ottweill and Shaw (12) found from electrophoretic mobility measurements and desorption of radioactive emulsifier that all of the emulsifier was removed by dialysis or at least that a constant surface charge was obtained. 

Later work (8,15) showed that the value of a increased with increasing electrolyte concentration and that it could be correlated with the electrophoretic mobility. Table IX shows that the electrophoretic mobility measured using the Micromeritics Mass Transport cell increased with increasing a as observed for IT, Na+, and Ba++ counterions. These results also show that the distribution of the counterions in the electric double layer is critically dependent upon the nature of the counterion, e.g.,... 

For quite some time, there have been indications for a phase-separation in the shell of polyelectrolyte block copolymer micelles. Electrophoretic mobility measurements on PS-PMAc [50] indicated that a part of the shell exhibits a considerable higher ionic strength than the surrounding medium. This had been corroborated by fluorescence studies on PS-PMAc [51-53] and PS-P2VP-heteroarm star polymers [54]. According to the steady-state fluorescence and anisotropy decays of fluorophores attached to the ends of the PMAc-blocks, a certain fraction of the fluorophores (probably those on the blocks that were folded back to the core/shell interface) monitored a lower polarity of the environment. Their mobility was substantially restricted. It thus seemed as if the polyelectrolyte corona was phase separated into a dense interior part and a dilute outer part. Further experimental evidence for the existence of a dense interior corona domain has been found in an NMR/SANS-study on poly(methylmethacrylate-fr-acrylic acid) (PMMA-PAAc) micelles [55]. 

Electrophoretic mobility measurement has been useful in this context, as the measurement is made at very low surface area-to-solution volume ratios, However, particle electrophoretic mobility is related to surface charge in complex ways depending upon particle size and morphology 2], Quantitative analysis... 

The soluble protein showed a single boundary peak in the Tiselius apparatus in buffers of ionic strength 0.02 at all pH values in the range pH 2-9, but its isoelectric point was markedly dependent upon the salt concentration. At ionic strength 0.2, in the presence of sodium chloride, the isoelectric point both from electrophoretic mobility measurements and membrane potential determinations was pH 3.9-4,0. At lower ionic strength (0.02) the protein was isoelectric at pH 4.8 in the electrophoresis experiments and pH 4.7 in membrane potential determinations. 

Figure 9.19. The diffuse double layer, (a) Diffuseness results from thermal motion in solution, (b) Schematic representation of ion binding on an oxide surface on the basis of the surface complexation model, s is the specific surface area (m kg ). Braces refer to concentrations in mol kg . (c) The electric surface potential, falls off (simplified model) with distance from the surface. The decrease with distance is exponential when l/ < 25 mV. At a distance k the potential has dropped by a factor of 1/c. This distance can be used as a measure of the extension (thickness) of l e double layer (see equation 40c). At the plane of shear (moving particle) a zeta potential can be established with the help of electrophoretic mobility measurements, (d) Variation of charge distribution (concentration of positive and negative ions) with distance from the surface (Z is the charge of the ion), (e) The net excess charge.

The so-called potential can be taken as a first estimate for the surface potential. The potential is the electrostatic potential at the hydrodynamic shear plane close to the particle surface. It can be determined from electrophoretic mobility measurements of the particles in an electric field (see for example Ref. [23]). The potential is zero when the charge within the shear plane is zero. This is the case as the surface charge plus the charge due to adsorbed ions other than hydrogen (for example AIOH2 in the case of alumina suspensions) is zero. This point is the iso-electro-point (i.e.p.) of the material in the dispersion medium. The suspension pH with respect to the i.e.p. is an important criterion for a first judgement of possible electrostatic stability. 

Dietrich, P.G. et al.. The characterization of sihca microparticles by electrophoretic mobility measurements, Chmmatographia, 44, 362, 1997. 

More details about the method of electrophoretic mobility measurement by means of dynamic light scattering are given in Section 5.9.2.1. 

The fixed charge is computed from alkalimetric titration curves and from analytical determination of the quantity of adsorbed. For the electrophoretic mobility measurement (bottoma ratio of P /FeOOH equal to that of the lower charge vs. pH curve (top) was used (equilibrium values of P in solution are approximately the same) (IS), (top) (O) Pt = 2 X 1.2 g FeOOHf... 

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