Electrokinetic properties

In foams with charged gas/liquid interface, various electrokinetic parameters can be obtained, such as streaming potential and zeta-potential. For example, the relationship between the volumetric flow of a liquid flowing through a capillary or membrane and the zeta-potential can be given by the Smoluchowski equation. 

The interpretation of electrokinetic results is complicated because of surface mobility and border and films elasticity, which causes large nonhomogeneities in density and border radii at hydrostatic equihbrium and Hquid motion. 

In foams with charged gas/liquid interfaces, as in other disperse systems, various electrokinetic phenomena are possible to occur. Such are the change in the transport numbers of ions, electroosmosis, streaming potential and surface conductivity. While these phenomena are largely studied in disperse systems with solid disperse phase, the first electrokinetic observations in foams have been reported only recently. 

The maximum electroosmotic pressure of the liquid flowing through a capillary or a membrane equals 

The first results about foam electrokinetics have been reported by Sharovamikov [62,63]. An electroosmotic liquid transport is observed in foams from solutions of ionic surfactants (NaDoS, CTAB, PO-3A, etc.) and it is larger than in systems with solid capillaries (specific transport from 1.6-1 O 6 to 210 6 m3 C 1). The maximum electroosmotic pressure depends on the initial pressure in borders and reaches 1 Pa. The addition of dedecanol to the NaDoS solution sharply decreases the electroosmotic transport but increases the electroosmotic pressure. To reduce the influence of border and film non-homogeneity that originates in a static foam under gravity, the electrokinetic studies have been performed in an advancing foam [62]. The specific electroosmotic transport depends on the capillary pressure and reaches a maximum value at pg = 0.5 kPa. The streaming potential (up to 10 mV) 

These studies were performed at high surfactant concentrations ( 0.01 mol dm 3) where the appearance of -potential was hard to explain. Unclear remained the fact that f-potential did not depend on the electrolyte concentration (NaCl) up to a value of 3%. Anomalously high values of -potential ( 0.5 V) have been reported by Laniquielli and Galembeck [64] who studied electrokinetic phenomena occurring at gravitational foam drainage. They attribute this anomaly to the electrolyte accumulation in the electrode space. The analysis of these results indicates that reliable values of the -potential in foams can be derived with an especially developed theory of the electrokinetic phenomena in foams that accounts for the peculiarities of these systems. Furthermore, new apparatus is needed to allow the study of foams with controlled border sizes and precisely defined border profile. 

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]. 

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Suspensions are generally evaluated with respect to their particle size, electrokinetic properties (zeta potential), and rheological characteristics. A detailed discussion on the methods/techniques and relevant instrumentation is given in Sec. VII. A number of evaluating methods done specifically with suspension dosage forms, such as sedimentation volume, redispersibility, and specific gravity measurements, will be treated in this section. 

Collins, Y.E. and Stotzky, G., Heavy metals alter the electrokinetic properties of bacteria, yeasts, and clay minerals, Appl Environ Microbiol, 58 (5), 1592-1600, 1992. 

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. 

In the present work, a constant composition method has been used to investigate the growth of HAP from solutions of low supersaturation and in the presence of different background electrolytes. The influence of magnesium and strontium ions both on the rate of crystallization and upon the electrokinetic properties of the crystallite surfaces has also been investigated. 

Brant J, Lecoanet H, Hotze M et al. (2005a) Comparison of electrokinetic properties of colloidal fullerenes (n-C60) formed using two procedures. Environ Sci Technol. 39 6343-6351. 

The Phenomenology of Mobile Electrified Interfaces Electrokinetic Properties... 

In on effort to establish the mechanism of coal flotation and thus establish the basis for an anthracite lithotype separation, some physical and chemical parameters for anthracite lithotype differentiation were determined. The electrokinetic properties were determined by streaming potential methods. Results indicated a difference in the characteristics of the lithotypes. Other physical and chemical analyses of the lithotypes were mode to establish parameters for further differentiation. Electron-microprobe x-ray, x-ray diffraction, x-ray fluorescent, infrared, and density analyses were made. Chemical analyses included proximate, ultimate, and sulfur measurements. The classification system used was a modification of the Stopes system for classifying lithotypes for humic coals. 

The electrokinetic properties were determined by streaming potential methods (I). Results indicated that the surface characteristics of the litho-types are different. Hydronium and hydroxyl ions appeared to be potentialdetermining ions for coal. Results of a typical streaming potential investigation are shown in Figure 2. Potential-determining ions may be loosely defined as those ions which participate in the the electrolytic reaction that establishes equilibrium at the solid-liquid interface. When the potential-determining ions... 

Fig. 2. Changes in electrokinetic properties of zeolite F9 and its modified form, Al3+-exchanged zeolite.

The ODN adsorption onto cationic microgel poly(N-isopropylacrylamide) particles was reported to be dramatically affected by the salinity of the incubation medium [9] as illustrated in Fig. 6. The observed result was related to (i) the reduction in attractive electrostatic interactions between ODN molecules and the adsorbent and (ii) the drastic effect of ionic strength on the physico-chemical properties of such particles [17, 27]. In fact, the hydrodynamic size, the swelling ability, the electrokinetic properties, and the colloidal stability are dramatically affected by pH, salt concentration, and the medium temperature [27]. 

Another well documented, but less widely used method for formulation development is the measurement of electrokinetic properties [14]. These tests require more disdnc-tive/elaborate sample preparation and are mosdy restricted to use during development. Also widely used and relied on are rheological measurements. The scope of rheological measurements ranges from viscosity measurements to the determination of yield points or oscillatory properties, such as the G and G -modules [15]. Since suspoemulsions are not ideal viscous but mostly viscoelastic or dilatant, a wide range of characterization techniques exists. Instrumentation required for this are simple rotary viscosimeters (Brookfield) or more sophisticated stress or shear controlled rotational viscosimeters. 

This section will focus mainly on characterization of the physical nature of the dispersed phase or its size distribution. Electrokinetic characterization techniques, which determine the electric double-layer properties of the dispersed phase, will be only briefly mentioned. Again, electrokinetic properties, their significance, and their measurement have been covered in review articles 44, 45). 

Washington, C. The electrokinetic properties of phospholipid stabilized fat emulsions. III. Interdroplet potentials and stability ratios in monovalent electrolytes. Int. J. Pharm. 1990, 64, 67-73. 

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