Microelectrophoretic mobility

Figure 6.3 Effect of carboxy group content on the microelectrophoretic mobility of microcrystalline cellulose (figures in brackets are negative).

The microelectrophoretic mobility (jUe) is related to zeta potential ( ) via one of two equations. When the diameter of the particle is small relative to the thickness of the electrical double layer, the Huckel equation applies ... 

Determination of microelectrophoretic mobility curves both in the absence and the presence of TMIS... 

The aforementioned general approach is schematized in Figure 2.6. It involves the application of several methodologies based on macroscopic adsorption data and potentiometric titrations as well as microelectrophoretic mobility or streaming potential measurements, the appKcation of spectroscopic techniques as well as the application of electrochemical (equilibrium) modeling, quantum-mechanical calculations and dynamic simulations. 

Methodologies Based on Macroscopic Adsorption Data and Potentiometric Titrations as well as on Microelectrophoretic Mobility or Streaming Potential Measurements... 

A successful modeling must describe the macroscopic adsorption data (adsorption isotherms, adsorption edges, the aforementioned plots amount of the H+ ions released (adsorbed) vs. amount of cationic (anionic) TMIS adsorbed , potentiometric titrations, microelectrophoretic mobility or steaming potential data over a wide range of pH, ionic strength and concentrations of the TMIS in the solution, using the minimum number of adjustable parameters. 

A completely independent test of a mechanistic model proposed for a deposition is the comparison of the variation, with pH, of the -potential determined experimentally using microelectrophoretic mobility measurements with the corresponding variation of the t -potential calculated by SURFEQL on the basis of the postulated deposition mechanism. Fig. 17 illustrates a typical example. The very good agreement observed supports the postulated mechanism. 

Addition of lysophosphatidylcholine (LPC) to an emulsion stabilized by a mixture of phosphatidylcholine and phosphatidyl ethanolamine (PC/PE) increased the charge on the droplets as well as the uptake by the phagocytosis systems. This uptake can be quantified by means of first order rate constant (Table 8.13). The results show clearly the importance of the nature of the surface layer. It is not clear whether the observed effects are due solely to surface charge, although there are correlations between uptake and microelectrophoretic mobility, or are acomposite of surface layer properties and charge effects. However, it may be concluded that the manner in which emulsion droplets are handled by both phagocytic systems can be altered readily by small formulation changes. 

Galisteo R, R J. De las Nieves, M. Cabrerizo, and R. Hidalgo-Alvarez. 1990. Effects of particle concentration, ionic strength, pH and temperature on the microelectrophoretic mobility of cationic polysterene latex. Progress in Colloid Polymer Science 82 313-320. 

Potentiometric titrations have been used to determine the surface charge density, Ojj(pC.cm ), in the absence and presence of the species to be adsorbed and the concentration of the surface groups [SOH neutral hydroxyls, SOH protonated hydroxyls, SO deprotonated hydroxyls]. Full details have been given elsewhere [14-17],Microelectrophoretic mobility measurements were used to determine the electrokinetic charge density, o (pC.cm ), in the absence and presence of the species to be adsorbed. Full details are given elsewhere [18]. 

Model particle mobility has been determinated with the Tiselius method (Tiselius, 1937, 1938). This method also allows the integration of the mobility of a large number of particles even if the refractive index is very close to that of the electrolyte medium, allowing to minimize the experimental errors inherent to the classical microelectrophoretic techniques. The electrophoretic mobilities will not be transformed into surface charges because the theoretical relationship between these parameters is highly dependant on the particle radius of curvature and the electrolyte concentration in the vicinity of the particle (Hunter and Wright, 1971). For both methods, the analytical error falls below 5 %, however, it increases up to 10 % for natural composite samples and/or low mobilities. 

Recent advances made in measuring particle charge and mobility in nonaqueous suspensions are reviewed. Microelectrophoretic techniques have been used to determine zeta potential and the measurements related to particle stability. 

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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