Origins of the Surface Charge

The method of equilibrium foam film allows to study the ( -potential at various aspects by means of the microinterferometric technique (see Chapter 2). For instance, to determine cpo at electrolyte solution/air interface (no surfactant) which is very hard to realise experimentally to find the origin of the surface charge in this case [186,187] to find the isoelectric points at the solution/air interface [173,188] to study the effect of the concentration of various kinds of surfactants [95,100,189,190] ionic effects influence of Na+... 

The origin of the surface charges, and of the double layers is probably different for different materials. Thus colloidal ferric hydroxide... 

The origin of the surface charge has not been deal with as yet. The charge is due to adsorption of ionic species and/or surface reactions these matters will be analyzed in the next chapters. 

The electrostatic forces are due to the fact that most particles are charged inside a medium, especially a polar one like water, which has a high relative permittivity. The origin of the surface charge can be complex and there are many mechanisms for this, e.g. adsorption of ions from the solution or dissociation of surface groups. Other repulsive forces, which help stability, exist especially at low interparticle distances (e.g. the so-called hydration or steric forces). 

Because ionic species adsorbed in response to coulombic attraction alone obviously cannot adsorb in amounts larger than those equivalent to the original surface charge—i.e., they cannot reverse the sign of the surface charge—charge reversal must occur in response to specific adsorption alone. Similarly at the isoelectric point, the surface charge is zero hence, adsorption by coulombic attraction alone will not occur. Thus, adsorption which results in a shift in the isoelectric point must be specific adsorption. 

The results of all of these measurements can be approximated by an empirical expression that explains most of the features of our experiments on oxides. In this paper, we will not attempt to give theoretical justification, but we and others (2, 4 ) have shown that most of the five characteristics given follow directly from equilibrium ion exchange considerations if certain assumptions are made relative to the origin of the lattice charge on the oxide. Other approaches have been used to explain these observations on hydrous oxides. Among these are approaches which associate enhanced sorption with hydrolysis of the nuclide (5-7) and with formation of surface complexes to specific sites (8-10). Some of these approaches are quite elaborate making extensive use of computer calculations and include double layer theory. The approach that we have used (2) is relatively simple, and explains many of the characteristics of sorption on hydrous oxide with equilibrium theory. 

Fig. 13. Possible sign combinations involving the sign of the surface charge at the metal—oxide interface and the sign of the charge of the field-driven mobile species originating at the metal—oxide interface, together with schematic diagrams of the concentration profiles for the mobile species, (a) Field-driven cation interstitial (or anion vacancy) transport (b) Field-driven electron transport.

The constant capacitance model and diffuse layer model are combined in the Stern model. This model was originally introduced for the mercury electrode. The interfacial region is modeled as two capacitors in series. A part of the surface charge is balanced... 

Stability is based. The attractive van der Waals force depends on the size of and the distance between two bodies. The repulsive electrostatic force originates from the surface charge of the colloidal particles. Apart from this, structural forces (e.g., hydrogen bonding) can stabilize or destabilize colloidal particles depending on the nature of the particles, and adsorption of polymer material at the sulfur-solvent interface can cause steric stabilization. 

Fig. 6.8. The effect of rubbing on the molecular distribution of poisoner segments at the film surface is illustrated to explain the origin of the observed charge anisotropies, which are characterized by the distribution factors illustrated in Fig. 6.7B.

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