Surface Charge and the Electric Double Layer

Solid particle surfaces can develop electric charge in three principal ways  

1) The charge may arise from chemical reactions at the surface. Many solid surfaces contain ionizable functional groups -OH, -COOH, -OPO3H2, -SH. The charge of these particles becomes dependent on the degree of ionization 

At low pH a positively charged surface prevails at high pH, a negatively charged surface is formed. At some intermediate pH the net surface charge will be zero. 

Charge can also originate by processes in which solutes become coordinatively bound to solid surfaces, for example, 

2) Surface charge at the phase boundary may be caused by lattice imperfections at the solid surface and by isomorphous replacements within the lattice. For example, if in any array of solid Si02 tetrahedra an Si atom is replaced by an Al atom (Al has one electron less than Si), a negatively charged framework is established  

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Figures 1 and 2 show that the intercepts Qi and Q° at t = 0 are not zero. Rapid initial mass transfer into solutiBn at short times is attributed to exchange between aqueous hydrogen ions and cations situated at or near the fresh silicate surface. Tamm ( ) and Garrels and Howard ( ) have demonstrated the reversibility of such surface exchange between hydrogen and potassium ions in potassium feldspars. However, in the case of magnesium silicates. Luce and others ( ) showed that such exchange was not stoichiometric, owing to establishment of surface charge and the electric double layer.

SURFACE CHARGE AND THE ELECTRIC DOUBLE LAYER Acquiring Surface Charge... 

A number of refinements and applications are in the literature. Corrections may be made for discreteness of charge [36] or the excluded volume of the hydrated ions [19, 37]. The effects of surface roughness on the electrical double layer have been treated by several groups [38-41] by means of perturbative expansions and numerical analysis. Several geometries have been treated, including two eccentric spheres such as found in encapsulated proteins or drugs [42], and biconcave disks with elastic membranes to model red blood cells [43]. The double-layer repulsion between two spheres has been a topic of much attention due to its importance in colloidal stability. A new numeri-... 

A classic definition of electrochemical ultracapacitors or supercapacitors summarizes them as devices, which store electrical energy via charge in the electrical double layer, mainly by electrostatic forces, without phase transformation in the electrode materials. Most commercially available capacitors consist of two high surface area carbon electrodes with graphitic or soot-like material as electrical conductivity enhancement additives. Chapter 1 of this volume contains seven papers with overview presentations, and development reports, as related to new carbon materials for this emerging segment of the energy market. 

Detailed numerical examples of the behaviour of the surface charge and surface potential when the electrical double layer of two identical amphoteric surfaces overlap and interact are available in the literature (8). Examples of the differences between the form of the interaction free energy under constant... 

In Chapter 5 we learned that, in water, most surfaces bear an electric charge. If two such surfaces approach each other and the electric double layers overlap, an electrostatic double-layer force arises. This electrostatic double-layer force is important in many natural phenomena and technical applications. It for example stabilizes dispersions.7... 

In electrokinetic phenomena such as electroacoustics, theoretical models need to consider the induced movement of charge within the electrical double layer (EDL), the surface current , Is, as well as the interaction of the outer portion of the double layer with the applied signal (acoustic or electric field) and with the liquid medium. Hydrodynamic flows generate surface current as liquid moving relative to the particle... 

Electrochemical capacitors, also called supercapacitors, are very attractive electricity sources because of their high power, very long durability, and intermediate energy between the classical dielectric capacitors and batteries. The performance of a typical electrochemical capacitor is based on the accumulation of charges in the electrical double layer without faradaic reactions (no electron transfer The electrons involved in double layer charging are the delocalized conduction-band electrons of the electrode material. As shown in Fig. 23.9, an electrochemical capacitor contains one positive electrode with electron deficiency and the second one with electron excess (negative). The capacitance C of one electrode due to a pure electrostatic attraction of ions is proportional to the surface area S of the electrode-electrolyte interface, according to the formula (23.3) ... 

It must also be recognized that, because of the usually much finer subdivision of natural ion exchangers in soils as compared with commercial synthetic resins, surface phenomena may play a more prominent role. Among these effects are water is more structured at the interface than in the bulk liquid ions and water molecules are less mobile at the interface because of stronger interactions the dielectric constant of water is lower than in the bulk solution and surface charges produce an electric double layer (Horst, 1990). 

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