Charged surface

The treatment in the case of a plane charged surface and the resulting diffuse double layer is due mainly to Gouy and Qiapman. Here may be replaced by d /dx since is now only a function of distance normal to the surface. It is convenient to define the quantities y and yo as... 

Charged surface plus diffuse double layer of ions —>... 

A number of electrokinetic phenomena have in common the feature that relative motion between a charged surface and the bulk solution is involved. Essen-... 

Just as with interaction energies, II can be regarded as the sum of several components. These include Ilm due to dispersion interaction, Ilf due to electrostatic interactions between charged surfaces, 11 due to overlapping adsorbed layers of neutral... 

Adsorption at a charged surface where both electrostatic and speciflc chemical forces are involved has been discussed to some extent in connection with various other topics. These examples are drawn together here for a brief review along with some more speciflc additional material. The Stem equation, Eq. V-19, may be put in a form more analogous to the Langmuir equation, Eq. XI-4 ... 

The adhesion between two solid particles has been treated. In addition to van der Waals forces, there can be an important electrostatic contribution due to charging of the particles on separation [76]. The adhesion of hematite particles to stainless steel in aqueous media increased with increasing ionic strength, contrary to intuition for like-charged surfaces, but explainable in terms of electrical double-layer theory [77,78]. Hematite particles appear to form physical bonds with glass surfaces and chemical bonds when adhering to gelatin [79]. 

A picture of the electrical lines of force is given in Fig. XV-12 [114] in the plane CD of the ionic groups, it will be a periodic field, whereas a little further into the solution the effect will be more that of a uniformly charged surface. The Donnan treatment is probably best justified if it is supposed that ions from solution penetrate into the region of CD itself and might in fact, lie between CD and AB. 

The Donnan effect acts to exclude like-charged substrate ions from a charged surface region, and this exclusion, as well as the concentration of oppositely charged ions, can be expressed in terms of a Donnan potential pD. Thus for a film of positively charged surfactant ions S one can write... 

Leimbach J, Sigg J and Rupprecht FI 1995 The existence of small surface aggregates-surface micelles on polar charged surfaces Colloids Surf. A 94 1-11... 

Related phenomena are electro-osmosis, where a liquid flows past a surface under the influence of an electric field and the reverse effect, the streaming potential due to the flow of a liquid past a charged surface. 

Figure 5 shows the enhanced concentration of oppositely charged ions near the charged surface, and the depleted concentration of similarly charged ions near the charged surface due to electrostatic attractions and repulsions. Both factors reduce the effective potential, /, as the distance from the surface, X, increases. The distance at which / drops to 1/ (37%) of its value at the Stem plane is called the counterion atmosphere decay distance,... 

Fig. 5. Variation of ion concentrations, C (-), and effective surface potential, / (-), with distance from a positively charged surface. The layer of...

Inorganic Ions. Because of electrostatic attraction, positive ions are attracted to negatively charged surfaces and have a higher concentration near the surface than in the bulk. Negative ions are repeUed from the negative surface and have a lower concentration near that surface. Ions which are very strongly bound (// ds Stem layer, whereas those that can move into and out of the ionic atmosphere < kT) are in the Helmholtz... 

Liquid toners are suspensions of toner particles in a fluid carrier. The carrier is typically a hydrocarbon. Dielectric, chemical, and mechanical properties of the Hquid must be compatible with the photoreceptor, the suspended toner particles, and the materials of the development equipment. Liquid toners are capable of producing higher resolution than dry toners because of the smaller (3—5 -lm) particle size achievable. Development of the latent image occurs as it passes through a bath of toner and the charged particles are attracted to the oppositely charged surface. 

Another class of DNA-binding proteins are the polymerases. These have a nonspecific interaction with DNA because the same protein acts on all DNA sequences. DNA polymerase performs the dual function of DNA repHcation, in which nucleotides are added to a growing strand of DNA, and acts as a nuclease to remove mismatched nucleotides. The domain that performs the nuclease activity has an a/P-stmcture, a deep cleft that can accommodate double-stranded DNA, and a positively charged surface complementary to the phosphate groups of DNA. The smaller domain contains the exonuclease active site at a smaller cleft on the surface which can accommodate a single nucleotide. 

It can be seen from these two factors, ie, particle charge and van der Waals forces, that the charge must be reduced or the double layer must be compressed to aUow the particles to approach each other closely enough so that the van der Waals forces can hold them together. There are two approaches to the accomplishment of this goal reaction of the charged surface sites with an opposite charge on an insoluble material and neutralization of... 

As the pH is iacreased or decreased from the isoelectric point, the particles acquire a charge (surface potential) that can enhance repulsion. Surface charge on the particle can be approximated by measuring 2eta potential, which is the electrostatic potential at the Stem layer surrounding a particle. The Stem layer is the thickness of the rigid or nondiffiise layer of counterions at a distance (5) from the particle surface, which corresponds to the electrostatic potential at the surface divided by e (2.718...). 

The physical separation of charge represented allows externally apphed electric field forces to act on the solution in the diffuse layer. There are two phenomena associated with the electric double layer that are relevant electrophoresis when a particle is moved by an electric field relative to the bulk and electroosmosis, sometimes called electroendosmosis, when bulk fluid migrates with respect to an immobilized charged surface. 

There are four related electrokinetic phenomena which are generally defined as follows electrophoresis—the movement of a charged surface (i.e., suspended particle) relative to astationaiy hquid induced by an applied ectrical field, sedimentation potential— the electric field which is crested when charged particles move relative to a stationary hquid, electroosmosis—the movement of a liquid relative to a stationaiy charged surface (i.e., capiUaty wall), and streaming potential—the electric field which is created when liquid is made to flow relative to a stationary charged surface. The effects summarized by Eq. (22-26) form the basis of these electrokinetic phenomena. 

This is used to measure charge, surface charge density, volumetric charge density or charge-to-mass ratio. It comprises an all-metal container, such as... 

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