Charge step methods principles

General Principles Electrostatic separation (of particles), also commonly known as high-tension separation, is a method of separation based on the differential attraction or repulsion of charged particles under the influence of an electrical field. Applying an electrostatic charge to the particles is a necessary step before particle separation can be accomplished. Various techniques can be used for charging. These include contact electrification, conductive induction, and ion bombardment. 

For clusters of higher nuclearity too, the kinetics method for determining the redox potential E°(M /M ) is based on the electron transfer, for example, from mild reductants of known potential which are used as reference systems, towards charged clusters M/. Note that the redox potential differs from the microelectrode potential E° (M. M /M ) by the adsorption energy of M onM (except for n = 1). The principle (Figure 5) is to observe at which step n of the cascade of coalescence reactions (14), a reaction of electron transfer, occurring between a donor S and the cluster M/ could compete with (14). Indeed n is known from the time elapsed from the end of the pulse and the start of coalescence. The donor S is produced by the same pulse as the atoms M", the radiolytic radicals being shared between M (reactions 1,7,8) and S (reactions 25, 26). 

An alternate route to assembly of nanoparficles as hollow spheres that does not require a polymerization reaction step is sequential electrostatic assembly. Electrostatic-mediated multilayer assembly of charged particles was first demonstrated by R. Her on planar surfaces, wherein he established the proof-of-principle to deposit particles sequentially onto soUd substrates [25]. Decher advanced this scheme by assembling alternately charged PEs (e.g., polycations and polyanions) onto solid supports. Ever since, this scheme has been used to form capsules by sequential electrostatic deposition of single or multiple coatings of materials on preformed colloidal templates and subsequent removal of the template by calcination or solvent dissolution. This constitutes the LBL method for assembly of hoUow spheres. 

The formation of ion pairs between two ions of relatively large size and low charge - one of which will be the active principle - provides water-insoluble products. The experimental procedure is very simple, but is also slow as the insoluble compounds formed must be extracted with an organic solvent for their spectrophotometric determination. Ion-pairing reactions also provide substantially improved sensitivity and selectivity. Most reported methods of this type use organic counterions to avoid the extraction step. Table 2 gives a representative list of examples. 

A bilayer-coated electrode has two electroactive films, each having different reduction potentials. The inner layer is in direct contact with the carrier electrode surface and acts as a mediator to the outer layer which is mainly in contact with the solution. Provided that the redox levels in the two layers are appropriate, the interface between the two polymer films acts analogously to a semiconductor junction as a charge rectifying junction. The method of preparing first the inner layer on the electrode and then, in a second step, the outer layer may in principle be deduced from the conventional methods already described. 

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