Charge stabilization

In electro-osmosis, the volume flow rate (dV/df) is measured through a capillary or a porous plug which can be treated as a series of capillaries. Using the Smoluchowski equation (Eq. 3.15), this is related to the zeta potential. For flow in a capillary of cross-sectional area A, we obtain 

This equation can also be expressed in terms of current using Ohm s law. 

The concentration and nature of the electrolyte also has a significant impact on the stability of charged colloid dispersions. This was discussed in Section 3.3.2, where the concept of electric double layers was introduced. The electric double layer results from the atmosphere of counterions around a charged colloid particle. The decay of the potential in an electric double layer is governed by the Debye screening length, which is dependent on electrolyte concentration (Eq. 3.8). In the section that follows, the stability of charged colloids is analysed in terms of the balance between the electrostatic (repulsive) forces between double layers and the (predominantly attractive) van der Waals forces. 

The valence of the counterion is the predominant influence in preventing coagulation of a colloidal dispersion. The nature of the counterion, the valence of the co-ion and the concentration of the sol are much less important, and the nature of the sol only has a moderate effect on stability. These empirical observations, made in the late nineteenth century, are known as the Schulze-Hardy rule. We are now able to interpret these effects using a quantitative model known as the Derjaguin-Landau-Verwey-Overbeek theory, discussed in the next section. 

Electrostatic interactions are not just important in stabilizing colloidal dispersions, but also influence emulsification, through interactions between head groups of ionic surfactants. The stabilization of emulsions is the subject of Section 3.13. 

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Behrens S H, Borkovec M and Schurtenberger P 1998 Aggregation in charge-stabilized colloidal suspensions revisited Langmuir 1951-4... 

Dimerization is reportedly catalyzed by pyridine [110-86-1] and phosphines. Trialkylphosphines have been shown to catalyze the conversion of dimer iato trimer upon prolonged standing (2,57). Pyridines and other basic catalysts are less selective because the required iacrease ia temperature causes trimerization to compete with dimerization. The gradual conversion of dimer to trimer ia the catalyzed dimerization reaction can be explained by the assumption of equiUbria between dimer and polar catalyst—dimer iatermediates. The polar iatermediates react with excess isocyanate to yield trimer. Factors, such as charge stabilization ia the polar iatermediate and its lifetime or steric requirement, are reported to be important. For these reasons, it is not currently feasible to predict the efficiency of dimer formation given a particular catalyst. 

The greater the charge stabilization, the greater will be the depth of the energy minimum at the intermediate-complex stage, and the greater its stability. The intermediate complex from an unactivated substrate is unlikely to be detectable or isolable and even from activated molecules may not reach appreciable concentrations. The energy minimum may frequently not be occupied for an appreciable... 

When nitrogen is in an " inductive position of an adjoining ring, the charge stabilization could be greater for a nitrogen atom at the center (5-position in 56) of the pentadienoid transition state than at the 7-position (57), even though the distances from the reaction site are... 

The low reactivity (Section IV,B,3,a) of 3-chlorocinnoline compared to that of its isomer 407 is the combined result of decreased activation by an ortho ring-nitrogen and of the effect of the bicyclic system on charge stabilization in 2,3-orientations. The 2-Le-3-aza effect alone is the reason that the reactivity of 3-chlorocinnoline (396) is much lower than that of its benzopyridazine isomer 403. 

Figure 16.18 Nucleophilic aromatic substitution on nitrochlorobenzenes. Only in the ortho and para intermediates is the negative charge stabilized by a resonance interaction with the nitro group, so only the ortho and para isomers undergo reaction.

When there is no conjugated substituent able to stabilize the positive charge development, the bromine atom is involved in the charge stabilization and the... 

At present, this rule fails only when functional neighboring substituents, capable of anchimeric assistance and in a convenient position with respect to the developing positive charge, can compete with bromine in the charge stabilization of the cationic intermediate (ref. 15). For example, the reaction of some unsaturated alcohols (ref. 16) goes through five- or six-membered cyclic oxonium ions, rather than through bromonium ions. 

So you only need one skill to completely master acid-base chemistry you need to be able to look at a negative charge and determine how stable that negative charge is. If you can do that, then acid-base chemistry will be a breeze for you. If you cannot determine charge stability, then you will have problems even after you finish acid-base chemistry. To predict reactions, you need to know what kind of charges are stable and what kind of charges are not stable. 

The most important factor for determining charge stability is to ask what atom the charge is on. For example, consider the following two structures ... 

Two major types of stabilization mechanisms are described for submicron particles (1) charge stabilization, where surface charge forms a repulsive screen that prevents the particles from flocculation, and (2) steric stabilization, where a surface repulsive screen is formed by solvent-compatible flexible polymeric chains attached to the particle s surface. 

The photoelectrochemical properties of 283 colloids prepared by chemical solution growth [193] have been demonstrated by carrying out oxidation and reduction processes under visible light irradiation. Charged stabilizers such as Nation were found to provide an effective microenvironment for controlling charge transfer between the semiconductor colloid and the redox relay. 

Meijer, E. J. Azhar, F. El, Novel procedure to determine coexistence lines by computer simulation, application to hard-core Yukawa model for charge-stabilized colloids, J. Chem. Phys. 1997,106, 4678-4683... 

Temperature Sensitivity. Samples of Au-acetone colloid were subjected to boiling and freezing. Upon returning to room temperature the colloids remained stable and no flocculation had occurred. These results indicate that steric stabilization33,43) (solvation) is a very important mechanism. Charge-stabilized colloids generally flocculate when subjected to such extremes of temperature.(56)... 

In solution, stabilization due to polarizability is of less importance, as the solvent will provide other charge stabilization mechanisms. However, recent work shows that polarizability is still of influence in solution 57,58,59). 

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