Isoelectric points hydroxylation

In the case of the epoxypolyamide varnish, however, as the pH increased the resistance of the film at first rose, then at about pH 8.8 it started to fall until at pH 11 the change-over in the type of conduction occurred. This suggests that the resin was acting as a zwitterion with an isoelectric point at about pH 8.8. Thus before the isoelectric point the membrane would be positively charged and an increasing concentration of hydroxyl ions would... 

The surface structure and characteristics (density and acidity) of the hydroxyl groups presented in Fig. 13.21 (using CrystalMaker 2.1.1 software) give very useful information to understand the reactivity of the surface of the particles, particularly when adsorption of another complex is desired to synthesize a bimetallic catalyst, or to control the interaction with an oxide carrier (the deposition step). The isoelectric point calculated with the model (5.9) is in fair agreement with the experimental value (4.3). 

If the solid diaphragm material adsorbs both hydrogen and hydroxyl ions it is evident that electric endosmose will cease when equal ionic adsorption has taken place, the double layer potential or electrokinetic potential being at this point zero and the diaphragm is at the isoelectric point. 

No substance examined appears to have an isoelectric point identical with pure water Ph = 1 where the solution contains identical concentrations of both hydrogen and hydroxyl ions and in general most materials adsorb hydrogen ions more readily than hydroxyl ions. 

Hydrolysis. The surfaces of metal oxides and hydroxides can take up or release H+ or OH- ions and become chaiged. Potentials as high as 100 mV may be sustained in aqueous solutions. For aqueous solutions this is a function of the pH the zeta potential for the particle is positive if the solution pH is below the particle s isoelectric pH (pH ), and negative if the pH is above pH. Isoelectric points for metal oxides are presented in several publications (22,23). Reactions of hydroxyl groups at a surface, Q, with acid and base may be written as follows ... 

All ions in the system, whether specifically adsorbed or not, must be expected to saturate partially the ion exchange capacity arising from these relatively pH-independent sources. For this reason an isoelectric point defined in terms of hydrogen and hydroxyl ion adsorption is hypothetical, and in any real system, the pH at which zero surface charge is observed will depend upon the system composition. 

The surface of uncalcined aluminas consists of amphoteric hydroxyl groups that are even less acidic than those in silica gel. Electrophoretic measurements by Stigter et al. (90) show that the isoelectric point for alumina is attained at a pH of 9, much higher-than the value of 2 obtained in the case of silica gel. Thus, hydroxylated aluminas are relatively non-acidic, unless they are promoted with acid-producing impurities. 

The electrical double layer at the metal oxide/electrolyte solution interface can be described by characteristic parameters such as surface charge and electrokinetic potential. Metal oxide surface charge is created by the adsorption of electrolyte ions and potential determining ions (H+ and OH-).9 This phenomenon is described by ionization and complexation reactions of surface hydroxyl groups, and each of these reactions can be characterized by suitable constants such as pKa , pKa2, pKAn and pKct. The values of the point of zero charge (pHpzc), the isoelectric point (pH ep), and all surface reaction constants for the measured oxides are collected in Table 1. 

The properties of the isolated peptides were quite similar in nature, whereby each peptide consisted of 12 amino acids in length and possessed a munber of residues with functional side groups that could stabilize nanoclusters. In many instances, these side chains were the hydroxyl-terminated side chains of serine, threonine, and tyrosine. In two of the peptides (AG3 and AG4), the location of the hydroxylated amino acids was conserved within two of the peptides. Similarly, one proline amino acid was conserved throughout all three of the sequences. Upon incubating each peptide in a solution of silver nitrate with no exogenous reductant, a clearly observable plasmon resonance peak arose at 440 nm for AG3 and AG4, but not with AG5. The peak was quite broad, indicative of a disperse size and shape distribution. The main difference between the active peptides and inactive AG5 was an overall basic isoelectric point for AGS The assays were performed at neutral conditions which would modulate the side-chain dynamics under acidic or basic conditions. 

Abrasive particles are a key component in CMP slurry. The most commonly used abrasive particles include silica, alumina, ceria, zirconia, titania, and diamond. Table 21.1 listed a set of information on each type of abrasive particles such as density, microhardness, and isoelectric points (lEP). It is important to point out that the specific values for these properties depend highly on the preparation techniques and the specific states of the samples. The values listed in the table represent an average of the most commonly reported data. For example, the isoelectric point for silica is a function of the number of hydroxyl groups, type and level of adsorbed species, metal impurity in the solid matrix, and the treatment history of the materials [1]. There are three major types of silica according to their preparation methods fumed, colloidal, and precipitated. The common sources for obtaining these abrasive particles are listed in Table 21.2. As examples, some of the more specific information on... 

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