Isoelectric state

As previously discussed, in anion gap metabolic acidosis, the isoelectric state is maintained because unmeasured anions are present. With a normal anion gap metabolic acidosis, the isoelectric state is maintained by an increase in the measured... 

The side chains of aspartic acid and glutamic acid contain acidic carboxyl groups. These amino acids have acidic isoelectric points around pH 3. An acidic solution is needed to prevent deprotonation of the second carboxylic acid group and to keep the amino acid in its neutral isoelectric state. 

Fig. 3.25 presents the aqueous solutions in the absence of a surfactant at constant ionic strength (HC1 + KC1) [186,197], It can be seen that at pH > 5.5, op-potential becomes constant and equal to about 30 mV. At pH < 5.5 the potential sharply decreases and becomes zero at pH 4.5, i.e. an isoelectric state at the solution surface is reached. As it is known, the isoelectric point corresponds to a pH value at which the electrokinetic phenomena are not observed. Since in the absence of the potential of the diffuse electric layer, the electrokinetic potential (zeta-potential) should also be equal to zero, the isoelectric point can be used to determine pH value at which aqueous solutions is mainly due to the adsorption of H+ and OH" ions. Estimation of the adsorption potential of these ions in the Stem layer (under the assumption that the amounts of both ions absorbed are equal) showed that the adsorption potential of OH" ions is higher. It follows that ( -potential at the solution/air interface appears as a result of adsorption of OH" ions. 

Isoelectric state and charge reversal at the surfactant solution/gas interface. The... 

The data of the electrophoretic behaviour of bubbles in aqueous electrolytes, obtained from Kelsey et a/. [185], Yon and Jordan [180] and Huddelston and Smith [181] are also in support to the isoelectric state found. The apparent negative charge on the bubbles in the surfactant-free electrolyte solution was ascribed to preferential adsorption of OH ions. 

An interesting option to study n(/ ) isotherm for non-ionic surfactants is to plot it at a constant ionic strength but different pH of the source solution. This possibility was in sight after finding the strong effect of pH on ( -potential, including the isoelectric states (pH )reached [173,186,188,284] (see also Section 3.3.2). 

Effect of the isoelectric state at the solution/air interface on the stability... 

The examples given with the two representatives of non-ionic surfactants, NP20 and Cio(EO)4 clearly indicate that the isoelectric state at the solution/air interface leads to foam film rupture and, respectively, to decrease in foam stability. This fact supports the idea about the role of foam films in the stability of foams. On the other hand, it provides an opportunity to regulate foam stability. 

It can be anticipated that further studies of the isoelectric state at the PEO polymer solution/air interface would indicate that the steric component of disjoining pressure (see Section 3.3.3) appearing in foam films from long chain PEO polymers would prevent the foam films from rupturing. 

An increase in the concentration of counterions in the medium leads to an increase in n — x), which produces a decrease in the charge of the particles. In the general case the conversion of micelles of iron hydroxide to the isoelectrical state is represented thus ... 

Golikova, E.V. etal.. Aggregation stability of aqueous dispersions of a-FejOj, a-FeOOH and Cr2O3 under conditions of the isoelectric state, Colloid J., 60, 166, 1998. 

From the ratio of polyampholyte and metal ion concentrations at which a maximum reaction rate is observed one can ascertain the composition of catalytically active complexes [82]. The effect of [polyampholyte]/[metal ion] composition on Vq at a constant metal ion concentration for several polyampholyte-metal systems is illustrated in Fig. 7. For a majority of the systems the ratio is not higher than 3. These results support the contention that catalytic activity ctm occur only in the presence of free sites in the coordination sphere of a metal ion [83]. An exception to this is the styrene-A/ N-dimethylaminopropylmonoamide of maleic acid/copper(II) complex for which a maximum rate of H2O2 decomposition was found at [polyampholyte]/ [Cu ] = 16 1 and pH = 8.5. Because the isoelectric state of the polyampholyte is attained at pH 6.4 it is unlikely that the compression of the macromolecule coil has affected the complex composition. Apparently it is the presence of hydrophobic styrene units in the copolymer that affects the reaction rate. 

The theory of solutions of flexible uncharged polymers with excluded volume is at present well developed, but the properties of polyelectrolytes and especially polyampholytes have been considered much less from the theoretical point of view. It is well known that polyampholytes exhibit a change in phase from the extended random flight configuration to a condensed microphase. The polyampholyte theory of Edwards et al. [6] considers the isoelectric state of polyampholytes as a microelectrolyte satisfying a Debye-Huckel-type of structure. The criterion of transition from the collapsed conformation to the extended one is described as follows ... 

Polyampholytes at the isoelectric state when the whole macromolecule is neutral tend to collapse, have a small hydrodynamic radius if soluble, and have the phase separation if insoluble. 

Thus also an unbalancing of the original electrical compensation characteristic of the isoelectric state, will be the result, which will increase in the order 1—2y 1—3, 1—4. Of course this unbalancing will in itself diminish the efficiency of the mutual... 

Vysotskii ZZ, Strazhesko DN (1973) Isoelectric state of disperse silicas and ion exchange in acid solutions, adsorption and adsorbents, 1st edn. WUey, New York, pp 55-71... 

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