Isoelectronic point

The isoelectronic points of solid oxides, solid hydroxides and aqueous hydroxo complex systems. G. A. Parks, Chem. Rev., 1965, 65, 177-198 (131). 

Isoelectric point (IEP), 20 479 Isoelectronic point, 3 674 selected inorganic particles,... 

The zeta potential determines the rate of gelation. If the charge is high, panicles effectively repel one another and avoid contact. If it is low, then thermal motion leads to collision and coalescence. These rates are highest at the isoelectronic point, where the zeta potential is zero. 

Furthermore, metal oxide electrode surfaces are sites for acid/base equilibria. This situation leads to a strong solution pH dependence of the surface charges and, therefore, the potential drop in the double layer. In the absence of specific ion adsorption other than H" and OH", there will exist a particular solution pH for which the excess surface charge on the metal oxide is zero. This value of pH is called the zero point of charge (not to be confused with the pzc). If specific adsorption of ions other than H" and OH" occurs, then the solution pH at which no excess surface charge exists is called the isoelectronic point or the point of zero zeta potential. ... 

A simple way to understand the importance of maintaining the pH of our blood within a narrow range is to look at the pAa of the amino acids that constitute the proteins. The ionic state of the protein is maintained by this value of pAig relative to the pH of the surrounding fluid. As an amino acid usually has two or three pAig values, it is often more useful to discuss it in terms of the isoelectronic point, denoted by p/, which is the pH at which the amino acid is neutral, i.e. the zwitterion form is dominant if the amino acid is stable, and the amino acid does not migrate in... 

The amino acid sequences of each of the live varieties of histones, taken from several animal species, have been determined. The histones have an exceptionally high content of basic amino acids, particularly lysine and/or arginine and this gives them a positive charge at neutral pH (isoelectronic point 10.8). 

Milk normally has a pH of 6.0-6.3. If bacteria grow in it they release acids such as lactic acid which lower the pH and render the milk sour. When the isoelectronic point for casein is reached at about pH 4.7, this phosphoprotein is precipitated and the milk curdles, enabling it to be separated into the traditional curds and whey. The latter corresponds roughly to milk serum (Figure 12.8). 

The behaviour of racemic compounds may also be influenced with the choice of pH value of the aqueous solution. The isoelectronic point of 2-phenylglycine (PG) is at neutral pH value (Ip = 7.0). The free racemic aminoacid with (S)-camphorsulfonic acid (CSA) forms a well crystallizing salt in water, but the amino acid in the precipitated diastereoisomer have almost racemic composition. When the crystallization is started from an aqueous solution containing half an equivalent amount of CSA and equivalent amount of hydrochloric acid (total 50% excess of acid to the PG as base ), the crystalline salt contains (S)-2-phenylglycine in high enantiomeric purity (and in good yield), while the almost pure (R)-2-phenylglycine was obtained from filtrate by crystallization of its hydrochloric acid salt." ... 

A general conclusion has been reached that drilling rate is a maximum, wear rate a minimum, and hardness a maximum in any environment at a pH that produces a zeta potential equal to zero. Thus the isoelectronic point is important however it is achieved Figure 4.10 is a visual demonstration of this viewpoint taken from the work of Swain et Chemisorbed or weakly physisorbed species lead to differing effects for example, weakly absorbed lauric acid does not shift the zeta potential from pH = 9.0 (fixed by strongly adsorbed OH" and that between them determine the surface potential) while oleic acid is strongly chemisorbed and so the isoelectronic point moves to pH 7.5 and drilling rates and hardness are consequently affected. 

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