I.e.p. = isoelectric point

GIAO gauge invariant atomic orbital i.e.p. isoelectric point... 

The esterases have also been classified according to p/ value, i.e., the isoelectric points assessed by electrophoretic mobility expressed in pH units. For many years, this was one of the major criteria used to distinguish esterases. The drawback of the classification based on pI values is that the isoelectric points can vary between species and even between strains of the same species. 

Fig. 4. Influence of pH on the plateau-value /T of adsorption isotherms of polyampholytes. At either side of the isoelectric point, i.e.p., the polyampholyte attains a net charge causing intra- and intermolecular electrostatic repulsion. As a result, the mass of adsorbed polyampholyte, that can be accommodated per unit area of the sorbent surface, decreases. Electrostatic interactions are suppressed by increasing ionic strength, yielding /T less sensitive to pH.

Thus, ihe yield stress ay is maximized at the isoelectric point. Note, however, in Fig. 7-20 that when the i.e.p. is shifted by the binding of ions to the particle surfaces, the maximum yield stress is reduced. As noted in Section 7.2.1, the binding of ions to the particle surfaces is likely to increase the thickness of the hydration layers on the particles that keep the particle surfaces from coming closer than a few nanometers from each other. Leong et al. (1993) have found a correlation between the size of the adsorbed anion and the magnitude of the decrease in the yield stress. However, the size of the hydrated ion, rather than the size of the ion itself, should, in principle, control the closest approach of the particles (Israelachvili 1991). 

Hunter and Nicol [5] studied the flocculation and restabilisation of kaolinite suspensions using rheology and zeta-potential measurements. Figure 21.11 shows plots of the yield value (cr ) and electrophoretic mobility (i) as a function of cetyl trimethyl ammonium bromide (CTAB) concentration at pH =9. increases in line with increases in CTAB concentration, reaching a maximum at the point where the mobility reaches zero (the isoelectric point, i.e.p., of the clay), and then decreases with further increases in CTAB concentration. This trend can be explained on the basis of flocculation and restabilisation of the clay suspension. 

Figure 5. Change in pH of the ferrous and ferric solution upon the addition of ammonia at I0°C. The isoelectric point (i.e.p.) of Fe(OH)., is at pH 7.7.

Figure 4. Schematic variation of the DLVO theoretical stability domain for a pHiep colloid of pH 2 (critical coagulation concentration, c.c.c. isoelectric point, i.e.p.). The insert shows this theoretical prediction compared to that observed...

Furthermore, on polystyrene it was found that the adsorption of HPA in the low surface coverage region increased with increasing temperature, except at the isoelectric point (i.e.p.) of the protein where the adsorption appeared to be independent of the temperature. According to Clapeyron s law a positive value for (6r/6T) implies an endothermic adsorption process under isosteric conditions. Although with protein adsorption isosteric conditions are difficult to establish, the qualitative conclusion is that at pHf i.e.p. the adsorption enthalpy is positive. Hence, under those conditions, adsorption must be entropically driven. We will return to this subject in section 5. 

The amphoteric colloids will at very low pH behave practically as " ionised colloids with basic character at high pH just as acid colloids , while in the intermediate region positive and negative charges occur together and just equilibrate one another at the isoelectric point (I. E. P.). 

The isolabile proteins do flocculate, coacervate or crystallise at their isoelectric point. For a large group of these proteins, which have received the name of globulins, it is characteristic that they only separate out at their isoelectric point, when the medium contains no, or sufficiently little, indifferent salt. The protein separated out at the LE.P. has the property of dissolving in dilute salt solutions. These properties give us a direct proof for the idea that the separation of the globulins at the I.E.P. is based on the existence of pronounced intermolecular complex relations. Indeed the property of indifferent salts of suppressing complex relations has already been met with very frequently in this chapter. It is then to be expected... 

Another important point, and one of fundamental significance, is that above the isoelectric point, although no precipitation is observed with added SDS, definite indications of interaction can still be obtained see, for example, the surface tension data for gelatin/ SDS presented below, calorimetric data for BSA/SDS (128), and other information to be given later. This means that proteins, like many HM-polyions, are able to interact with ionic surfactants against an unfavorable (overall) electrical gradient. For this reason many studies involving anionic surfactants have been executed above the i.e.p. of the protein, as illustrated below. 

The change in charge with pH of amphoteric surfactants affects their properties, such as wetting, detergency, foaming, etc. At the isoelectric point (i.e.p.), the properties of amphoterics resemble those of non-ionics very closely. Below and above the i.e.p. the properties shift towards those of cationic and anionic surfactants, respectively. Zwitterionic surfactants have excellent dermatological properties. They also exhibit low eye irritation and are frequently used in shampoos and other personal care products (cosmetics). 

The surface of hair has both acidic and basic groups (i.e. amphoteric in nature). For unaltered human hair, the maximum add combining capacity is approximately 0.75 mmol per g hydrochloric, phosphoric or ethyl sulphuric acid. This corresponds to the number of dibasic amino acid residues, i.e. arginine, lysine or histidine. The maximum alkali combining capacity for unaltered hair is 0.44 mmol per g of potassium hydroxide, which corresponds to the number of acidic residues, i.e. aspartic and glutamic side-chains. The isoelectric point (i.e.p.) of hair keratin (i.e. the pH at which there is an equal number of positive, -NH+ and negative, -COO groups) is pH 6.0. However, for unaltered hair, the i.e.p. is at pH 3.67. 

Fe203) at pH 7.0 [34]. The isoelectric point (i.e.p.) of FczOs has been determined to be 8.1, in agreement with the 8.5 value determined by Breeuwsma and Lyklema [35]. Consequently, both polymer and surfactant molecules can adsorb on the solid surface at the experimental pH. Competitive adsorption takes place as soon as the two species are present in the system. The magnitude of the adsorption changes depends on the order of the addition of polymer and surfactant and is probably linked to the kinetics of the surfactant and polymer segment adsorption on the solid surface. 

Ma et al. studied the adsorption behavior or PVP-SDS mixtures at two different mineral surfaces, Ti02 and FezOs [47,49]. They determined an isoelectric point of pH 5.1 for both surfaces. This result is in line with other i.e.p. determinations [54,55]. The abnormally low value recorded for natural hematite compared to synthetic FezOa means that the former is contaminated by SiOa. 

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