Isoelectric point suspensions

Modifications of surface layers due to lattice substitution or adsorption of other ions present in solution may change the course of the reactions taking place at the solid/liquid interface even though the uptake may be undetectable by normal solution analytical techniques. Thus it has been shown by electrophoretic mobility measurements, (f>,7) that suspension of synthetic HAP in a solution saturated with respect to calcite displaces the isoelectric point almost 3 pH units to the value (pH = 10) found for calcite crystallites. In practice, therefore, the presence of "inert" ions may markedly influence the behavior of precipitated minerals with respect to their rates of crystallization, adsorption of foreign ions, and electrokinetic properties. 

The unstable sol evidently possess a maximum stability at the isoelectric point in contrast to the suspensions. 

Some like casein, salt free globulin and acid albumin are not heavily solvated in solution and are thus readily precipitated at the isoelectric point in a manner similar to the suspensions already considered. Others, e.g. glutin, gelatine and natural albumin, are solvated like silica which at the isoelectric point are not necessarily precipitated being maintained in the sol form by the solvent. On removal of the stabilising water however by the addition of alcohol or neutral salts precipitation will occur and this most readily at the isoelectric point. 

The isoelectric point (IEP(s)) and the zero point of charge (ZPC) are convenient references for predicting the charge-dependent behavior of oxide minerals and their suspensions. 

Size enlargement of fine particles in liquid suspension can be accomplished in a number of ways. Electrolytes can be added to a suspension to cause a reduction in zeta potential and allow colliding particles to cohere. Examples include the use of trivalent aluminum and iron ions to flocculate the particles responsible for the turbidity of many water supplies and the flocculation of metallurgical slimes by pH adjustment to the isoelectric point. Alternatively, polymeric flocculants can be added to suspensions to bridge between the particles. A wide range of such polymeric agents [1] is available today to aid the removal of fine particles from water. 

The second mechanism, first proposed by van Olphen (8,11-14), assumed structure formation in bentonite gel to be due to edge-to-flat surface asssociation of the plate-like particles as a result of electrostatic attraction between the oppositely charged double layers at the surface. This so-called "house of cards structure" is likely to occur provided the pH of the suspension is below the isoelectric point of the edges, which are then positive and become attracted to the negatively charged faces. 

A specific example of this behavior is shown in Figure 12.9 [23] where the viscosity of AlgOg suspensions is plotted as a function of pH. Near the isoelectric point, the viscosity is hi due to the colloid instability and the formation of floccs, and away from the lEP the viscosity is low due to colloid stability. 

FIGURE 12.9 Viscosity (at low shear rate) of AI2O3 suspensions at different pH values adjusted by the addition of various amounts of 0.2 M AICI3. Near the isoelectric point the suspension is not colloidally stable, giving a high viscosity. Data taken from Reed [23]. 

Chen and Soucie (96) showed that treatment of soy protein isolate with hydro-xylated lecithin lowered the isoelectric point, increased electrophoretic mobility, and signihcantly increased protein dispersibility and suspension stability. Nielsen (97) investigated the interaction of peroxidized phospholipids with several proteins under N2. His findings demonstrated a covalent attachment of phospholipids to proteins whose molecular size is increased. 

Figure 7.21 pH at which the yield stress Uy is maximum ( ) and pH at the isoelectric point (O) measured electroacoustically, versus concentration of phosphate, for suspensions described in Fig. 7-20. (From Leong et al. 1993, reproduced by permission of The Royal Society of Chemistry.)...

To estimate the total exchange capacity of the AAH, a method of potentiometric suspension titration was employed. The values of TEC that were determined below the isoelectric point are shown in Fig. 2. The value of TEC above 8 eq/kg AI2O3 indicates a great number of surface active groups present as well as a well-developed interfacial surface of active aluminium hydroxide. 

Figure 3. Determination of the isoelectric point of tin by adhesion method. Results are obtained by suspension of negative ( ) and positive ( ) latex particles [46].

Hackley, VA. et al.. Analysis of the isoelectric point in moderately concentrated alumina suspensions using electroacoustic and streaming potential methods, J. Dispersion Sci. Technol., 23. 601. 2002. 

Franks, G.V., Zeta potentials and yield stresses of silica suspensions in concentrated monovalent electrolytes Isoelectric point shift and additional attraction, 7. Colloid Intetf. Sci., 249,44, 2002. 

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. 

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