Isoelectric point colloid stability

At the pH = Jt there is a balance of charge and there is no migration in an electric field. This is referred to as the isoelectric point and is determined by the relative dissociation constants of the acidic and basic side groups and does not necessarily correspond to neutrality on the pH scale. The isoelectric point for casein is about pH = 4.6 and at this point colloidal stability is at a minimum. This fact is utilised in the acid coagulation techniques for separating casein from skimmed milk. 

The hydrogen ion concentration at which a colloidal system is electrically neutral the addition of acidic substances to, for example, rubber latex causes the pH value to move towards the isoelectric point, which is the region of minimum stability, and coagulation may take place. 

The W values [65] for a dispersion of AI2O3 as a function of pH and KNO3 salt concentration are shown in Figure 10.27. The AI2O3 particles are colloidally stable far away from their isoelectric point (i.e., pH 8.9). As the salt concentration is increased the zeta potential decreases and the colloid stability ratio, W, decreases. Near the isoelectric point there is no electrostatic repulsion, giving a rapid coagulation. 

FIGURE 10.27 (a) Zeta potential as a function of pH for A1203 in an indifferent 1 1 electrolyte solution (i.e., l Os). (b) Colloid stability ratio for the same AI2O3 sol as a function of pH. The minimum values correspond to the isoelectric point at pH == 9. Data from Wiese and Healy [65]. 

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. 

The pH of a slurry has a profound influence on its colloidal stability and CMP performance. Strong correlations have been established between the particle isoelectric point (lEP) and the optimal pH for slurry stability. The general rule is that the slurry is more stable at a pH that is away from the lEP, so the zeta potential of the particles is greater than 20 mV. The focus of this section is on the influence of pH on the slurry performances such as material removal rate and defectivity. In order to examine the impact of slurry pH on these two important performance features, we first take a closer look at the interaction between abrasive particles and the surface to be polished. There is a vast amount of literature on the interaction between abrasive particles and silicon dioxide surface [26]. The discussion below will focus on the interaction between ceria abrasive particles and the silicon dioxide surface to be polished. The basic principles and conclusions can be easily extended to other pairs of abrasive particles and surfaces. 

Colloidal dispersions owe their stability to a surface charge and the resultant electrical repulsion of charged particles. This charge is acquired by adsorption of cations or anions on the surface. For example, an ionic precipitate placed in pure water will reach solubility equilibrium as determined by its solubility product, but the solid may not have the same attraction for both its ions. Solid silver iodide has greater attraction for iodide than for silver ions, so that the zero point of charge (the isoelectric point) corresponds to a silver ion concentration much greater than iodide, rather than to equal concentrations of the two ions. The isoelectric points of the three silver halides are ° silver chloride, pAg = 4, pCl = 5.7 silver bromide, pAg = 5.4, pBr = 6.9 silver iodide, pAg = 5.5, pi = 10.6. For barium sulfate the isoelectric point seems to be dependent on the source of the product and its de ee of perfection. ... 

The stability of latex is due to a thin layer of proteins on particles, which acts as a colloid stabilizer. Natural rubber is practically obtained by the precipitation and drying of the latex. The precipitation is done with acids (acetic acid is commonly used for this purpose) when the isoelectric point of the protecting protein is reached (pH 4.6). The macromolecules have a MW between 5 10 to 3 10 Dalton and contain between 600 to 50,000 units of isopentene. Due to the double bond, both cis and trans isomers are possible for the monomer units. It was determined that natural rubber is an isotactic polymer formed exclusively from cis units and has the following (idealized) structure (in reality the polymer is not perfectly planar) ... 

The kinetic potential is usually denoted as the zeta (0 potential and it is determined from the electrophoretic mobility of the extremely dilute particles in an electric field. More recently, the nse of electrokinetic sonic amplitude (ESA), acoustosizer (AZR), or colloid (or ultrasonic) vibration potential (CVP) has become available for the determination of the potential in rather concentrated particle suspensions. Again the potential may be measured as a function of either the metal concentration or the pH. In the latter case the point where the mobility ceases is denoted the isoelectric point (pH,Ep Fignre 8.27). It correlates particnlarly well with the stability of the sol. 

The interfacial electric polarizability y, being an important dynamic characteristic of the particle surface charge, can be easily determined from the electro-optical effect dependence on the square of the electric field strength (Eq. 6). A significant increase in the particle dimensions as well as the low surface charge of the colloid-polymer complex complicate the electric polarizability determination near to the system s isoelectric point (Figure 2). The electric polarizabilities are calculated in this review only for polymer covered particles in stabilized suspensions. One way to obtain correct values... 

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...

The caseins exist in milk as polydisperse aggregates ranging in size from ca. 40 to 220nm (3), but the size distribution of micelles depends upon the method of measurement. These casein micelles scatter light and are responsible for the whitish, opaque nature of skim milk. The casein micelles are also associated with a colloidal apatite comprised of calcium-phosphate-citrate (CPC) which has a stabilizing influence on the micelle structure. The colloidal CPC is in equilibrium with soluble CPC in the milk serum phase and is solubilized as the pH is reduced. Thus, as the pH is reduced to the isoelectric point of the caseins (4.6), the colloidal CPC solubilizes, and the caseins precipitate (143). This phenomenon should be kept in mind during some of the following discussions. 

The -potential plays an important role in that it is widely used as a measure of the stability of colloidal suspensions. Suspensions prepared at pH values close to the isoelectric point (lEP) may flocculate fairly rapidly because the repulsion may not be sufficient to overcome the van dw Waals attraction. Farther away from the lEP, we should expect the rate of flocculation to be slower. In practice, for good stability, suspensions are often prepared at pH values comparable to those of the plateau regions of the -potential or electrophoretic mobility curve. For the data shown in Fig. 4.18, this corresponds to pH values of <5 or >7. 

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