Colloid surface charge, stabilization

A separate class of materials, known as protective colloids, exerts a stabilizing influence by acting as a bridge between the continuous phase and the particles which they envelop. In many instances the optimum stabilizing effect is achieved when the protective colloids are used in conjunction with a compatible interfacial tension depressant. The protective colloid must have an affinity for the continuous phase. When stabilization occurs through protective colloidal action, the particles lose their surface property identities in respect to charge, agglomeration, etc., and assume the properties of the protective colloid. 

The sorbent materials are supplied as finely dispersed colloidal particles, whose surfaces are smooth. Some of their properties are presented in Table 3. The sorbents cover different combinations of hydrophobicity and sign of the surface charge. Thus, the model systems presented allow systematic investigation of the influences of hydrophobicity, electric charge, and protein structural stability on protein adsorption. 

But time is money. The waste industry, therefore, breaks the colloid artificially to remove the particulate solid from the water. They employ one of two methods. Firstly, they add to the water an inorganic polymer such as silicate. The colloid s thermodynamic stability depends on the surface of its particles, each of which has a slight excess charge. As like charges repel (in consequence of Coulomb s law ... 

As we shall see colloid stability can be affected by electrolytes and by adsorbates that affect the surface charge of the colloids and by polymers that can affect particle interaction by forming bridges between them, or by sterically stabilizing them. 

The surface charge of metal oxides (due to surface protonation) as a function of pH can be predicted if their pHpzc are known with the help of the relationship given in Fig. 3.4. Fig. 7.6 exemplifies the effect of various solutes on the colloid stability of hematite at pH around 6.5 (pH = 10.5 for Ca2+ and Na+) (Liang and Morgan, 1990). 

The use of surface charge to provide colloid stability to particles dispersed in dilute electrolytes in aqueous solution, or even in media of intermediate polarity, is an effective means of stabilising particles against van der Waals forces of attraction. Figure 3.16 shows typical potential... 

The choice of carrier liquid is primarily based on the suspension stability for colloids or solvent goodness for macromolecule solutions. Moreover, surface-charged colloidal particles are also sensitive to ionic strength and addition of surfactants. 

Second, nucleation and growth of Stober silica particles is modeled by a controlled aggregation mechanism of subparticles, a few nanometers in size, as for example presented by Bogush and Zukoski (19). Colloidal stability, nuclei size, surface charge, and diffusion and aggregation characteristics are the important parameters in this model. 

Electrostatic and electrical double-layer forces play a very important role in a number of contexts in science and engineering. As we see in Chapter 13, the stability of a wide variety of colloids, ranging from food colloids, pharmaceutical dispersions, and paints, to colloidal contaminants in wastewater, is affected by surface charges on the particles. The filtration efficiency of submicron particles can be diminished considerably by electrical double-layer forces. As we point out in Chapter 13, coagulants are added to neutralize the electrostatic effects, to promote aggregation, and to enhance the ease of separation. 

The results in Table 13.1 have been collected for colloids bearing both positive and negative surface charges. One of the earliest (1900) generalizations about the effect of added electrolyte is a result known as the Schulze-Hardy rule. This rule states that it is the valence of the ion of opposite charge to the colloid that has the principal effect on the stability of the colloid. The CCC value for a particular electrolyte is essentially determined by the valence of the counterion regardless of the nature of the ion with the same charge as the surface. The numbers listed in parentheses in Table 13.1 are the CCC values in moles per liter for counterions of the... 

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