Comminution or Dispersion Methods

While the presence of the liquid normally facilitates the comminution process, the resulting dispersion still may not exhibit the stability necessary to make the process viable. That is, the dispersed particles may begin to flocculate or coagulate rapidly once the comminution process is halted (Fig. 10.2). The solution for the second problem is normally the addition of new components (surfactant, polymer, small particles, etc.) that adsorb at the solid-liquid interface and provide an electrostatic or steric barrier that retards or prevents sticky collisions between particles, thereby making the dispersion more stable. In the case of particulate colloids such additives are termed dispersing aids or agents. The mechanisms of action of such materials will be discussed in more detail in the context of specific colloidal phenomena. 

The comminution of liquid phases is a special case of the above and is generally referred to as emulsification. Because of the nature of liquid systems, emulsification has a number of additional variations not generally available for the formation of dispersions. These include spontaneous emulsification, electroemulsification, and spontaneous microemulsion formation. 

Some important processes for the formation of sols involve first the formation of an emulsion or a liquid aerosol. In suspension or dispersion polymerization, a monomer or monomer mixture is emulsified to a drop size approximately the same as that of the final desired particle. Polymerization is then initiated using an initiator soluble in the monomer, so that chain growth occurs within each individual drop. The result (with luck) is a dispersion of polymer particles with the same average size as the original monomer emulsion. Normally, some type of stabilizer system is employed in the emulsification stage—a surfactant or very small particles of some material such as silica. 

Another process involves the formation of a liquid aerosol or mist of the precursor to the solid. This is sprayed into a vapor atmosphere containing a 

FIGURE 10.2. In the preparation of colloids by comminution, it is usually necessary to include a surfactant or polymeric stabilizer to inhibit the rapid flocculation of the newly formed particles. 

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