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Content of Aerosil (%)

Figure 7.34 Influence of the polarity of the medium on the increase in viscosity attainable with Aerosil 200, a coagulated silica sol.

Reproduced from Degussa's Technical Bulletin No. 4 on Aerosil.

Oleogels

Lipophilic ointment bases and nonaqueous suspensions may be thickened with materials such as Aerosil, a coagulated silica sol. Incorporation of the silica into an oil leads to an increase in viscosity, which is brought about by hydrogen bonding between the silica particles. Silica (Aerosil 200) at 8-10% imparts a paste-like consistency to a range of oils such as isopropyl myristate, peanut oil and silicone oil. The degree to which viscosity is increased is a function of the polarity of the oil, the silica being more effective in nonpolar media (see Fig. 7.34). Suspensions of silica in oils are thixotropic; on storage for several days the viscosity increases owing to the slow aggregation of the silica particles, shown schematically in Fig. 7.35.

7.4.6 Adhesion of suspension particles to containers: immersional, spreading and adhesional wetting

When the walls of a container are wetted repeatedly, an adhering layer of suspension particles may build up, and this subsequently

Figure 7.35 Schematic representation of the interaction between two Aerosil particles (top), the formation of a chain structure (centre), and the development of a 'chickenwire' structure as well as thixotropy (bottom). The mean particle diameter of Aerosil is 10 nm.

Reproduced from Degussa's Technical Bulletin No. 4 on Aerosil.

Figure 7.35 Schematic representation of the interaction between two Aerosil particles (top), the formation of a chain structure (centre), and the development of a 'chickenwire' structure as well as thixotropy (bottom). The mean particle diameter of Aerosil is 10 nm.

Reproduced from Degussa's Technical Bulletin No. 4 on Aerosil.

dries to a hard and thick layer. In Fig. 7.36 three types of wetting are shown. Where the suspension is in constant contact with the container wall, immersional wetting occurs, in which particles are pressed up to the wall and may or may not adhere. Above the liquid line, spreading of the suspension during shaking or pouring may also lead to adhesion of the particles contained in the spreading liquid. Adhesional wetting occurs when a liquid drop remains suspended, like a drop of water on a clothes line. Evidently the surface tension of the suspension plays a part in the spreading and wetting processes.

Adhesion increases with increase in suspension concentration, and with the number

Figure 7.36 Adhesion. (a) Three types of wetting giving rise to adhesion of suspension particles. (Modified from H. Uno and S. Tanaka, Kolloid Z. Z. Polym., 250, 238 (1972).) Suspended particles adhering to the surface of a glass vial can be seen in Figure 7.27 on page 255.

of contacts the suspension makes with the surfaces in question.

Additives, especially surfactants, will modify the adhesion of suspension particles. They will act in two ways: (a) by decreasing the surface tension; and (b) by adsorption modifying the forces of interaction between particle and container. The example illustrated in Figs. 7.37 and 7.38 refers to the addition of benze-thonium chloride to chloramphenicol suspensions. Benzethonium chloride converts both the glass surface and the particles into positively charged entities (see Fig. 7.37). Adhesion in the presence of this cationic surfactant is concentration-dependent, the process being akin to flocculation. At low concentrations the surfactant adsorbs by its cationic head to the negative glass and to the suspension particle. The glass is thus made hydrophobic. At higher concentrations hydro-phobic interactions occur between coated particle and surface (Fig. 7.38). Further increase in concentration results in multilayer formation of surfactant, rendering the surfaces hydro-philic. In this condition the particles repel, reducing adhesion.

Miscellaneous colloidal systems

Several colloidal systems have not been mentioned in this chapter because they are dealt with elsewhere. These include nanoparticle suspensions used in drug delivery and targeting, and vesicular dispersions (liposomes,

Chloramphenicol

Chloramphenicol

Glass (powder)

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