Surfactants dispersing action

Watkinson [1988] lists some surfactants used for their dispersing action in organic liquids. He includes amongst them organic and metal sulphonates metal phenolates metal dialkyl dithiophosphates sodium dialkyl sulphosuccinates polyoxyethylene alkyl and alicyclic amines monethanol ammonium phosphate salts co-polymers of N-substituted formamide fatty acid phosphates... 

Along with the classification by chemical nature, one can also classify surfactants with respect to the mechanism of their behavior at interfaces. Two main mechanisms of surfactant action at S/L interfaces are of importance in their relevance to contact interactions between particles that we will address broadly throughout this book. These are (1) the control of the wetting of a solid surface by a liquid (see Figures 2.10 and 2.11) and (2) the dispersion action, which facilitates the fracture of solids, which we will discuss in detail in the second part of this book. 

The selection of an optimum surfactant, or combination of surfactants, was necessary primarily to prevent re-agglomeration of the dispersed sample while settling. In addition, it was demonstrated that the use of a blender (Waring) was an effective aid in dispersing these two components. The violent action of the blender did not cause a change in the concn of coarse particles in the dispersion with increasing blending times up to 17 minutes... 

In the detergency process, fatty materials (i.e. dirt, often from human skin) are removed from surfaces, such as cloth fibres, and dispersed in water. It is the surfactants in a detergent which produce this effect. Adsorption of the surfactant both on the fibre (or surface) and on the grease itself increases the contact angle of the latter as illustrated in Figure 4.7. The grease or oil droplet is then easily detached by mechanical action and the surfactant adsorbed around the surface of the droplet stabilises it in solution. 

Foams may contain not just gas and liquid (and usually surfactant), but also dispersed oil droplets and/or solid particles. Figure 1.5 shows a practical aqueous foam that contains dispersed oil droplets within the foam lamellae. This can occur, for example, when a foaming solution is used for detergent action in a cleaning process (see Section 12.2) or when a foam is propagated through an underground oil reservoir as part of an enhanced oil recovery process (See Section 11.2.2). 

Properly compounded PTFE dispersions are suitable for impregnation because of their low viscosity, very small particles, and ability to wet the surfaces. The surfactant aids the capillary action and wetting interstices in a porous material. After the substrate is dipped and dried, it may or may not be sintered. This depends on the intended application. In fact, the unsintered coating exhibits sufficiently high chemical resistance and antistick property. If required, the coated substrate may be heated to about 290°C (555°F) for several minutes to remove the surfactant. Lower temperatures and longer times are used if the substrate cannot tolerate such a high temperature. In some cases, the impregnated material is calendered or compressed in a mold to compact the PTFE resin and to hold it in place. 

The stabilizing function of macromolecular surfactants in solid-liquid systems is exercised through protective colloid action. To be effective, they must have a strong solution affinity for hydrophobic and hydrophilic entities. In liquid-liquid systems, surfactants are more accurately called emulsifiers. The same stabilizing function is exercised in gas-liquid disperse systems where the surfactants are called foam stabilizers. 

The kinetic factors of foam stability as well as of the stability of any disperse system, are mainly determined by the stabilising ability of the surfactant adsorption layers. This action is spread over all structural elements of the foam. 

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