Surfactants and Highly Divided Systems

Highly divided systems, in which constituent particles are very small, have a correspondingly large interfacial area. Indeed, if a certain volume V is dispersed in the form of spherical droplets of radius Ri or, in another dispersion radius i 2, the ratio of their respective interfacial areas Si and S2 is 

Interfacial area is thus inversely proportional to the radii of the dispersed objects. Since interfacial tension 7 is a factor limiting dispersion, the interfacial energy is increased correspondingly. The idea then is to introduce a surfactant system that will considerably lower 7, and if possible cancel it altogether. We have already seen that 7 depends on the area occupied by each molecule (cf. Fig. 4.7). Consequently, there is a simple geometrical relation between the dispersed volume 4 per unit volume of solution, the droplet radius i , the required number Ug of surfactant molecules (of molecular area ao) per unit volume and the number of droplets no  

For equal interfacial tensions, the required quantity of surfactant is inversely proportional to particle radius. The area oq coming in to this expression is the area in the interfacial film when the two immiscible liquids, and possibly some cosurfactants, are present. 

The problem of obtaining zero interfacial tension is also one of formulation, i.e., choosing one or more surfactant molecules with the right solubilities relative to the two phases. These must be such as to induce a high degree of adsorption at the interface (rather than a micellar solubilisation in one of the solvents), by creating a high film pressure. Box III, entitled Enhanced Petroleum Recovery, sums up the industrial research which led to discovery of microemulsions. 

An oilfield is generally part of a sedimentary geological formation, made up of porous and permeable rocks, and bounded by impermeable barriers (see Fig. 4.10). The latter prevent oil migration. Fluids (water, oil and possibly gas) are contained inside pores (1-10 gm) in the rock where they are trapped by capillary forces, as shown in Fig. 4.11. For this reason, oil extraction from such a field raises several problems. 

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