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Other nonionics are adsorbed at oil-water interfaces. These compounds consist of an oil-soluble (lipophilic) chain of atoms linked to a water soluble (hydrophilic) chain. The lipophilic portion dissolves on the oil side of the interface, and the hydrophilic on the water side. An enormous number of such compounds can be synthesized from polyhydric alcohol anhydrides and polyoxyethylene to suit various applications. Two factors help determine their suitability for particular application: the chemical identity of the two chains and the HLB numberThe latter is defined as the ratio by weight ot the hydrophilic part of the molecule to the lipophilic; the greater the HLB number, the more water soluble is the molecule. Figure 7-6 shows how the molecule is pulled more into the water phase as the length of the polyoxyethylene chains, and consequently the HLB number, increases. Note that the two factors—HLB number and chemical identity—serve only as guides in selecting nonionic surfactants. Final selection must be based on experimental evidence.

Many surfactants perform dual functions, e.g., they may act as an emulsifier and as a wetting agent. Alternatively, blends of compatible surfactants may be used to accomplish several purposes.

Emulsions

The interfacial tension between oil and water is very high, so if the liquids are mixed together mechanically they separate immediately the agitation ceases, to minimize the interfacial area. Lowering the interfacial tension with a surfactant enables one liquid to form a stable dispersion of fine droplets in the other. The lower the interfacial tension, the smaller the droplets and the more stable the emulsion. The interfacial tension between mineral oil and water is about 50 dynes/cm, and a good emulsifier will lower it to about 10 dynes/cm.

In most emulsions, oil is the dispersed phase and water is the continuous phase (see Figure 7-7), but "invert emulsions," in which water is the dispersed phase, can be made with a suitable emulsifier.

Besides lowering interfacial tension, the emulsifier stabilizes the emulsion, because its molecules are adsorbed at the oil/water interfaces, forming a skin around the droplets (see Figure 7-8). This skin acts a physical barrier, preventing the droplets from coalescing when they collide.

Emulsion droplets may carry a small electrostatic charge. The consequent mutual repulsion contributes to the stability of the emulsion, but the charge can only be maintained in low salinity (i.e., low conductivity) water.

The stability of an emulsion increases with increase in viscosity of the continuous phase because the number of collisions between the droplets is decreased. Similarly, the stability decreases with increase in temperature because the number of collisions increases.

partly because the droplets are not uniform in size but also because they are deformable. However, it becomes increasingly difficult to form a stable emulsion as the proportion of the disperse phase is increased above 75%.

Because the creation of new surfaces involves work, some degree of mechanical agitation is required to form an emulsion, but the lower the interacial tension, the less the amount of work required. In certain cases the emulsifier lowers the interfacial tension so much that merely pouring the constituents together provides sufficient agitation to form the emulsion. In other cases, a high input of energy is required. The most important requirement of a mixer is a high shear rate, which may be obtained either by a high speed rotor with a narrow clearance between the blades and the housing, or by forcing the components through a small orifice under high pressure.

Figure 7-7. Two percent oit emulsion mud magnified 900 times. (Courtesy Dresser-Magcobar.)
Figure 7-8. Protective skin of surfactant molecules around oil droplet. (Schematic).

Whether an oil-in-water (O/W) or a water-in-oil (W/O or "invert") emulsion is formed depends on the relative solubility of the emulsifier in the two phases. Thus a preferentially water-soluble surfactant, such as sodium oleate, will form an O/W emulsion because it lowers the surface tension on the water side of the oil-water interface, and the interface curves towards the side with the greater surface tension, thereby forming an oil droplet enclosed by water. On the other hand, calcium and magnesium oleates are soluble in oil, but not in water, and thus form W/O emulsions. Similarly, a nonionic surfactant with a large hydrophilic group (HLB number 10 to 12) will be mostly soluble in the water phase, and thus forms an O/W emulsion, whereas a nonionic surfactant with a large lipophilic group (HLB number about 4) will form a W/O emulsion.6

Typical O/W emulsifiers used in fresh water muds are alkyl aryl sulfontes and sulfates, polyoxyethylene fatty acids, esters and ethers. A poly-oxyethylene sorbitan tall-oil ester,7 sold under various trade names, is used in saline O/W emulsions, and an ethylene oxide derivative of nonylphenol, C9H19C6H40(CH2 — CH2 — O)30H, known as DME, is used in calcium-treated muds.4 Fatty acid soaps, polyamines, amides, or mixtures of these are used for making W/O emulsions.

An O/W emulsion can be broken by adding a small amount of a W/O emulsifier, and vice versa. In either case, the emulsion will be reversed if too much of the contrasting emulsifier is added.

Whether a given emulsion is oil-in-water or water-in-oil can easily be determined by adding some of the emulsion to a beaker of water. Because its continuous phase is water, an O/W emulsion will disperse readily in the water, whereas a W/O emuslion will remain as a separate phase.

Stable emulsions can be formed without the presence of a surfactant by the adsorption of finely-divided solids, such as clays, CMC, starch, and other

Particle mostly Particle half Particle mostly in water in water in oil

Figure 7-9. Idealized diagram showing effect of contact angle on immersion of particle. The most stable emulsion is formed when 0 = 90° (Schematic).

colloidal materials, at the oil-water interfaces. A skin of solid particles is thus for med around the dispersed droplets, which prevents their coalescence. Since the particles do not significantly lower the interfacial tension, they are known as mechanical emulsifiers.

Strongly water-wet particles will remain wholly in the water phase, and strongly oil-wet particles will remain wholly in the oil phase, so that in neither case will they act as mechanical emulsifiers. In order to form stable emulsions, the particles must be somewhere between slightly oil-wet and slightly water-wet, so that they remain partly in each phase, as shown in Figure 7-9. Ideally the most stable emulsion is formed when the contact angle is 90°.

Most aqueous drilling fluids contain the finely-divided particles required to form mechanical emulsions, and the electrochemical conditions are such that the particles are adsorbed at oil-water interfaces. Dispersed clays and various colloidal additives, especially lignosulfonates in alkaline solution,* act as mechanical emulsification agents, so that quite stable O/'W emulsions are formed merely by adding oil and providing sufficient mechanical agitation. As a rule, however, mechanical emulsions are not as stable as chemical ones. When sufficient stability is not achieved, the emulsion may be stabilized by adding small quantities of a suitable chemical emulsifier.

Oil-Wetting Agents

Nitrogen compounds with long hydrocarbon chains are the most frequently used oil-wetting agents. N-alkyl trimethylene diamine chloride is a typical example. The salt ionizes in water to yield a large organic cation and two chloride anions:

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