Emulsifying agents interfacial tension

It is quite clear, first of all, that since emulsions present a large interfacial area, any reduction in interfacial tension must reduce the driving force toward coalescence and should promote stability. We have here, then, a simple thermodynamic basis for the role of emulsifying agents. Harkins [17] mentions, as an example, the case of the system paraffin oil-water. With pure liquids, the inter-facial tension was 41 dyn/cm, and this was reduced to 31 dyn/cm on making the aqueous phase 0.00 IM in oleic acid, under which conditions a reasonably stable emulsion could be formed. On neutralization by 0.001 M sodium hydroxide, the interfacial tension fell to 7.2 dyn/cm, and if also made O.OOIM in sodium chloride, it became less than 0.01 dyn/cm. With olive oil in place of the paraffin oil, the final interfacial tension was 0.002 dyn/cm. These last systems emulsified spontaneously—that is, on combining the oil and water phases, no agitation was needed for emulsification to occur. 

One may rationalize emulsion type in terms of interfacial tensions. Bancroft [20] and later Clowes [21] proposed that the interfacial film of emulsion-stabilizing surfactant be regarded as duplex in nature, so that an inner and an outer interfacial tension could be discussed. On this basis, the type of emulsion formed (W/O vs. O/W) should be such that the inner surface is the one of higher surface tension. Thus sodium and other alkali metal soaps tend to stabilize O/W emulsions, and the explanation would be that, being more water- than oil-soluble, the film-water interfacial tension should be lower than the film-oil one. Conversely, with the relatively more oil-soluble metal soaps, the reverse should be true, and they should stabilize W/O emulsions, as in fact they do. An alternative statement, known as Bancroft s rule, is that the external phase will be that in which the emulsifying agent is the more soluble [20]. A related approach is discussed in Section XIV-5. 

Larch gum is readily soluble in water. The viscosity of these solutions is lower than that of most other natural gums and solutions of over 40% soHds are easily prepared. These highly concentrated solutions are also unusual because of their Newtonian flow properties. Larch gum reduces the surface tension of water solutions and the interfacial tension existing in water and oil mixtures, and thus is an effective emulsifying agent. As a result of these properties, larch gum has been used in foods and can serve as a gum arabic substitute. 

This block copolymer acts as an emulsifying agent in the blends leading to a reduction in interfacial tension and improved adhesion. At concentrations higher than the critical value, the copolymer forms micelles in the continuous phase and thereby increases the domain size of the dispersed phase. 

Thus, in the relatively simple case of oil in water emulsions, where a surface active agent such as a soap is used as the emulsifying agent, it is known that the soap adsorbed on the surface of the oil particles decreases the interfacial tension, thus stabilizing the emulsion. The adsorbed soap ions also give a net electrostatic charge to the dispersed oil droplets, serving to repel other oil droplets, with the net effect that flocculation is hindered (and stability is increased). It is even possible to measure the amount of adsorbed soap ions and to calculate the values of the surface potential. 

The selection of a suitable emulsifying agent and its appropriate concentration are matters of experience and of trial and error. It is not necessary to use emulsifier amounts above the required quantities to produce complete interfacial films, unless an increase in the viscosity of the dispersion medium is intended. Reducing the interfacial tension makes emulsification easy but does not by itself prevent coalescence of the particles and resultant phase separation. Frequently, combinations of two or more emulsifying agents are used [2] to (a) adequately reduce the interfacial tension, (b) produce a sufficiently rigid interfacial film, and (c)... 

Lipids are insoluble in water and an interfacial tension therefore exists between the phases when lipids are dispersed (emulsified) in water (or vice versa). This tension in toto is very large, considering the very large interfacial area in a typical emulsion (section 3.7). Owing to the interfacial tension, the oil and water phases would quickly coalesce and separate. However, coalescence (but not creaming) is prevented by the use of emulsifiers (surface active agents) which form a film around each fat globule (or each water... 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

The energy plotted in Figure 3.3 was obtained by multiplying the total area by the interfacial tension. Suppose now that a small quantity of a surfactant was added to the water, possibly a few tenths of a percent, that lowered the interfacial tension to 0.35 mN/m. This would lower the amount of mechanical energy needed in the above example by a factor of 100. From the area per molecule that the adsorbed emulsifying agent occupies, the minimum amount of emulsifier needed for the emulsion can also be estimated. 

As material parameters, the densities and the viscosities of both phases as well as the interfacial tension o must be listed. We incorporate the material parameters of the disperse phase p,i and pa in the relevance list and note separately the material numbers p/pd and p/pd. Additional (dimensionless) material parameters are the volume ratio of both phases, emulsifying agent (surfactant) [e.g. given in ppm]. 

Surfactant Substance that adsorbs to surfaces or interfaces to reduce surface or interfacial tension may be used as wetting agent, detergent, or emulsifying agents Benzalkonium chloride, nonoxynol 10, oxtoxynol 9, polysorbate 80, sodium lauryl sulfate, sorbitan monopalmitate... 

Surface-active agents. Surface-active agents such as emulsifiers and surfactants play a very significant role in the stability of emulsions. They greatly extend the time of coalescence, and thus they stabilize the emulsions. Mechanisms by which the surface-active agents stabilize the emulsion are discussed in detail by Becher (19) and Coskuner 14). They form mechanically strong films at the oil-water interface that act as barriers to coalescence. The emulsion droplets are sterically stabilized by the asphaltene and resin fractions of the crude oil, and these can reduce interfacial tension in some systems even at very low concentrations (i7, 20). In situ emulsifiers are formed from the asphaltic and resinous materials found in crude oils combined with ions in the brine and insoluble dispersed fines that exist in the oil-brine system. Certain oil-soluble organic acids such as naphthenic, fatty, and aromatic acids contribute to emulsification 21). 

In the formation of an emulsion, one of the two immiscible liquids is broken up into droplets which are dispersed in the other liquid. The dispersion of one liquid in another immiscible liquid leads to a large increase in interfacial free energy because of the increase in the area of the interface. The emulsifying agent stabilises the emulsion by adsorbing at the liquid-liquid interface as an oriented interfacial film. This film reduces the interfacial tension between the liquids and also decreases the rate of coalescence of the dispersed droplets by forming mechanical, steric and/or electrical barriers arormd them. 

Surface-active agent. (surfactant). Any compound that reduces surface tension when dissolved in water or water solutions, or that reduces interfacial tension between two liquids, or between a liquid and a solid. There are three categories of surface-active agents detergents, wetting agents, and emulsifiers all use the same basic chemical mechanism and differ chiefly in the nature of the surfaces involved. 

The application of surfactants or emulsifying agents may decrease interfacial tension and assist solubilization of sorbed HOCs from soils, thereby making the hydrophobic compounds more available for microbial degradation (22, 23). However, the experimental observations of the effects of surfactant addition on microbial degradation of HOCs are not always consistent, nor has a general explanation been advanced for their influence. Some of the varied observations of surfactant effects on biodegradation are summarized in Table I (24-37). 

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