Emulsions and microemulsions
 One of the most important uses of surfactants is to stabilize emulsions.
 Emulsions are dispersions of one liquid into another.
 They are usually prepared by mixing the two liquids under high shear or by sonification.
 The dispersed liquid exists in the form of droplets.
 The size of the droplets may vary from a few microns to several hundred microns.
 The surfactant molecules adsorb at the surface of the droplets.
 When the droplets come very close to one another, a thin liquid film separates them.
 If this film is unstable, it ruptures easily resulting in coalescence of the droplets.
 If coalescence occurs rapidly, the emulsion becomes unstable and the two liquids separate into
clear immiscible phases.
 The role of surfactant is to stabilize the film so that coalescence is prevented.
 The surfactant achieves this by two mechanisms.
 The adsorbed surfactant molecules slow down the hydrodynamic drainage of the film.
 This drainage process is important for the films of viscous liquids.
 The second mechanism by which the surfactant molecules prevent the film from rupture is by
exerting interfacial repulsive force between the droplets.
 The properties of the surfactant govern the nature of this repulsion.
 The ionic surfactants exert electrostatic repulsion whereas the nonionic surfactants exert steric
repulsion.
 Due to this repulsion between the droplets, they cannot approach each other beyond a certain
separation, which varies between 5 and 50 nanometers.
 However, the film can rupture only when the separation between the droplets (i.e., the thickness
of the film) is only a few nanometers.
 The rupture of the thin film is caused by the van der Waals force.
 Therefore, if the surfactant molecules can generate sufficient repulsion between the approaching
droplets, they are prevented from coalescence.
 These phenomena are crucial for the stability of emulsions.
 The properties of the surfactant and its concentration are important parameters for stabilizing
emulsions.
 Sometimes the emulsion needs to be destabilized so that the two liquids are separated.
 Examples are dewatering of crude oil during the removal of salt.
 Certain chemical additives are used to destabilize the ‘tough’ films made of organic compounds
present in petroleum crude (e.g., asphaltenes).
 Surfactant micelles can solubilize organic liquids, but the amount of this solubilization is usually
small.
 Under certain circumstances a large amount of the organic compound can be dissolved in an
aqueous solution of surfactant, or vice versa.
 Such a solution is very transparent and stable. It is known as microemulsion.
 This term was first used by Schulman (1959).
 The interfacial tension between the aqueous and organic phases needs to be reduced to nearzero values (~ 1  10 6 N/m, or less) to form a microemulsion.
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 In addition to the surfactant, a cosurfactant (e.g., an alcohol such as hexanol) is usually added
to prepare the microemulsion.
 At some temperatures, certain nonionic surfactants can form microemulsion without requiring
the cosurfactant.
 A variation in temperature or addition of some compound can destabilize a microemulsion.
 The size of the droplets in microemulsions is very small (~ 1 – 100 nm).
 The microemulsions form spontaneously, just by simple mixing.
 Microemulsions are also viewed as ‘swollen micelles’ and the concept of solubilization of
organic compound by micelles is used to describe them.
 At low interfacial tension, a large swollen micelle can form, which can take up a considerable
amount of organic liquid.
 The solution remains transparent and stable.
 At this stage, one can view the organic liquid either as emulsified in the aqueous phase or
solubilized: both terminologies are used by the scientists.
 The term ‘solubilized oil’ is also used in the literature.
 Microemulsions have several important commercial uses.
 Water-in-oil microemulsions are used in some dry-cleaning processes.
 Many floor polishes, cleaners, personal care products, and pesticide formulations are actually
microemulsions.
 An application of great commercial value is in enhanced recovery of petroleum trapped in
porous sandstones.
Foams
 Foam is a dispersed system in which gas bubbles are separated by a liquid medium.
 If we shake an aqueous surfactant solution in presence of a gas, foam is formed.
 Like the emulsions, foams are stabilized by surfactants.
 Different surfactants stabilize foams in different ways.
 In absence of a surfactant, foams hardly have any stability.
 A combination of salt and ionic surfactant produces stable foams.
 Foams have been of great practical interest because of their widespread occurrence in everyday
life such as food products, detergents, personal-care products, industrial applications, and
hazard management.
 Foams are present in almost every part of petroleum production and refining process.
 In some situations foams are undesirable (e.g., in distillation and fractionation tower, paper
production, industrial water purification, and beverage production).
 Foams are characterized by well-defined interfaces.
 The same factors which stabilize the emulsion droplets (i.e., hydrodynamic drainage of the thin
liquid film, and the interfacial forces) govern the stability of the foams as well.
 The mechanism of collapse of the foams is also similar to the coalescence of droplets in
emulsions.
 When the foaminess is required to be reduced, certain additives are used.
 These are known as antifoams.
 Antifoams prevent the formation of foams.
 Another class of additives, called defoamers, causes the already-formed foams to collapse.
 The presence of oil droplets (e.g., silicone oil) and/or hydrophobic solid particles (e.g.,
hydrophobized silica particles) in the foaming solution can act as antifoam agents.