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Types of Surfactants, their Synthesis, and Applications

Definition of a Surfactant

Surfactant is a widely used contraction for surface active agent, which literally means active at a surface. The term surface active means that the surfactant reduces the free energy of surfaces and interfaces. Expressed differently, they reduce the surface and the interfacial tensions. This is not an unique quality, however. Most water-soluble organic compounds give a reduction of the surface and interfacial tensions when added to an aqueous solution but the effect is normally much less pronounced than for surfactants. The unique behavior of a surfactant is that it self-assembles at interfaces and forms tightly packed structures: monolayers at the air-water and the oil-water interface, and monolayers and aggregates at the solid-water interface. Such self-assembled layers drastically change the character of the interface. Surfactant self-assembly at the air-water interface, commonly referred to as the "surface," is dealt with in Chapter 12; the assembling at the oil-water interface, which is key to formation of emulsions, is treated in Chapter 24; and assembly at the solid-water interface, adsorption, is described in depth in Chapter 8. Surfactants also self-assemble in water, usually forming micelles at very low concentration and other aggregates, called surfactant liquid crystals, at higher concentration. These are treated in Chapters 4 and 6, respectively.

The term surfactant is usually associated with relatively low molecular weight substances. The molecular weight is typically below 500?Da but may be larger for nonionic surfactants with long polyoxyethylene tails. There also exist polymeric surface active agents and these may be called polymeric surfactants. However, more often they are referred to as surface active polymers and that terminology is used in this book. Several chapters deal with surface active polymers.

Surfactants are amphiphilic molecules. The word has a Greek origin with amphi meaning "both" and phil meaning "like"; that is, surfactants are molecules that like both a polar and a nonpolar environment. This is due to their structure. All surfactants have at least one polar headgroup that wants to be in water and at least one hydrophobic tail that prefers to be in an apolar environment; hence, the tendency to go to interfaces. Figure 1.1 shows the structure of a surfactant with one polar head group and one hydrophobic tail.

Figure 1.1 Schematic illustration of a surfactant

A surfactant may be viewed as a molecule consisting of a lyophilic and a lyophobic part. The lyophilic moiety is soluble in a specific fluid whereas the lyophobic moiety in insoluble in this fluid. When the fluid is water, which is usually the case, the terms hydrophilic and hydrophobic parts are normally used.

The term amphiphile or amphiphilic compound is sometimes used in the same sense as the word surfactant. Amphiphilic compounds are also very common in nature. All biological systems contain surface active substances. However, these molecules are usually called polar lipids rather than surfactants. Thus, implicit in the name is that a surfactant is a man-made compound in some sense, although the molecule may have a natural origin, as is discussed later in this chapter.

The hydrophobic tail is almost always based on one or more carbon chains and the chains may be linear, branched, or cyclic. The only exception to carbon chains as hydrophobic backbone is siloxane chains. There exist both high molecular weight and low molecular weight amphiphilic compounds based on the -Si-O- unit; these are discussed in Chapters 10 and 20, respectively.

Surfactants Adsorb at Interfaces

The term interface denotes a boundary between any two immiscible phases; the term surface indicates that one of the phases is a gas, usually air. Altogether five different interfaces exist:

1. Solid-vapor surface
2. Solid-liquid
3. Solid-solid
4. Liquid-vapor surface
5. Liquid-liquid

The driving force for a surfactant to adsorb at an interface is to lower the free energy of that phase boundary. The interfacial free energy per unit area represents the amount of work required to expand the interface. The term interfacial tension is often used instead of interfacial free energy per unit area. Thus, the surface tension of water is equivalent to the interfacial free energy per unit area of the boundary between water and the air above it. When that boundary is covered by surfactant molecules, the surface tension (or the amount of work required to expand the interface) is reduced. The denser the surfactant packing at the interface, the larger is the reduction in surface tension.

This book is concerned with events at interfaces that involve at least one liquid phase, which means three out of the five interfaces listed above. The liquid is usually, but not always, water. Phenomena occurring at these three interfaces, as well as in the bulk liquid, may be referred to as wet surface chemistry. Dry surface chemistry is also very important-heterogeneous catalysis is a prime example-but that science is not discussed here. Examples of the different "wet" interfaces and of products in which these interfaces are important are given in Table 1.1.

Table 1.1 Examples of interfaces involving at least one liquid phase

The systems indicated in Table 1.1 are all examples of dispersions, that is, systems with one phase, called the dispersed phase, finely distributed in another phase, the continuous phase. In many formulated products several types of systems are present at the same time. Water-based paints and paper coating formulations are examples of familiar but, from a colloidal point of view, very complicated systems, containing both solid-liquid (dispersed pigment particles) and liquid-liquid (latex or other binder droplets) interfaces. In addition, foam formation is a common (but unwanted) phenomenon at the application stage. All the interfaces are stabilized by surfactants.

As mentioned above, the tendency to accumulate at interfaces is a fundamental property of a surfactant. In principle, the stronger the tendency, the better is the surfactant. The tendency for a surfactant to accumulate at a boundary depends on the surfactant structure and also on the nature of the two phases that meet at the interface. Therefore, there is no universally good surfactant, suitable for all uses. The choice will depend on the application. A good surfactant should have low solubility in the bulk phases. Some surfactants (and several surface active macromolecules) are only soluble at the oil-water interface. Such compounds are difficult to handle but are very efficient in reducing the interfacial tension.

There is, of course, a limit to the surface and interfacial tension lowering effect by the surfactant. Usually that limit is reached when micelles start to form in the bulk solution. Table 1.2 illustrates what a good surfactant can do in terms of reducing surface and interfacial tensions. The values given are typical of what is attained by normal light-duty liquid detergents. With special formulations, so-called ultra-low interfacial tension, that is, values in the range of 10-3?mN/m or below, can be obtained. An example of a system giving ultra-low interfacial tension is a three-phase system comprising a microemulsion in equilibrium with excess water and oil phases. Such systems are of interest in oil recovery and also in detergency; these are discussed in Chapter 25.

Table 1.2 Typical values of surface and interfacial tensions at room temperature (mN/m)

Surfactants Aggregate in Solution and at Interfaces

As discussed, one characteristic feature of surfactants is their tendency to adsorb at interfaces. Another fundamental property of surface active agents is that unimers in solution tend to form aggregates, so-called micelles. (The free or unassociated surfactant is referred to in the literature either as monomer or unimer. In this text unimer is used and the term monomer is restricted to...