Surfactants
Authors: Tadros Tharwat 
Published: July 2006

Surface active agents (usually referred to as surfactants) are amphipathic molecules consisting of a nonpolar hydrophobic portion, usually a straight or branched hydrocarbon or fluorocarbon chain containing 8–18 carbon atoms, which is attached to a polar or ionic portion (hydrophilic). The hydrophilic portion can, therefore, be nonionic, ionic or zwitterionic, accompanied by counter ions in the last two cases. The hydrocarbon chain interacts weakly with the water molecules in an aqueous environment, whereas the polar or ionic head group interacts strongly with water molecules via dipole or ion‐dipole interactions. It is this strong interaction with the water molecules that renders the surfactant soluble in water. However, the cooperative action of dispersion and hydrogen bonding between the water molecules tends to squeeze the hydrocarbon chain out of the water and hence these chains are referred to as hydrophobic. The balance between hydrophobic and hydrophilic part of the molecule gives these systems their special properties, eg, accumulation at various interfaces and association in solution (to form micelles).

The driving force for surfactant adsorption is the lowering of the free energy of the phase boundary. The interfacial free energy per unit area is the amount of work required to expand the interface. This interfacial free energy, referred to as surface or interfacial tension, γ, is given in mJm
−2
or mNm
−1
. Adsorption of surfactant molecules at the interface lowers γ and the higher the surfactant adsorption (ie, the more dense the layer is) the larger the reduction in γ. The degree of surfactant adsorption at the interface depends on surfactant structure and the nature of the two phases that meet the interface.

Surface active agents also aggregate in solution forming micelles. The driving force micelle formation (or micellization) is the reduction of contact between the hydrocarbon chain and water, thereby reducing the free energy of the system. In the micelle, the surfactant hydrophobic groups are directed towards the interior of the aggregate and the polar head groups are directed towards the solvent. These micelles are in dynamic equilibrium and the rate of exchange between a surfactant molecule and the micelle may vary by orders of magnitude, depending on the structure of the surfactant molecule.

Surfactants find application in almost every chemical industry of which the following may be worth mentioning: detergents, paints, dyestuffs, cosmetics, pharmaceuticals, agrochemicals, fibers, plastics, etc. Moreover, surfactants play a major role in the oil industry, for example, in enhanced and tertiary oil recovery. They are also occasionally used for environmental protection, eg, in oil slick dispersants. Therefore, a fundamental understanding of the physical chemistry of surface active agents, their unusual properties and their phase behavior is essential for most industrial chemists. In addition, an understanding of the basic phenomena involved in the application of surfactants, such as in the preparation of emulsions and suspensions and their subsequent stabilization, in microemulsions, in wetting spreading and adhesion, etc, is of vital importance in arriving at the right composition and control of the system involved. This is particularly the case with many formulations in the chemical industry mentioned above.

It should be stated that commercially produced surfactants are not pure chemicals, and within each chemical type there can be tremendous variation. This can be understood, since surfactants are prepared from various feedstocks, namely petrochemicals, natural vegetable oils and natural animal fats. It is important to realize that in every case the hydrophobic group exists as a mixture of chains of different lengths. Products that may be given the same generic name could vary a great deal in their properties and the formulation chemist should bear this in mind when choosing a surfactant from a particular manufacturer.