Fig. 1.1 - The compounds that lower the surface tension between two phases, like between two liquids, a gas and a liquid, or a liquid and a solid are called surface active agents or surfactants.
In aqueous solutions surfactants behave like organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (their tails) and hydrophilic groups (their heads). The arrangement of the hydrophilic head is at the interface of water and the hydrophobic groups aligns toward oil. Therefore, a surfactant contains both a water-insoluble (or oil-soluble) component and a water-soluble component. Surfactants will diffuse in water and adsorb at interfaces between air and water or at the interface between oil and water, in the case where water is blended with oil. This property allows surfactants to act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.
Many important surfactants include a polyether chain terminating in a polar anionic group. The polyether groups often comprise ethoxylated (polyethylene oxide-like) sequences inserted to increase the hydrophilic character of a surfactant. Polypropylene oxides conversely, may be inserted to increase the lipophilic character of a surfactant. Surfactant molecules have either one tail or two; those with two tails are said to be double-chained. Surfactant classification according to the composition of their head falls under nonionic, anionic, cationic, and amphoteric. A nonionic surfactant has no charged groups in its head. The head of an ionic surfactant carries a net positive, or negative charge. If the charge is negative, the surfactant is more specifically called anionic; if the charge is positive, it is called cationic. If a surfactant contains a head with two oppositely charged groups, it is termed zwitterionic. Zwitterionic or amphoteric surfactants have both cationic and anionic centers attached to the same molecule.
Two major surfactants, are linear alkylbenzene sulfonates (LAS) and the alkyl phenol ethoxylates (APE). Anionic surfactants contain anionic functional groups at their head, such as sulfate, sulfonate, phosphate, and carboxylates. Prominent alkyl sulfates include sodium lauryl sulfate (sodium dodecyl sulfate, SLS, or SDS), and the related alkyl-ether sulfates sodium laureth sulfate (sodium lauryl ether sulfate or SLES), and sodium myreth sulfate.
Nonionic surfactants have covalently bonded oxygen-containing hydrophilic groups, which are bonded to hydrophobic parent structures. The water-solubility of the oxygen groups is the result of hydrogen bonding. Hydrogen bonding decreases with increasing temperature, and the water solubility of nonionic surfactants therefore decreases with increasing temperature. The characteristic feature of cloud point (CP) of nonionic surfactants is the temperature at which the surfactant separates out from an aqueous solution due to the weakening of hydrogen bonds between the surfactant and water molecules. Nonionic surfactants are less sensitive to water hardness than anionic surfactants, and they foam less strongly.
Most anionic and nonionic surfactants are nontoxic, having LD50 comparable to sodium chloride. The toxicity of quaternary ammonium compounds, which are antibacterial and antifungal, varies. Prolonged exposure to surfactants can irritate and damage the skin because surfactants disrupt the lipid membrane that protects skin and other cells. Skin irritancy generally increases in the series nonionic, amphoteric, anionic, cationic surfactants.
Fig. 1.2 - Classification/Types of surface active agents or surfactants.
Surfactants play an important role in personal care products such as cosmetics, shampoos, shower gel, hair conditioners, and toothpastes. The property of providing cleaning, wetting, and dispersing, emulsifying, foaming and anti-foaming effects are used in many practical applications and products. Surfactants with different HLB are used in detergents, fabric softeners, soaps, paints, adhesives, inks, emulsions, anti-fogs, ski waxes, snowboard wax, deinking of recycled papers, in flotation, washing and enzymatic processes. Agrochemical formulations such as some herbicides, insecticides, biocides (sanitizers), and spermicides also contain Surfactants. Surfactants find use in firefighting and pipelines as liquid drag reducing agents. Alkali Surfactant polymers are used to mobilize oil in oil wells for exploration.
