Entry by Haifei Zhang, AP 225, Fall 2009
What is surfactant
Surfactant is a combination of three words: Surface Active Agents. Surfactants are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids. Surfactants are usually organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (their "tails") and hydrophilic groups (their "heads"). Therefore, they are soluble in both organic solvents and water.
As shown in the figure on right, the polar "heads" of the micelle, due to favourable interactions with water, form a hydrophilic outer layer that in effect protects the hydrophobic core of the micelle. The compounds that make up a micelle are typically amphiphilic in nature, meaning that not only are micelles soluble in protic solvents such as water but also in aprotic solvents as a reverse micelle
Surfactants reduce the surface tension of water by adsorbing at the liquid-gas interface. They also reduce the interfacial tension between oil and water by adsorbing at the liquid-liquid interface. Many surfactants can also assemble in the bulk solution into aggregates. Examples of such aggregates are vesicles and micelles. The concentration at which surfactants begin to form micelles is known as the critical micelle concentration or CMC. When micelles form in water, their tails form a core that can encapsulate an oil droplet, and their (ionic/polar) heads form an outer shell that maintains favorable contact with water. When surfactants assemble in oil, the aggregate is referred to as a reverse micelle. In a reverse micelle, the heads are in the core and the tails maintain favorable contact with oil. Surfactants are also often classified into four primary groups; anionic, cationic, non-ionic, and zwitterionic (dual charge). Thermodynamics of the surfactant systems are of great importance, theoretically and practically. This is because surfactant systems represent systems between ordered and disordered states of matter. Surfactant solutions may contain an ordered phase (micelles) and a disordered phase (free surfactant molecules and/or ions in the solution). Ordinary washing up (dishwashing) detergent, for example, will promote water penetration in soil, but the effect would only last a few days (many standard laundry detergent powders contain levels of chemicals such as sodium and boron, which can be damaging to plants and should not be applied to soils). Commercial soil wetting agents will continue to work for a considerable period, but they will eventually be degraded by soil micro-organisms. Some can, however, interfere with the life-cycles of some aquatic organisms, so care should be taken to prevent run-off of these products into streams, and excess product should not be washed down.
Surfactants play an important role in many practical applications and products, including:
- Fabric softener
- Emulsifiers and Emulsions
- Soil remediation
- Ski wax, snowboard wax
- Deinking of recycled paper, both in flotation, washing and enzymatic processes
- Foaming agents
- Agrochemical formulations
- Herbicides some
- Quantum dot coating
- Biocides (sanitizers)
- Hair conditioners (after shampoo)
- Spermicide (nonoxynol-9)
- Pipeline, Liquid drag reducing agent
- Alkali Surfactant Polymers (used to mobilize oil in oil wells)
- Leak Detectors
A particular type of molecular structure performs as a surfactant. This molecule is made up of a water soluble (hydrophilic) and a water insoluble (hydrophobic) component. The hydrophobe is usually the equivalent of an 8 to 18 carbon hydrocarbon, and can be aliphatic, aromatic, or a mixture of both. The sources of hydrophobes are normally natural fats and oils, petroleum fractions, relatively short synthetic polymers, or relatively high molecular weight synthetic alcohols. The hydrophilic groups give the primary classification to surfactants, and are anionic, cationic and nonionic in nature. The anionic hydrophiles are the carboxylates (soaps), sulphates, sulphonates and phosphates. The cationic hydrophiles are some form of an amine product. The nonionic hydrophiles associate with water at the ether oxygens of a polyethylene glycol chain. In each case, the hydrophilic end of the surfactant is strongly attracted to the water molecules and the force of attraction between the hydrophobe and water is only slight. As a result, the surfactant molecules align themselves at the surface and internally so that the hydrophile end is toward the water and the hydrophobe is squeezed away from the water.
Because of this characteristic behaviour of surfactants to orient at surfaces and to form micelles, all surfactants perform certain basic functions. However, each surfactant excels in certain functions and has others in which it is deficient.
Foaming agents, emulsifiers, and dispersants are surfactants which suspend respectively, a gas, an immiscible liquid, or a solid in water or some other liquid. Although there is similarity in these functions, in practice the surfactants required to perform these functions differ widely. In emulsification, as an example - the selection of surfactant or surfactant system will depend on the materials to be used and the properties desired in the end product. An emulsion can be either oil droplets suspended in water, an oil in water (O/W) emulsion, water suspended in a continuous oil phase, water in oil (W/O) emulsion, or a mixed emulsion. Selection of surfactants, orders of addition and relative amounts of the two phases determine the class of emulsion.
Each of these three functions is related to the surfactant absorbing at a surface, either gas, liquid or solid with the hydrophilic ends of the molecules oriented to the water phase. The surfactants form what amounts to a protective coating around the suspended material, and these hydrophilic ends associate with the neighbouring water molecules. In addition to surfactant effects the stability of these suspensions is related to the particle size and density of the suspended material.
Solubilisation is a function closely related to emulsification. As the size of the emulsified droplet becomes smaller, a condition is reached where this droplet and the surfactant micelle are the same size.
At this stage, an oil droplet can be imagined as being in solution in the hydrophobic tails of the surfactant and the term solubilisation is used. Emulsions are milky in appearance and solubilised oils, for example - are clear to the eye.
The function of detergency or cleaning is a complex combination of all the previous functions. The surface to be cleaned and the soil to be removed must initially be wet and the soils suspended, solubilised, dissolved or separated in some way so that the soil will not just re-deposit on the surface in question