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The Physics of Cleaning, Part 7: Surfactants
By Ed Kanegsberg, BFK Solutions LLC
Oil and water don’t mix. This is not news. In the 1700s, Ben Franklin observed that a small drop of oil deposited in a pond would spread rapidly to make a smooth surface (Gray, 2009). So how can we get oily substances to dissolve in water?
In the last installment of this series (The Physics of Cleaning, Part 6), I discussed how water and oils don’t mix. It is because the water-to-water and oil-to-oil molecular attractive forces are stronger than water–to-oil forces. However, we know that aqueous processes are used for cleaning including personal hygiene, dishwashing and complex critical cleaning in industrial processes.
One way to mix oil and water is to introduce an intermediary, a molecule that is attracted to both. A surfactant (derived from surface active agent) is such a molecule. One end of the molecule is attracted to water (hydrophilic); the other end is repelled by water (hydrophobic) but is attracted to oil. With the addition of surfactants, oil droplets break up and are dispersed in water. The oil is not truly dissolving in water. A structure, still at the molecular scale, consisting of water+surfactant+oil is dispersed within the oil and water system.
In the beginning
When surfactants are first added to water, they are monomers, single molecules (Figure 1), sometimes referred to as amphiphiles. The energy (here’s the physics part) associated with the attraction of the hydrophilic ends to water causes the monomers to congregate at the surface, the “sur” in surfactant, where the hydrophobic ends face the air interface and the hydrophilic ends face the water bath below. This has the effect of reducing the surface energy and surface tension which results in letting the water more easily wet a surface or penetrate into small spaces.
Figure 1. Surfactant amphiphile (From Quitmeyer, 2010)
The plot thickens
As the amount of surfactant in the bath increases, a point is reached when any additional surface concentration will not lower the surface energy. Above this concentration, the Critical Micelle Concentration, CMC, the way the surfactant molecules minimize their energy is to gather together to form intra-liquid cells called micelles, with the hydrophilic ends facing outward and the hydrophobic ends inward (Figure 2).
So what does this have to do with cleaning?
If oil or other non-polar substances are present, surfactant molecules in a micelle sort of gang up on the oil, squeezing it out of the water into the center where it is trapped. This then is the macro structure that, by separating the oil from the water, allows oil to be dispersed in water (as opposed to lurking on the part you are trying to clean).
Figure 2. Surfactant Micelle (by Emmanuel Boutet, from Wikipedia, at http://en.wikipedia.org/wiki/Micelle)
Surfactant Classification
Surfactants are classified by the electronic charge in the hydrophilic “head”. The four common classifications are nonionic, anionic, cationic, and amphoteric.
Non-ionic surfactants have no charged groups. They still have some polar properties and can form hydrogen bonds. The hydrogen bonds provide hydrophilic characteristics. (See May the Forces Be With You, The Physics of Cleaning, Part 2). Examples of non-ionic surfactants are glycerides and polysorbates (frequently used as oil emulsifiers in food). The electrical neutrality of non-ionic surfactants is important in that they impart a lower sensitivity to electrolytes in the chemical system. In terms of cleaning this means that if there are salts, metal ions etc. in the system, non-ionic surfactants may assist in providing a longer bath life.
Anionic surfactants carry a negative charge. Soaps and most fatty acids are anionic. They are found in basic solutions, but also in some neutral solutions. Historically, they were used early on and they continue to be useful. Formulators often choose them because they work well; they clean effectively. Certain ones may also be selected for their environmental attributes, such as low aquatic toxicity.
Cationic surfactants carry a positive charge. Examples are Pyridines and Quaternary ammonium compounds. The latter are used in many anti-bacterial disinfectant cleaning agents. They are also used in corrosion inhibitors.
Amphoteric surfactants carry a charge that varies depending on the pH of the solution. At low pH (acidic conditions), they act as cationic surfactants while at high pH (basic), they act as anionic surfactants. When both charge groups are permanent, the surfactants are sometimes also called Zwitterionic. Lecithin, a naturally occurring fatty substance from plants and animals (for example, to reduce surface tension in our lungs), is an amphoteric surfactant. Lecithin is used as a component of anti-stick sprays for cooking surfaces. In cleaning solutions, amphoteric surfactants tend to have antibacterial properties. They tend to be high foaming.
Surfactant structure is complex and varied; and surfactants are far from the only class of additives to aqueous cleaning agent. In fact, for some applications, surfactant-free cleaning agents may be preferable. The next installment in this series will discuss additional chemicals to aid in aqueous cleaning processes.
References
D. Gray, Organic and Biomaterials Chemistry - Lecture 21 [Internet]. Version 4. Knol. 2009 Dec 9. Available from: http://knol.google.com/k/david-gray/organic-and-biomaterials-chemistry/2cobdudsrjajv/50.
J. Quitmeyer, “Cleaning Agent Chemistry”, Handbook for Critical Cleaning, Second Edition, B. Kanegsberg and E. Kanegsberg, eds., CRC Press, in preparation—publication expected 2010.
