The Journal of Topical Formulations
(ISSN 1554-0197)
published by Scribionics Katvah

Rosemarie L. Coste, General Editor
Elishalom Yechiel, Ph.D., Scientific Editor

An Online Journal on
Topicals:

- Formulation
- Processing
- Intra-Dermal and
Trans-Dermal Vehicles
- Nano-Encapsulation
- Nano-Emulsion
- Topical Medicinals
- OTC
- Skincare
- Naturals
- Cosmeceuticals
- NanoBioTech

Feature Article: Detergents and Surfactants in Soaps, Shampoos, and Other Cleansers
by Elishalom Yechiel, Ph.D.

Detergents and surfactants are sometimes treated as synonyms, in scientific literature and elsewhere; however, there is an important distinction between them. While detergents are also surfactants, not every surfactant can act as a detergent. For example, monoglycerides are surfactants but their detergency is so poor that they cannot be practically used as detergents.

What are surfactants?

Surfactants are molecules which can reduce the surface tension of water. Surfactants lower the surface tension of water by disrupting hydrogen bonds between water molecules, thus increasing water molecules’ ability to contact and wet a surface.

Some very easy home experiments will demonstrate the nature of surfactants.

Experiment 1: Pour faucet water into two cups and add a few drops of food coloring to each cup. Into one of them add a teaspoon of liquid dishsoap and stir gently so that no bubbles are created. Take two nails of identical size and identical points and dip one into each cup. Pull the nails out slowly from the cups and you will notice that each nail has a water drop at its point. You will clearly notice that the water drop from the cup with the dishsoap is more elongeted than the water drop from the cup with the plain water, which is more round.

Experiment 2: Take a needle and try to slowly press against the water drops. You will notice that the needle does not penetrate the drop immediately but rather creates a slight depression in the water drop. Only after applying increased pressure will the needle penetrate the water drop. Penetration into the plain water drop requires more force than penetration into a same-size drop from the cup which contains dishsoap.

To understand the results of these experiements, consider the following. The water layer which surrounds the water drop is organized in a different way than the water molecules which are deeper in the water drop. They are at the surface, and unlike other water molecules which are surrounded with outer water molecules, they touch water on one side and air on the other side. This special environment causes them to organize as an “air/water barrier”. The water surface layer acts as a membrane around the water drop. The bulk water in the drop is also organized as a network of molecules which adhere to each other via hydrogen bonds. The hydrogen bond network also creates resistance to a needle penetrating into the drop. In the presence of detergent, hydrogen bonds are interrupted and the water molecules are not organized in a rigid network and are therefore, soft water. The smaller the drop, the larger its surface-per-volume area which creates a limit of size, below which the surface tension would be thermodynamically too high to be stable. The higher the surface tension, the more water can be contained in a drop. There are thermodynamic considerations which help determine the stable size of a water drop under given conditions, but they are beyond the scope of this discussion. Warming the water as well as adding a surfactant will reduce hydrogen bonds between water molecules and reduce surface tension, which prevails in cold water or water without surfactants. Distilled water will have less surface tension than “hard water”.

Why is it important to lower water surface tension in a cleaning product?

In the presence of surfactants, individual water molecules have free mobility and are not part of a hydrogen bond network, and thus can access more of the surface area of oil deposits and chip away small oil droplets. Surfactants interfere with the hydrogen bond network and can help maintain small oil droplets which are chipped from the oil deposit in a semi-stable state, by surrounding the small oil droplets and keeping them from rejoining into a bulk oily deposit. However, oil does not mix in water and the action of lifting oil deposits with “soft water” may not result in total removal of the oil deposit. For complete removal of oil, we need a surfactant which is also a potent detergent.

surface tension In the red (upper) box, the water drop on the left containing detergent (dish soap, in this case) tends to flatten and disperse widely, creating a much larger spot than the drop on the right. The water drop on the right, containing no detergent, has a higher surface tension and tends to occupy a narrower but taller (more ball-like) space. Blue food coloring was added to improve visibility; the drop containing detergent appears to be a lighter color because the same amount of color is spread over a larger area. In the green (lower) box, the drops were allowed to dry at room temperature for two hours. The drop on the right, without detergent, created a sharp ring upon drying, indicating that the water in the drop did not have significant contact with the surface due to the water's own surface tension. On the left, the drop with detergent is larger and created a color smear throughout its drying surface, indicating better contact with the surface while drying.

What are detergents?

Detergents, like other surface active molecules, have a polar region and a hydrophobic region. However, the “polarity” of the polar region, the length of the hydrophobic region, and the ratio between the two regions are critical in determining the detergency of the molecule. The polar region points towards the water and the hydrophobic region can attach itself to the surface of an oil droplet and suspend it in water. Detergents form a surface coating on oil droplets and provide excellent breakdown, suspension, and removal of oils.

Detergents must be water-miscible and must have certain structural and physical characteristics to create and maintain stable suspensions of oil in water. All detergent types have a similar mechanism of action.

In interacting with an oil deposit via its hydrophobic region, a detergent, with the help of water, chips away molecules from the oil deposit. This process continues until the entire oil deposit is used up and converted into small, detergent-coated oil droplets, dispersed in the water phase. The detergent further incorporates oil molecules into its micellar structure while interacting with water via its molecular polar region. Thus, the detergent suspends the oil molecules in water.

Determining detergent activity of surface active molecules involves many and complex calculations which will not be discussed in this article. However, there are many available detergents, some of which are very popular and widely used. I will summarize important features of some of these detergents which may help create a better informed view of a group of molecules which have a critical role in modern life.

Detergents should be compatible with other ingredients with which they are formulated so that they are not chemically changed by other chemicals in the formulation or at their target area. Detergents mobilize, relocate, and reorganize other molecules. There are several groups of detergents: anionic detergents, cationic detergents, zwitterionic detergents, non-ionic detergents.

What are soaps?

Soap is the oldest known detergent. It is a very strong detergent but has some limitations. Soaps are anionic in nature. They are fatty acid salts made from digestion of oils by very strong base. Their main limitation is that soap has a narrow pH range of activity and is not very effective in low pH, which is the natural pH of healthy skin. In fact, most “old fashioned” soaps have a pH of 9-10 which is not very good for skin, to say the least; some soaps are adjusted to a more neutral pH but are more greasy. In addition, soap interacts with calcium and magnesium in water and creates “soap scum”, which is an undesirable attribute of soap. Soap is not very soluble in water and soap scum builds up greasy deposits on skin and scalp which may require extensive rinsing. Soap does not foam well in hard water and deteriorates with time. The great advantage of soap is that it is very inexpensive.

Anionic detergents have a larger effective pH range than soap, do not leave calcium or magnesium scum deposits like soap, and are very foaming. Anionic detergents will also foam in hard water and are very stable. Foaming is not required for the cleaning action of detergents but is a feature desired by consumers. Some anionic detergents are: sodium lauryl sulfate, triethanolamine lauryl sulfate, and ammonium lauryl sulfate. These are excellent main detergents but can leave hair and skin dry if not supported by other moisturizing ingredients. Another main group of detergents are sodium laureth sulfate, triethanolamine laureth sulfate, and ammonium laureth sulfate. These too are excellent detergents. Though milder than the first group, they produce rich foam, are good cleansers, and leave hair with a soft feel. As secondary detergents, lauryl sarcosine and sodium lauryl sarcosinate are sometimes used. These are too mild to be very effective cleaners but are excellent co-detergents and conditioners.

Cationic detergents are effective in low pH and are anti-static; that means they will leave hair feeling soft. They do not create significant foam and are very effective bacteriostats and disinfectants. However, they are very poor detergents when used alone and have to be fortified with non-ionic detergents. In addition, cationic detergents pose more risk to aquatic life than anionic detergents and so they may create environmental considerations if used in large quantities. Because they are not compatible with anionic detergents, preparations containing cationic detergents are not very common. However, when it comes to hair, cationic detergents are excellent in leaving hair soft and manageable. Some cationic detergents are: cetylpyridinium chloride monohydrate and hexadecyltrimethylammonium bromide.

Non-ionic detergents are effective in a broad pH range of preparations; they are not very foaming but can increase foaming of anionic detergents. They are very mild and not as effective as anionic detergents. Alkanolamides, polyoxyethylene fatty alcohols, and polyoxyethylene sorbitol esters are non-ionic detergents.

Zwitterionic detergents are very potent at high pH and very mild at low pH. They are very safe for use and can be added to other detergents. They can be used as shampoo at a high pH and as conditioner at a low pH. This makes them a bit complex for practical use; they are also quite costly. Cocamidopropyl betaine and sodium lauraminopropionate are amphoteric detergents. They are non-irritating to the eyes and are thus found in baby shampoos.

zwitterionic detergent N-Alkyl-N,N-dimethylglycine betaine is a zwitterionic detergent.

Myths and facts

Myth: Shampoos with much foam clean better.
Fact: Foaming does not contribute to cleaning.

Myth: Thick shampoo is more concentrated.
Fact: Thick shampoo can be obtained by adding thickeners rather than beneficial ingredients.

Myth: Opaque shampoos are richer in beneficial ingredients.
Fact: Opaque shampoo can be obtained by adding opacifiers rather than beneficial ingredients.

pH considerations

Some shampoos claim to be pH balanced. The best shampoos are pH 7 or below. Most shampoos are alkaline, which can cause hair shaft swelling and increase hair’s susceptibility to damage. People with hair problems should avoid alkaline shampoos and choose shampoos with balanced or mildly acidic pH.

Hair type considerations

Shampoos with lauryl sulfate as detergent base provide excellent cleaning for people with thick, normal-oily hair which does not have a tendency to tangle. For people with fine hair with a tendency to tangle, lauryl sulfate based shampoos should be supplemented with moisturizers or alternatively, they can use instead mild detergent based shampoos such as dry-hair shampoos, baby shampoos (laureth sulfate), conditioning shampoos, or shampoos which contain sulfosuccinate and amphoteric—self conditioning—detergents or co-detergents. These shampoos can be used daily.

Conclusion

Detergents are at the core of almost every aspect of modern life. Detergents symbolize technological evolution as much as the computer chip. It is impossible to imagine modern medicine, pharmaceutical industries, food technologies, thin-layer coating technologies, hygiene, composite material handling, and many other technological fields without detergents. It is therefore important to be well informed about the advantages and shortcomings of different detergents and learn how to use them wisely.

In this issue:
Welcome to the Journal of Topical Formulations

Feature Article: Detergents and Surfactants in Soaps, Shampoos, and Other Cleansers

The Formulator's Bookshelf

What Does It Mean?

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