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Structured Surfactants

By: Steve Herman
Posted: December 6, 2010, from the December 2010 issue of GCI Magazine.

“Learning and innovation go hand in hand. The arrogance of success is to think that what you did yesterday will be sufficient for tomorrow.” —William Pollard

Surfactants are the engines that power most personal care products, from emulsions to shampoos and body washes. They are molecules that combine within themselves both polar and nonpolar sections. Some types make oil and water coexist in lotions, others help cleanse hair and skin. Emulsions can hold high oil concentrations, but do not foam and cleanse, while cleansing products traditionally cannot hold or deposit significant amounts of oil-based ingredients. But having the best of both worlds, holding a significant amount of oil while cleansing, is now possible. New technology based on the concept of “structured surfactants” has opened the door to a new range of products never before possible, finally combining cleaning with high delivery of water-insoluble actives. [More on emulsions is available in the July 2010 issue’s The Mysteries of R&D, Part I, by Art Rich, PhD.]

Down to the Molecular Level

In emulsions, surface-active molecules create micelles that contain the oil phase. In cleansers such as shampoos and body washes, the micelles are essentially empty. Trying to add oil to these systems, and then to deposit them on the skin, is very difficult because they are designed to wash oil off the hair and skin. Some cream conditioners are hybrid products, where emulsion phases are imbedded in the cleansing base, but stability and performance of these products is less than ideal.

Structured surfactants have come to the rescue, but they involve understanding a higher level of molecular organization. Traditionally, knowing if a surfactant was nonionic, ionic or amphoteric (if the surfactant had an electrical charge, whether that charge was negative or positive, and its pH) was enough to know a lot about how it would behave in a system. These new structures require combinations of ingredients and processing conditions to achieve the desired characteristics, and you must look at the system from a larger perspective.

To understand structured surfactants, it is instructive to start with a single surfactant molecule and see what happened to it in different environments. The polar head will enter the water, but the nonpolar tail is repelled, since it prefers to stay outside. The tail doesn’t like air, but it likes water even less. As more surfactant molecules are added, they accumulate on the surface until space runs out. Then they are forced into the water, but float around in a random way. Finally, even the water is filling up with surfactant molecules and they get forced together in spherical shapes. This point is called the critical micelle concentration (CMC). At the CMC, a variety of properties, such as detergency and conductivity, abruptly change.