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“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.
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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.
In structured surfactants a different geometry is encouraged: lamellar phases. These are bilayer sheets of surfactants, with the polar heads on the outside and nonpolar tails in the middle. By changes of salt content, pH and the application of high shear mixing, the sheets can be forced into spheres. These so-called spherulites have concentric bilayers resembling an onion with water trapped between the layers. The spherulite structure is fundamentally different from the micelles of conventional cleansers.
Properties and Possibilities
Structured surfactants have two critical rheological properties (properties such as consistency and flow): shear-thinning and the presence of yield value. They are both easy to understand intuitively. When using ketchup, the bottle must not just be turned upside down during use, it must be hit. There is a minimum force needed to generate flow. This minimum force is called yield value. Systems that have yield value are able to suspend particles. This is important for products containing insoluble actives. When a force is applied to a liquid, it moves. If the movement is directly proportional to the force, it is called Newtonian. Water and glycerin and mineral oil are Newtonian. Shear-thinning materials lose viscosity and flow more easily when a force is applied. This is important in almost all personal care products—when you squeeze a bottle, you want the product to come out. This is also important in production. If the opposite were true (shear thickening), material could not be pumped out of the manufacturing tank. Ingredient supplier Rhodia’s Miracare SLB-365 is an example of a commercially available structuring surfactant. One proposed benefit of this product as a shampoo base is the retention of color in permanent color-treated hair compared to the frequent use of conventional cleansing systems. Rhodia has prototype formulas such as a body wash with 15% rapeseed oil and another with 35% petrolatum, surely a system not previously viable.
Impossible Yesterday, Possible Today
Surfactants as individual molecules have been the foundation of cosmetic formulation for more than a century. Until recently, these chemicals have been used for their properties as individual molecules, not for their ability to form more complex structures. The recent advances in structured surfactants for cleansing products are a quantum leap in personal care formulation. The ability to deposit high levels of moisturizing oils and water-insoluble actives makes a new generation of products technically feasible.
Structured surfactants show that for whatever is in the chemical toolbox, there is always the possibility for innovation beyond the obvious. What was impossible 10 years ago is now reality, and what is impossible now should be explored today to lay the groundwork for the “miracle” products of the future.
Steve Herman is president of Diffusion LLC, a consulting company specializing in regulatory issues, intellectual property, and technology development and transfer. He is a principal in PJS Partners, offering formulation, marketing and technology solutions for the personal care and fragrance industry. He is an adjunct professor in the Fairleigh Dickinson University Masters in Cosmetic Science program and is a Fellow in the Society of Cosmetic Chemists.