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Structured Surfactants

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

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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.