Sodium laureth sulfate, or SLS, is the least irritating of the surfactant detergents, the chemicals used to alter the surface tension of water and facilitate efficient cleaning. The lauryl fatty acids in sodium laureth sulfate originate in natural oils, such as coconut oil, but are industrially treated with petroleum-based sulfated oxyethylene to create cleaners and cosmetics that will dissolve and foam in hard water. Beyond cleaning, SLS creates flow properties that give texture to commercial products.
How Detergents Clean
All detergents are chemical surfactants. Natural surface tension forms water into droplets or larger bodies that do not easily dissolve substances into solution. Because surfactants dissolve by binding to water at one end of their molecule, they are called hydrophilic, meaning "water loving." The other end of the surfactant molecule is a fat that will form a coating on substances, suspend them in solution and prevent them from returning to the item being cleaned.
Flow Property
Surfactants assemble in solution as micelles of different shapes determined by the concentration of the surfactant in the solution. Each of these shapes creates products with unique thickness, or viscosity, that give commercial soaps, toothpastes and cosmetics the familiar flow properties that consumers expect and value. Based on percent of solution by weight, micelles may form as globs, rods, bundles and lamellar sheets. Sodium laureth sulfate displays all four stages of micelle shape and behavior.
Low Concentrations by Weight
At very low concentrations, such as used for antimucus function in vaginal douches, sodium laureth sulfate does not form micelles in solution. As formulas increase--up to 20 percent total surfactant by weight--sodium laureth sulfate and other surfactants form round globular micelles. This percent surfactant is used for rapid-dry soaps and spray cleaners that are not thicker than water. Between approximately 20 to 32 percent, surfactants form rod-shaped micelles that thicken the product and make it easier to handle, as in conditioning shampoos or shower gels.
Stiff Gels
At concentrations of approximately 33 to 55 percent sodium laurel sulfate by weight, the rod-shaped micelles in solution form hexagonal bundles that greatly reduce the flow property of the product and create a stiff gel. At this concentration, no additional substances can be dissolved in the solution.
Creams
Beyond 55 percent concentration, sodium laureth sulfate forms lamellar sheets of molecules. Products of this concentration have a flowing, creamy texture because the sheets of surfactant slide smoothly past each other. Cosmetic creams thickened with sodium laureth sulfate will dissolve additional substances within the cream and can be used for cleaning and as carriers for other ingredients.
References
- Analysis of Surfactants; Thomas M. Schmitt; 2001
- Kiwi Web Chemistry and New Zealand: Surfactants: Surface Active Agents
- Handbook of Detergents: Formulation; Michael S. Showell; 2005.
- U.S. Food and Drug Administration: Morgan, Lewis & Brokius Letter, June 28, 2004
- Handbook Of Detergents: Part C--Analysis, Volume 123; Heinrich Waldhoff and Rudiger Spilker; 2004
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