Surfactants and Cleansing Products

By: Eric Abrutyn, TPC2 Advisors Ltd., Inc.

Posted: June 30, 2009, from the July 2009 issue of Cosmetics & Toiletries.

Much has been written about surfactant chemistry and structural functionality.^{1, 2} In general, products exhibiting some degree of surface activity and the ability to remove soil from a substrate are referred to as cleansing products. Body cleansing has been practiced for thousands of years, primarily with bar soap based on the alkaline salt of a fatty acid. Today, cleansing products are available in solid bar, liquid and gel forms and can be based on alkaline salts and synthetic and natural surfactants. When substituted for alkaline salts, synthetic and natural surfactants improve the foam structure and mildness of formulas.

Surfactant molecules rest at the water interface and form a thermodynamically stable system that, by virtue of its two distinct active sites (hydrophilic and hydrophobic), prevents polar and nonpolar solvents from contacting each other. Types of thermodynamically stable systems include: micelles, lamellae, micro-emulsions, emulsions and liquid crystals. The polar and nonpolar components of a surfactant provide different affinities that attract specific solvents. The nonpolar component is hydrophobic and generally insoluble in water—consisting typically of linear or branched alkyl and alkylphenyl groups.

The polar component is hydrophilic, and it is this portion of the structure that determines the surfactant’s classification. A nonionic classification includes polyalkoxylate, glucose, sucrose and amine oxide; anionic consists of sulfate, sulfonate, carboxylate and phosphate; cationic includes alkylammonium salts; and zwitterionic includes both anionic and cationic groups. See Table 1 for surfactant classes and their properties.

In general, the mechanism of surface activity is similar between all types but the intended use ultimately determines the appropriate surfactant or surfactant combination choice. Everyday uses include:

• detergency to remove soil—e.g., in shampoos and soap;
• wetting to improve the contact angle between a solution and a substrate—e.g., in hair coloring or permanent wave lotions;
• foaming for visual effects—e.g., in shampoos, bubble bath and laundry detergents;
• emulsification to form a stable mixture of two incompatible phases such as oil-in-water, water-in-oil and multiple phases; and clear micro, alcoholic, nano- and refractive index matching—e.g., in skin and hair creams and lotions; and
• solubilization of insoluble components to improve their compatibility—e.g., in perfumes and flavors.

Development Issues
The key to maximizing on the unique properties of surfactants lies in understanding how and when to use them. Surfactants can easily be incorporated into personal care formulas but this requires skillful execution of surfactant-related processes. One important step to consider is how the surfactants will be incorporated to optimize the product’s performance and processing. While the primary performance properties of cleansers are to provide foam and cleansing activity, the level of foam and structure of the foam bubble (e.g., creamy, loose, tight, quick-breaking, etc.) are equally important.