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.
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.
Fundamentals to formulating a successful cleanser construction include determining which part of the body is to be cleaned; foam size and structure; ease of foam building; feel during application and after rinse-off; viscosity during dispensing and use; and deposition of active ingredients. In addition it is necessary to development an aesthetically pleasing product that consumers will continue to use. Finally, since in-process foaming is a concern, consideration of the equipment to be used during formulation is of importance; and since surfactants need to dissolve quickly and fully, variables such as equipment, order of addition of ingredients (to include surfactants), mixing rate and temperature must be considered.
Selecting a surfactant system is difficult due to the diversity of options available. To determine the appropriate system, formulators must consider the interaction of ingredients and how surfactant-based cleansing systems will be positioned in the marketplace. Also not to be forgotten is the purchase of ingredients from reliable suppliers. Since no two raw material manufacturers use the same process(es), some manufacturer variations exist that could impact the quality and performance of ingredients. Surfactant specifications are critical to ensure viscosity control and color, odor, pH, salt content and foaming/cleansing characteristics.
While constructing a cleanser, it is helpful for the formulator to divide the formula into its functional segments:
- Primary surfactant(s): workhorse ingredient(s) required to remove soil from a substrate
- Co-surfactant(s): to add structure or foam density to a formula; these materials, such as alkanolamide MEA and betaines, are conducive to forming a micellar structure that confers higher viscosity
- Rheology modifier(s): consisting of the two types—polymeric and high melting point wax. Polymeric thickeners include acrylate chemistries, cellulose and gums such as guar, xanthan and locust. High molecular weight/melting point waxes such as stearyl alcohol and PEG esters produce crystalline structures that provide suspension for insoluble components.
Performance properties of rheology modifiers include: controlling rheology and yielding stress, modifying the formula’s appearance, flow and texture to alter the pour and at-rest characteristics; stabilizing oils and suspended particles; thickening surfactants—i.e., those that do not thicken with the addition of salt; modifying aesthetics to impart a modified feel during application; and stabilizing viscosity—i.e., preventing viscosity drift during long-term and high-temperature conditions.
- Preservative(s): since cleansing products are generally based on aqueous systems at a relatively neutral pH, preservatives are critical to maintaining a system that is free from micro-organisms.
- pH Adjuster(s): alkaline and/or acidic (e.g., sodium hydroxide and citric acid)
- Miscellaneous functional ingredients: for conditioning, pearlizing, etc.
- Color and fragrance
- Commercial Products
Regarding cleansers, recent advances have been made in milder and softer-feeling surfactants. With the advent of naturalness as a desired attribute, change is continuing. Formulas 1-6 are commercial examples of various cleansing systems on the market. These formulas were taken from sources of public domain and are estimates of the ingredient percentages and procedures used, to provide readers with a starting point for their own formulation work. It is suggested that readers perform a patent search to ensure they are not infringing on any existing and protected technologies.
Incorporating a surfactant system is one of the first steps to making a cleansing product. Minimal issues arise when incorporating sodium or ammonium lauryl sulfate since the surfactant is provided in a relatively dilute solution (28–30%). Using sodium laureth sulfate (SLES) instead presents added challenges since SLES typically is provided in a concentrated solution of 70%, and acts more like a gel than a solution. Therefore, formulators should consider how to incorporate SLES into a formula. Following are some tips, provided by Barry Salka of Surfactants Inc., to incorporating SLES so as to minimize process issues. It should be noted that these suggestions can be combined to facilitate faster processing with good solubilization of the surfactants:
1. If NaCl will be added to the formula, add it directly to the SLES. For example, if the formula contains 20% SLES and 0.5% NaCl, add the 0.5% NaCl directly to the SLES so that it represents a high salt concentration in the SLES and helps to break up the concentrated SLES gel; then add the NaCl/SLES to the water and follow with the remaining ingredients.
2. Consider adding other surfactants to the water first to create co-micellae; then add the SLES so the gel breaks up easier.
3. Warm the water and SLES first to around 35–45˚C each, then add the SLES to the water phase.
4. If using a rheology modifier, consider incorporating a portion of the water into it as a separate phase, then adding this mixture to the solubilized surfactant system-water phase. Chelators, antioxidants and preservatives can be added to this mixture before neutralization. It is better to neutralize/pH-adjust the formula after most of the ingredients are incorporated to facilitate easier processing—before the system builds too much rheology.
Outlook on Cleansing Materials
As noted, the growing trend in the marketplace is focused on natural ingredients and parallel to this is a push for eliminating marketing references to chemical preservatives, as well as anything to do with sulfates or synthetic surfactants. With this trend, biosurfactants have emerged as a new category of products since these materials are created via fermentation processes incorporating enzymes and bacteria. They behave much like synthetic surfactants and are available in a variety of ingredient classifications. However, biosurfactants tend to provide lower process yields than conventional synthetic processes and thus cost more. In addition, they can contain higher levels of impurities or remnants of the bacteria used to manufacture them.
The movement for nature-derived surfactants also is generating cost issues for manufacturers, retailers and consumers. These surfactants, like biosurfactants, are based on natural starting materials but have been modified. Again, these materials are more costly than non-naturally derived synthetic surfactants but as more of these materials are used, it is possible that their costs will be driven down.
1. Harry’s Cosmeticology, 8th ed., M Rieger, ed, Chemical Publishing: New York (2000), 187– 211; 485–501
2. The Chemistry and Manufacture of Cosmetics: Vol I—Formulating, M Schlossman ed, Allured Business Media: Carol Stream, IL (2006)