Cleansing Liquid Surfactants
Liquid cleansers often combine anionic and amphoteric surfactants. Anionic surfactants commonly used in liquid cleansers include soaps (salts of fatty acids) and synthetic surfactants such as alkyl ether sulfate, alkyl acyl isethionates, alkyl phosphates, alkyl sulfosuccinates, and alkyl sulfonates. Cocoamidopropyl betaine and cocoamphoacetate are the typical amphoteric or zwitterionic surfactants used.
Notably, nonionic surfactants such as alkyl polyglucoside and amino acid-based surfactants like acyl glycinates, alkyl glutamates, and sarcosinates are being increasingly incorporated as primary surfactants in cleanser systems for their mildness-enhancing activity (Surfactants in Cosmetics, New York, Marcel Dekker, 1997, pp. 427–71).
Although most liquid cleansers are formulated in the neutral to acidic pH range, products that contain soap (alkyl carboxylate) as the main active ingredient typically exhibit an alkaline pH.
Structurants and Other Ingredients
With cleansing bars, structurants such as long-chain fatty acids, waxes, and alkyl esters are necessary to maintain the solid format and facilitate the complex manufacturing process. In liquids, structurants impart the right rheology and consistency to the product for optimal dispensing and in-use experience. Structurants also ensure the physical stability of dispersed and suspended phases and confer moisturizing effects.
A moisturizing effect is provided in cleansing systems by water-soluble humectants such as glycerin. Emollients are included in cleansers to reduce the drying effects of surfactants. In moisturizing shower gels, typical emollients and occlusives include triglyceride oils, lipids, petrolatum, waxes, and mineral oil.
Other functional ingredients may be found in cleansers formulated for specific benefits. For example, bactericidal actives such as triclosan or triclocarban are contained in antimicrobial cleansers.
The Food and Drug Administration regulates synthetic cleansers and those designed to achieve antibacterial or other druglike effects. The Consumer Product Safety Commission regulates pure soap products.
Mild, Moisturizing, Cleansing Agents
Delivering lipids, emollient oils, and occlusives under cleansing conditions is one of the primary approaches to reducing visible signs of skin dryness and improving hydration. Incorporating high levels of emollients into a stable cleansing formulation and depositing the emollients on the skin during washing are achieved through specially structured surfactant formulations with cationic polymers to aid deposition and retention of oils and occlusives.
Emollient and occlusive ingredients used in cleansing liquid formulations include vegetable oils (soybean or sunflower seed) and petroleum jelly.
Hydrophobic emollients are more often included in cleansers because they are easier to deliver to skin than water-soluble moisturizers such as glycerin and other humectants.
Paradoxically, cleansing often leads to a weakening of the skin barrier. Consequently, for most skin disorders, cleansing with commonly used soap-based products may prove problematic and aggravate a patient's particular skin condition. In addition, prolonged daily use of cleansers that induce short-term damage can lead to xerosis, scaling, flaking, erythema, and pruritus. Therefore, mild cleansing is recommended for the management of compromised skin conditions such as acne, rosacea, atopic dermatitis, and photodamage.
Conclusion
Soap has an interesting and extensive history and has long been the primary cleansing agent. In recent decades, innovations have led to a marked increase in the variety and versatility of products used for cleansing and beautifying purposes.
Underlying many of these developments was the motivation to formulate products that would not cause irritation. Subsequently, agents have been developed that are more suitable for use on dry or sensitive skin or with compromised skin conditions.
Antibacterial soaps have also been formulated, but could conceivably contribute to the growing problem of antibacterial resistance. This information is available in more detail in the second edition of my book, “Cosmetic Dermatology: Principles and Practice” (McGraw-Hill Professional, 2009).