Surfactant Types Commonly Used in Personal Cleansers
While some new cleanser technologies can combine effective cleansing with the potential to improve skin condition, the focus for the majority of personal cleansing products remains on minimizing the potential for skin damage. Surfactants make up the bulk of most personal cleansing products and are primarily responsible for a product’s in-use
Figure 3 Expert visual dryness and erythema results from a hand washing pilot study comparing a petrolatum-depositing hand wash product to a water control. The hand wash product improved dry skin condition, even when used for washing hands 20 times daily.
properties, e. g., lather, and for its effects on skin. While all surfactant molecules are amphiphilic, there are distinct surfactant types. A surfactant’s dissociation behavior in water provides a convenient basis for classification.
Anionic Surfactants
These surfactants dissociate in water to yield a surfactant with a negatively charged hydrophilic group and a cation that is usually an alkali metal (sodium or potassium) or a quaternary ammonium species. Anionic surfactants are used in a wide variety of bar and liquid personal cleansing products and account for about 50% of worldwide surfactant production (21,22). Soap, which is chemically the alkali salt of a fatty acid, is the best – known anionic surfactant, but a variety of synthetic (i. e., non-soap) anionic surfactants are commonly used in personal cleansing products, including the acyl isethionates, alkyl sulfates, and alkyl ether sulfates (AES). The acyl isethionates have good skin compatibility and are good detergents and lime soap dispersants, viz they inhibit the formation of hard water scum. Sodium cocoyl isethionate is an example; this surfactant is a common primary surfactant in “mild” cleansing bars. The alkyl sulfates are widely used in cosmetic products ranging from skin cleansers to toothpastes. They have good foamforming properties and produce creamy lather but do not perform well in hard water. Alkyl sulfates have a marked potential to irritate skin. Sodium lauryl sulfate (SLS), an alkyl sulfate found in many personal care products, is often used as a model irritant. The AES are similar to the alkyl sulfates but their hydrophobic portion comprises ethylene oxide units rather than a straight-chain hydrocarbon. This gives AES a number of advantages over alkyl sulfates, including better lather formation in hard water and better lime soap dispersion. AES are also less irritating than alkyl sulfates, and their skin compatibility is improved by increasing the degree of ethoxylation (23,24). Sodium laureth sulfate is an example of an AES that is commonly found in personal cleansing products.
Cationic Surfactants
These surfactants dissociate in water to yield a surfactant with a positively charged hydrophilic group and an anion. Fatty amine or ammonium salts and quaternary ammonium salts are examples. Cationic surfactants are generally not good detergents or
foaming agents, and they are usually incompatible with anionic surfactants. However, being positively charged they adsorb to biological (and other) surfaces, which tend to have a net negative charge at a neutral pH. This property makes cationic surfactants useful as antistatic agents in hair conditioning products. The quaternary ammonium compounds have marked antibacterial activity and are often found in toiletries such as deodorants and mouthwashes.
Nonionic
These surfactants do not dissociate in water. Instead, their hydrophilic group is commonly an alcohol, phenol, ether, ester, or an amide. Alcohol ethoxylates and alkylphenyl ethoxylates are two common examples of this type of surfactant. A “new” class of nonionic surfactants employs various sugars as hydrophilic groups. The uncharged nature of nonionic surfactants makes them compatible with other surfactant types, and they also show reduced sensitivity to conditions such as water hardness or salinity and to formulation pH. Common examples are the sorbitan esters (marketed as SPAN) and their ethoxylated counterparts (marketed as TWEEN). As a class, nonionic surfactants tend to exhibit good skin compatibility (15,25), but they still have a potential to interact with and negatively impact the stratum corneum (26).
Amphoteric Surfactants
These surfactants have two functional groups, one anionic and one cationic. Their character is determined by the pH of their environment; amphoteric surfactants are anionic under alkaline conditions and cationic near or below their isoelectric point, i. e., the point at which the surfactant molecule carries no net charge. Betaines, which actually carry a positive charge in both acidic and alkaline media (27), are among the most widely used amphoteric surfactants and are found in both bar and liquid cleanser formulations. Betaines are used to improve lather quality or to increase the viscosity of liquid formulations. They generally show good skin compatibility and can decrease the skin irritation potential of harsher anionic surfactants when used in combination with them (24,28). But betaines are not without issues. There are a number of reported cases of contact allergy to cocamidopropyl betaine (29-32), one of the most commonly used surfactants in this group, and this surfactant was named the contact allergen of the year in 2004 (30). However, the effective incidence of issues is low given the widespread use of this surfactant in personal care products. Still, manufacturers may be able to reduce the risk of contact allergy by using a higher grade of betaine material as data suggest the allergic response is caused by impurities rather than by the surfactant itself (32-34).