SURFACTANT PENETRATION 93 surfactants (2,8). Anionic surfactants account for the vast majority of surfactants used in cleaning products, including (i) soap surfactants: soap surfactants can be prepared by hydrolysis of triglycerides (TS) into a mixture of various long-chain carboxylates, and this process is called saponifi cation. Soap has strong cleaning and foaming abilities, but it is not gentle to skin. Other drawbacks of soap surfactants include high pH (usually greater than 10) use condition and incompatibility with hard water. Soap precipitates at low pH or forms precipitations with magnesium and calcium ions in hard water (8) (ii) synthetic sulfate, sulfonate, and isethionate surfactants, such as sodium dodecyl sulfate (SDS), sodium lauryl sulfate (SLS), sodium laureth sulfate, sodium C14-16 olefi n sulfo- nate, and sodium lauroyl isethionate. They are slightly milder to the skin than soap surfactants, especially for isethionate surfactants. Unlike soap, they can be used in hard water and in a wide pH range of 3–11 and (iii) amino acid–derived surfactants, espe- cially acyl glutamate and acyl glycinate, which are popular ingredients in mild skin- cleansing products. Nonionic surfactants are the second largest surfactant class. They are not ionized in the solution, and thus are not sensitive to the ions in hard water, are not easily affected by pH, and have good compatibility with other types of surfactants, such as anionic surfactants, to formulate skincare products or cleaning detergents (8). Fatty alcohol ethoxylates or CmEn is one of the most important types of nonionic surfactants. They possess stable chemical structures and strong cleaning abilities. One of the most important or interesting features of fatty alcohol ethoxylates is that they exhibit reverse solubility versus temperature behavior in water, i.e., they have cloud point—the temperature at which the solution becomes cloudy. The cloud point generally increases when the hydrophilic part or the number of oxyethylene units becomes larger and is strongly dependent on cosolutes in- cluding electrolytes and polyols (1). Cationic surfactants, the third largest group of surfactants, are actually not as much used as anionic and nonionic surfactants in skin care. They are often added to skin products as preservatives, softener, and conditioner (6). The hydrophilic parts of the cationic surfactants are usually amine and quaternary ammonium based. The amines only function as a surfac- tant in the protonated state, and thus they are not compatible with high pH, whereas Figure 1. Diagram of surfactant monomer.

JOURNAL OF COSMETIC SCIENCE 94 quaternary ammonium compounds are not pH sensitive. Cationic surfactants show higher toxicity than most other classes of surfactants, and therefore they are often used as preservatives (9,10). Amphoteric surfactants are the smallest surfactant class probably because of the high price. Their hydrophilic part contains two charged groups. As a result, amphoteric surfac- tants are stable in both acidic and basic conditions, but pH change will affect their phys- icochemical properties. When pH is at the isoelectric point, the amphoteric surfactants possess similar properties as the nonionic surfactants. When pH is above or below the isoelectric point, the amphoteric surfactants show properties resembling those of anionic or cationic surfactants, respectively. In addition, amphoteric surfactants are mild to skin and eyes, enabling them to have a wide range of applications (11). Some typical examples of the four surfactant categories are presented in Table II. Thanks to the particular physicochemical properties, anionic surfactants and nonionic sur- factants are used as the main surfactants in skin-cleansing formulations. As mentioned Figure 2. Diagram of surfactant micelle.