Amphoterics are surfactants with ionic charge and they can change between anionic properties, the isoelectric neutral stage and the cationic properties depending on the pH value. Amphoteric surfactants have characteristics of stability against electrolytes, acids, alkalis, and hard water. Anionic, cationic and non-ionic surfactants are compatible with amphoteric surfactants. The major amphoteric surfactants are alkylamidopropylamine N-oxide (APAO), alkyldimethylamine N-oxide (AO), alkylbetaine (Bt) and alkylamidopropylbetaine (APB). Cocamidopropyl betaine, cocoamphoacetate and cocoamphodiacetate are also some commonly-used amphoteric surfactants. The amphoterics are dermatologically mild surfactants owing to their behaviour and protein-like structure. They can form complexes with anionic surfactants, show good surface-active functions over a wide range of pH and are able to reduce their irritative properties, and as a result they are mainly used as mild surfactants in cosmetics, toiletries and hand dishwashing liquids. Amphoterics surfactants have many effects such as cleansing, foaming, emulsifying, solubilizing, low toxicity, easy-biodegradation and so on. In addition, the application of amphoteric surfactants is related closely to the synergistic effects of amphoteric surfactant with other surfactants. Amphoteric surfactants can cooperate with other surfactants such as non-ionic surfactants, anionic surfactants. In a word, amphoteric surfactants form part of special surfactants available for formulators to improve or design new formulations in response to environmental, toxicity, safety and performance demands.
Fig.1 Chemical structures of the AO, APAO, Bt and APB homologues
Amphoteric surfactants have following excellent surfactant properties such as its low irritation to skin, eyes and mucous membranes, a moderate antimicrobial activity, the lack of toxicity, good mildness-enhancing ability, wetting power, cleansing ability, foaming power, hard water tolerance, and lime soap dispersibility, stability in extreme pH conditions, compatibility with other ingredients. Amphoteric surfactants are widely applied in personal care products such as moisturizing body wash products, shaving products, shampoos, toothpastes, contact lens detergents and other skin care and hair care cosmetics. Amphoteric surfactants strongly contribute to the viscosity build-up in cosmetic cleansing formulations. This is surely due to the sodium chloride content but also due to synergistic interaction between the anionic primary surfactant and the neutral or positive charged amphoteric.
The reasons for the wide utilization of amphoterics in personal care products are their good cleansing power and lather characteristics, and compatibility with different pH. There are also other excellent properties that lead to the application of amphoteric surfactants in domestic detergents. The properties are as follows: (1) easy-biodegradation (2) strong anti-electrolytes (3) low toxicity and high detoxification (4) high efficiency at low concentration (5) improvement of scourability. Amphoteric surfactants play role in these formulations as hosphate builders, enzyme stabilizers, bleaching agents and color-protecting reagents. In addition, they can be used as the main surfactants in mild dishwashing detergents.
Amphoteric surfactants can ensure the compatibility of each ingredient in liquid products under strong alkaline conditions. Amphoteric surfactants are stable in strong alkali and have high alkali solubility, so amphoteric surfactants can be used in water-based alkaline cleaning agents. When the molecules of amphoteric surfactants contain multiple anionic groups and exhibit the properties of anions or those of cations according to the pH of the solution, they are more favorable for the application of strong alkali. Amphoteric surfactants have strong hydrophilic rejection in that circumstances, show the function of water dissolving and increase the contact between the product and the substrate.