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.
Wang, Zonghua, et al. Electrochimica Acta 160 (2015): 288-295.
An intelligent zwitterionic surfactant, sodium lauroyl sarcosinate (SLA), was introduced into the construction of 3D graphene micro-nano clusters and utilized for the synthesis of platinum nanoparticles, resulting in Pt/3D graphene micro-nano composites. Due to the unique pH-induced charge transition and micelle arrangement properties of the zwitterionic surfactant, the high-order assembly of 3D porous graphene-based structures can be readily achieved from stacked graphene oxide layers. Graphene oxide (GO) was synthesized using the improved Hummers method. For the preparation of the composite catalyst, 50 mg of GO was dissolved in 50 mL of a 1:1 mixture of ethylene glycol and water, stirring to form a GO suspension with a measured pH of 4.2. Then, a 10 mL 5 wt% SLA aqueous solution was added dropwise under vigorous stirring. The pH of the resulting solution was adjusted to 11.0 using 0.01 M NaOH, followed by 2 hours of ultrasonication. Subsequently, 2 mL of 38 mM H2PtCl6 was added dropwise under vigorous stirring. The mixture was then transferred to a stainless steel autoclave and heated at 160 °C for 7 hours to prepare the 3D porous Pt/graphene structure. The resulting black precipitate was filtered, washed several times with an ethanol-water solution, and collected after vacuum drying at 60 °C for 24 hours to obtain the Pt/3D GN micro/nano composites.
Li, K., Lin, S., Li, Y., Zhuang, Q., & Gu, J. (2018). Angewandte Chemie, 130(13), 3497-3501.
A mesoporous Zr-based metal-organic framework (mesoMOF) with uniform mesoporous channels and crystalline microporous frameworks was constructed using a zwitterionic surfactant as a template in an aqueous system. The aqueous synthesis ensured the formation of rod-like surfactant micelles. Simultaneously, the carboxylate groups of the zwitterionic surfactant provided anchoring for the bridging of Zr-oxo clusters and the surfactant components. As a result, the MOF's oriented crystallization occurred around cylindrical micelles, producing hierarchical microstructures and mesopores. The size of these mesopores could be easily tuned by altering the alkyl chain length of the surfactant used.
The key design element in this strategy is the use of zwitterionic surfactants, such as Cocoamidopropyl Betaine (CAPB) or Oleamidopropyl Betaine (OAPB), as templates for constructing the mesoMOF. The results indicated that the good interaction between the MOF precursor and the surfactant is critical for the successful growth of these surfactant-directed mesoporous MOFs. Weak interactions would lead to preferential crystallization of the ligand and metal salts, despite the presence of surfactants. The zwitterionic surfactants have a carboxylic acid functional group and a quaternary ammonium salt as the hydrophilic head group. The carboxylic acid functional group facilitates tight binding with the metal clusters in the MOFs, while the quaternary ammonium salt group provides high solubility to otherwise insoluble fatty acids. Theoretically, the bifunctional nature of the zwitterionic surfactant allows it to act not only as a template but also as a coordination ligand chemically attached to the metal precursors during the self-assembly process, effectively preventing phase separation between non-mesoporous MOFs and the surfactant assemblies.
Zhu, Yongxin, et al. Journal of Power Sources 515 (2021): 230646.
This study reports the inhibitory effects of two zwitterionic surfactants, betaine (BS) and dodecyl dimethyl betaine (BS-12), as additives in 4 M NaOH for aluminum-air batteries.
The zwitterionic surfactants BS and BS-12 are utilized as electrolyte additives in the 4 M NaOH solution of aluminum-air batteries. Hydrogen gas release tests and electrochemical measurements indicate that BS-12 is a superior corrosion inhibitor. The optimal corrosion current inhibition efficiency of 1 mM BS-12 is 81%. BS-12 also enhances the electrochemical performance of the aluminum anode. The chemical adsorption of the hydrophilic groups in BS-12 and the interaction of the long alkyl chains may retard the diffusion of OH− and Al3+ ions along the aluminum alloy/solution interface. The presence of BS-12 effectively slows down the self-corrosion reaction of the aluminum anode.
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