The introduction of formulations comprising blends of amphoteric and anionic surfactants is regarded as one of the most significant innovations in personal cleansing products over the past six decades.1–4 This can be attributed to the synergistic interaction of anionics and amphoterics, which enables formulators to simultaneously increase the viscosity and foaming ability of cleansers while dramatically decreasing irritation potential to the skin and eyes. “True” amphoterics are surfactants that can exist either in anionic, zwitterionic or cationic form (see Figure 1) depending upon the solution pH. Betaines, on the other hand, can only exist in cationic or zwitterionic forms due to the presence of a quaternary ammonium group, yet they are still frequently referred to as amphoterics.
Although their use has been far surpassed by the more cost-effective betaines, amphoterics still remain an important class of surfactants in personal care, home care and industrial applications due to their mildness and versatility. The present column features one of the oldest and most well-known members of the amphoteric family: sodium lauroamphoacetate (SLAA).
Chemistry and Manufacture
The term amphoacetate refers to the monocarboxymethyl derivatives of alkylamido alkylamines prepared specifically from the reaction of aminoethylethanolamine (AEEA) with fatty acyl compounds, exemplified by the chemical structures of SLAA shown in Figure 1.5 Given that the proper chemical name for SLAA based on the International Union of Pure and Applied Chemistry’s nomenclature is sodium 2-[2-dodecanamidoethyl-(2-hydroxyethyl)amino]acetate, INCI names such as Sodium Lauroamphoacetate are certainly welcome synonyms.
The chemical formula of SLAA is C18H35N2O4Na, corresponding to a molecular weight of 366.47 g/mol. SLAA is a true amphoteric surfactant because its hydrophilic head group is comprised of a weakly acidic carboxylic acid moiety (pKa ≈ 2) and a weakly basic tertiary amino moiety (pKa ≈ 8 when protonated). The zwitterionic form of SLAA, i.e., bearing both a negatively charged carboxylate group and a positively charged ammonium group, is most prevalent in the isoelectric range of pH = 4–7, with the cationic and anionic forms dominating at more acidic and more alkaline pH values, respectively.4
The key starting materials required for the synthesis of SLAA include lauric acid (LA), AEEA and chloroacetic acid. The C12 LA is usually isolated via distillation of coconut or palm kernel fatty acids, which are obtained from saponification and hydrogenation of the respective oils. AEEA is the reaction product of ethylenediamine and ethylene oxide, both of which are commodity chemical intermediates derived from ethylene gas.6 Chloroacetic acid is derived from the direct chlorination of synthetic acetic acid with chlorine gas in the presence of red phosphorous as a catalyst.6 For the synthesis of SLAA, chloroacetic acid is employed in the sodium salt form, sodium chloroacetate (SCA), which is usually generated in situ by neutralization with sodium hydroxide (NaOH).
The synthesis of SLAA, shown in Figure 2, involves two principal reactions: 1) the condensation of LA and AEEA to yield lauryl hydroxyethyl imidazoline (LHI), and 2) ring opening and carboxy-methylation of LHI with SCA.3, 7–10 In the first step, LA is reacted with an excess of AEEA at 150–185°C to initially yield a secondary amide intermediate and one equivalent of water. Excess AEEA is necessary to promote the complete conversion of the LA to the amide and to help prevent the formation of unwanted diamide by-products. The reaction is then driven further by heating under reduced pressure to condense the secondary amino group with the secondary amide group, yielding the heterocyclic imidazoline ring and liberating a second equivalent of water. Excess AEEA is then removed by vacuum distillation. The resulting LHI is a stable compound, which can be isolated and has utility as a cationic surfactant.
The LHI is converted to SLAA via reaction with SCA in alkaline aqueous solution at temperatures ranging from 80–95°C. During this process, the imidazoline ring opens via base-catalyzed hydrolysis, cleaving at the 2,3-double bond and rearranging to yield the secondary amide as the major product. Upon nucleophilic attack of the SCA by the resulting secondary amine, the amine nitrogen is alkylated with a carboxymethyl group, producing SLAA and an equivalent of sodium chloride by-product.
Modern amphoacetate production processes have been optimized to deliver high-purity SLAA, yet it is important to note that this synthetic route always yields a mixture of SLAA and disodium lauroamphodiacetate (DSLADA) due to the existence of at least two possible reaction pathways.3–4, 9–10 DSLADA forms when the imidazoline ring of LHI opens via cleavage of the 1,2-single bond to yield the tertiary amide instead of the secondary amide. The resulting primary amine is readily alkylated by two equivalents of SCA to produce DSLADA, as shown in Figure 3. The ratio of SLAA to DSLADA can be tuned to yield an SLAA reactor product that is more or less rich in DSLADA, though SLAA will always dominate. This is accomplished by the careful selection and control of reaction conditions, including the SCA:LHI ratio, pH and temperature.4, 10
SLAA is typically supplied as clear, yellow to amber-colored, viscous aqueous solutions containing 25–33% w/w active SLAA and 6–9% w/w residual NaCl.11, 12 These SLAA solutions exhibit alkaline pH values in the range of 9–11 and do not require the addition of preservatives for microbiological stability. Depending on the concentration of SLAA in the solution, the fluid viscosity may be as high as 5000 cP at 25°C. Dilute solutions of SLAA foam readily and exhibit foaming power similar to that of betaines. In addition to NaCl and DSLADA by-products, other impurities commonly found in SLAA can include: nonalkylated amidoamines, sodium glycolate (from the hydrolysis of SCA), fatty acid salts, diamides and trace (ppm) levels of mono- and dichloroacetic acids and AEEA.
SLAA is recognized as a mild surfactant with relatively low irritation potential.13 Although the Cosmetic Ingredient Review Expert Panel has not formally reviewed SLAA, the panel has concluded that cocoamphoaceate—the coconut fatty acyl analog of SLAA, which comprises ca. 49% SLAA—is safe for use in cosmetics.14 In terms of environmental safety, SLAA is readily biodegradable under aerobic and anaerobic conditions and demonstrates relatively low aquatic toxicity relative to other surfactants, such as cocamidopropyl betaine and linear alkylbenzene sulfonates.15
Technology and Applications
SLAA exhibits robust performance characteristics enabling its deployment in a wide range of skin and hair care applications. It functions primarily as a surfactant for cleansing and foam boosting, yet it also has been reported to function as a hair conditioning agent.5 SLAA is soluble and stable over a wide pH range and can tolerate multivalent ions, such as the calcium and magnesium ions found in hard water. The material exhibits excellent compatibility with a broad selection of anionic, cationic, zwitterionic and nonionic surfactants, and it is also compatible with many types of water-soluble polymers such as anionic rheology modifiers and cationic conditioning polymers.
Historically, SLAA has been employed primarily to reduce the irritation potential of foaming cleansing compositions based on anionic surfactants. Indeed, the most famous application of SLAA was its use in the first-ever baby shampoos, which were designed to be nonstinging to the eyes. The surprising ability of SLAA and related amphoacetates to reduce the irritation and eliminate the eye sting associated with anionic surfactants was discovered by Mannheimer and patented in 1957.16 Shortly thereafter, Masci and Poirier applied Mannheimer’s amphoteric-anionic blends in combination with ethoxylated nonionic surfactants, e.g., polysorbate 20, to develop shampoo compositions that were nonirritating and nonstinging to the eyes.17, 18 These formulas evolved into the first generation of baby shampoo with the first “No More Tearsa” claim. To this day, SLAA still finds application in the formulation of gentle foaming cleansers for face, hair and body.
More recently, SLAA has played an important role in the development of structured surfactant systems for high-performance cleansing formulations.19–21 These surfactant blends are comprised of SLAA in combination with an anionic surfactant such as sodium trideceth sulfate, and optionally other structurants, such as cocamide MEA. The blends are designed to form lyotropic liquid crystalline phases, e.g., lamellae and multilayer vesicles, that exhibit high yield value for suspending and stabilizing dispersed phases, such as insoluble oils or particulates. Structured surfactant systems have proven especially useful for the formulation of cleansers containing high loads (10–15% w/w) of emollients. During in-use dilution, these structured formulations “break” to deliver the emollients for conditioning skin or hair while simultaneously providing good lathering, even when loaded with high levels of oil phase that would normally depress foam.
1. R Diez, A review of the progress of foaming cleansing products during the last fifty years, J Cosmet Sci 61(2) 187–188 (Mar/Apr 2010)
2. T Schoenberg, Formulating with betaine and amphoteric surfactants, HAPPI 34(10), 73–74, 76, 78–79 (1997)
3. HI Leidreiter, B Grüning and D Kaseborn, Amphoteric surfactants: Processing, product composition and properties, Int J Cosmet Sci 19(5) 239–253 (1997)
4. R Vukov, D Tracy, M Dahanayake, PJ Derian, JM Ricca and F Marcenac, A new generation of imidazoline-derived amphoteric surfactants, in Proceedings of the World Conference on Lauric Oils: Sources, Processing and Applications, AOCS Press: Champaign, IL USA (1994) pp 147–154
5. Sodium Lauroamphoacetate, Monograph ID 1468, in the International Cosmetic Ingredient Dictionary and Handbook, 13th edn, Personal Care Products Council: Washington, DC, USA (2010)
6. HA Wittcoff, BG Reuben and JS Plotkin, Industrial Organic Chemicals, John Wiley and Sons, Ltd: Hoboken, NJ, USA (2004)
7. US 2528378, Metal salts of substituted quaternary hydroxyl cycloimidinic acid metal alcoholates and process for preparation of the same, HS Mannheimer, assigned to JJ McCabe Jr and HS Mannheimer (Oct 31, 1950)
8. US 2970160, Process for making amphoteric surface active agents, RA Walker, assigned to Johnson and Johnson (Jan 31, 1961)
9. A Behler, M Biermann, K Hill, H-C Raths, M-E Saint Victor and G Uphues, Industrial surfactant synthesis, in Reactions and Synthesis in Surfactant Systems, Surfactant Sci Ser, vol 100, J Texter, ed, CRC Press: Boca Raton, FL, USA (2005) pp 1–44
10. DT Floyd, C Schunicht and B Gruening, Zwitterionic and amphoteric surfactants, ch 15 in Handbook of Applied Surface and Colloid Chemistry, vol. 1, K Holmberg, ed, John Wiley and Sons, Ltd: West Sussex, England (2001) pp 349–372
11. Miranol Ultra L-32, Rhodia product data sheet N000762, Rhodia Novecare, Cranbury, NJ, USA (Dec 2010)
12. Miranol HMD, Rhodia product data sheet N002190, Rhodia Novecare, Cranbury, NJ, USA (Mar 2007)
13. A Mehling, M Kleber and H Hensen, Comparative studies on the ocular and dermal irritation potential of surfactants, Food Chem Tech 45(5) 747–758 (2007)
14. Final report on the safety assessment of cocoamphoacetate, cocoamphopropionate, cocoamphodiacetate and cocoamphodipropionate, J Am Coll Toxicol 9(2) 121–142 (1990)
15. MT Garcia, E Campos, A Marsal and I Ribosa, Fate and effects of amphoteric surfactants in the aquatic environment, Envir Int 34(7) 1001–1005 (2008)
16. US 2781376, Detergent sulphonic acid and sulphate salts of certain amphoteric detergents, HS Mannheimer, assigned to JJ McCabe Jr and HS Mannheimer (Feb 12, 1957)
17. US 2999069, Detergent composition, JN Masci and NA Poirier, assigned to Johnson and Johnson (Sep 5, 1961)
18. US 3055836, Detergent composition, JN Masci and NA Poirier, assigned to Johnson and Johnson (Sep 25, 1962)
19. US 8029772, Stable surfactant compositions for suspending components, S Frantz, PL Cotrell and SA Warburton, assigned to Rhodia, Inc (Oct 4, 2011)
20. Miracare SLB-205/N, Rhodia product data sheet N002012, Rhodia Novecare, Cranbury, NJ, USA (May 2010)
21. Miracare SLB-365/G, Rhodia product data sheet N000939, Rhodia Novecare, Cranbury, NJ, USA (May 2010)