Alkylamidopropyl betaines

The wide availability of relatively inexpensive dimethylaminopropylamine (DMAPA) allows surfactant producers to convert economic triglycerides, fatty acids and methyl esters into amido -functional tertiary amines that may then be quaternized with sodium chloroacetate to produce alkylamidopropyl betaines (see Figure 6.15). The most economically significant of these is cocamidopropyl betaine which can be produced from a variety of feedstocks and lauramidopropyl betaine which is generally produced from lauric acid. These are widely used secondary surfactants in consumer products such as shampoos, bath products, washing up liquids and other cleaners. 

Over the past decade, studies by DeGroot [6] and others have indicated that free amines present in cocamidopropyl betaine appear to be sensitizers. It has not been clear as to whether the problem was due to free DMAPA or cocamidopropyl dimethylamine but recent evidence suggests it maybe due to both. The producers of these products reacted to the problem and the typical alkylamidopropyl betaine produced today contains less than 10 ppm of free DMAPA and less than 0.5% of cocamidopropyl dimethylamine (Table 6.9). 

Measurements of commercial alkylamidopropyl betaines, in contrast to the values reported for the pure alkyl betaines, give significantly lower surface tensions at concentrations above the CMC (Table 15.7) (57). 

These results show that with commercial alkylamidopropyl betaines, characterized by fatty acid compositions (instead of one singe fatty acid) and in cases containing by-products derived from the synthesis, very low surface tensions are achievable. This effect is certainly related to the known effects in surfactant mixtures, which usually show lower surface tensions than the pure products. 

Table 15.7. Surface tension of various alkylamidopropyl betaines (57)...

The alkylamidopropyl betaines, as a rule, are compatible with the anionic detergents over a wide range of concentration. The simpler alkyl betaines show some incompatibilities with alkyl sulfates at concentrations at which the betaine s concentration is about one half that of sodium lauryl sulfate [15-17]. The nature of these incompatibilities is largely dependent on the nature of anionic species present. The pH of the system also plays a role in compatibility. At the pHs normally encountered in shampoos, a competitive reaction occurs between the presumed zwitterionic inner salt and the complex salt formation from the interaction of the anionic detergent with the quaternary nitrogen. 

The workhorse amphoteric surfactants are the alkyl- and alkylamidopropyl betaines, containing C8-C18 linear chain distributions that are derived from coconut or palm kernel oil, or ethylene-based alpha olefins. The alkyl betaines are prepared from ADMA feedstocks that are typically derived from alpha olefins and dimethyl amine throngh hydrohalogenation and alkylation reaction steps. The alkylamidopropyl betaines are based on tertiary amines derived from whole triglycerides or their fractionated derivative fatty acids or methyl esters reacted with DMAPA. 

SCHEME 1.1 Preparation of alkyldimethyl betaines and alkylamidopropyl betaines. 

The amphoteric surfactants are relatively expensive products compared to anionic surfactants. Thus, it is not surprising that they are primarily being utilized at low concentrations in cosmetic formulations. A review of 438 shampoos of the U.S. market reveals that appreciable quantities, for example, >5% of alkylamido betaines and imidazolinium surfactants were found in only 8.7% and 13.5%, respectively, of the investigated shampoos. Alkyl betaines were found in a limited number of cases, whereas sulfo betaines were not found in this stndy [4,5]. Since these studies were made, the market has focused on mild products. The change in the U.S. market from bar to liquid soaps has increased the use of alkylamidopropyl betaines. 

The alkylamidopropyl betaines exhibit many useful properties in personal care formulation. Some are listed below ... 

Analogous to the betaine surfactants are the sultaines and hydroxysultaines. Both are derived from either alkyl dimethylamine or alkylamidopropyl dimethylamine. Rather than being the reaction product of sodium chloroacetate, they are manufactured from a tertiary amine and either propane sultone or sodium propanechlorohydrin sulfonate (CHPS). 

---