Soaps are salts of fatty acids and, not in the distant past, were the mainstay of shampoo products. In soft water, they lather copiously, cleanse well, and leave the hair in a well-conditioned style. Unfortunately, in hard water the lather is poor, and as the soap combines with calcium or magnesium salts present in hard water it deposits on hair a dulling film. The introduction of synthetic surfactants brought about the end of soap-based shampoos,
although some products still contain a small quantity of soap to exploit its conditioning property.
Alkyl sulfates are the most widely used anionic in shampoos, displaying excellent foaming and cleansing properties unaffected by hard water. Lauryl sulfate is the dominate ingredient being present in most shampoo formulations in the form of its ammonium or triethanol ammonium salt at a level of 6 to 18% w/w. Although very effective cleansers, the alkyl sulfates, particularly at high concentrations, have a tendency to irritate the scalp and remove some lipid constituents of hair cuticle. To make the alkyl sulfate-based shampoos milder, they are frequently modified by incorporation of less-irritating alkyl ether sulfates or amphoteric surfactants.
Alkyl ether sulfates are sulfated products of ethoxylated fatty alcohols. They are more water soluble than alkyl sulfates, are excellent solubilizers for fragrances and other oleophilic additives, and are particularly suitable for formulations of clear shampoos. As alluded to earlier, these surfactants are less irritating than the alkyl sulfates and are used, at a higher degree of ethoxylation, in baby shampoos.
Alpha-olefin sulfonates are complex mixtures resulting from sulfonation of alpha-olefins. These detergents exhibit excellent foaming in the presence of sebum, are effective over a wide range of pH, and compare favorably with other surfactants in dermal and eye irritation .
Other anionic surfactants worthy of note include alkyl monoglyceride sulfates and alkyl sulfosuccinates. Both are very mild to the skin and, although the former are good foamers and can be used in shampoo formulation in their own right, the latter are primarily used in combination with alkyl sulfates.
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