Amino acid-based surfactants: more than just ‘natural’!

 Additionally, amino acids can be linked together in different combinations to create shorter (peptides) and longer (proteins) chains. Both fulfil a variety of different vital functions in the human body: Collagen, for instance, is the most frequently occurring protein in the human body. It is composed of three intertwined chains (triple helix) to lend structure to connective tissue. Simply for spatial arrangement reasons in this protein the smallest amino acid, glycine (Fig 1b), is the main component. 

The water solubility of amino acids also makes them an attractive choice for the hydrophilic part of surfactants. When an amino acid is linked with a triglyceridederived fatty acid (e. g. from coconut oil), surfactants (N-acyl derivatives, “amino acid surfactants“, Fig 2a) exclusively based on natural components are created. When the amino acid is produced via a fermentation process starting from glucose or its derivatives, the surfactants even are completely composed of renewable raw materials (e. g. cocoyl glutamate). Those surfactants are an ideal choice for natural cosmetic applications. 

Contrary to expectations, the development of amino acid surfactants is not a result of the current trend towards ‘green’ surfactants. The first examples were patented in the 1930s.1 Already at that time one candidate, a derivative of sarcosine (Figs 1c, 2a), was introduced to the market. The original idea behind this development was to create surfactants alternative to soaps (Fig 2b) to overcome their drawbacks (precipitation in pH-neutral aqueous solutions, sensitivity to water hardness).2 The ‘trick’ is to insert a suitable spacer between the hydrophobic part and the carboxylate group (Fig 2). By the way, in the 1930s also another solution was found: To introduce a sulfo-group and to “block” or replace the carboxylic group of fatty acids (e. g. acyl isethionates2 and fatty alcohol sulfates).3