Physical chemical characteristics of dicarboxylic amino acid-based surfactants

Three dicarboxylic amino acid-based surfactants, the disodium salts of dodecylaminomalonic acid, dodecylaspartic acid and dodecylglutamic acid, having one, two and three carbon atoms, respectively between the carboxyl groups, have been studied with respect to solution behavior and adsorption at the air-water interface, as well as on a number of solid surfaces. The dicarboxylic surfactants had 4-5 times higher CMC than the dodecylglycinate, which only has one carboxyl group. The role of the amide bond for the self-assembly process was assessed by comparing sodium dodecylglycinate with sodium dodecylsarcosinate. The former can build hydrogen bonds between adjacent molecules via the amide groups but the latter which has an N-methyl group, cannot, It was found that the ability to form intermolecular hydrogen bonds lead to tighter packing both at the air-water interface and in micelles. The CMC was not affected, however. Dodecylglutamate was found to be very calcium tolerant compared to the two other dicarboxylic surfactants. This was attributed to differences in chelating calcium. Whereas dodecylaminomalonate and dodecylaspartate can form intramolecular chelates with calcium ions, dodecylglutamate prefers to form an intermolecular complex. Formation of an intermolecular complex leads to tight packing at the air-water interface and to very low surface tension values. The ability to form intramolecular chelates is advantageous for adsorption at calcium-containing surfaces, however. Whereas dodecylaminomalonate and, in particular, dodecylaspartate adsorb strongly on hydroxyapatite, dodecylglutamate shows very weak adsorption. On other surfaces, where there exist no specific interactions involving the surfactant headgroup, the three surfactants gave similar adsorption behavior. (C) 2011 Elsevier BM. All rights reserved.