Unique interfacial adsorption behavior of a hydroxy group-containing amino acid surfactant

The interfacial adsorption properties of the novel hydroxy-group-containing amino-acid-type surfactant, N-alkanoyl N-(2-hydroxyethyl)-beta-alanine (C-n-EtOH-beta Ala, where n is the length of the alkyl chain; n = 8, 10, 12, 14, and 16; EtOH is the hydroxyethyl group; beta Ala is beta-alanine), and the hydroxy-group-free conventional amino-acidtype surfactant, N-dodecanoyl-N-methyl-beta-alanine (C-12-Me-beta Ala), were investigated for comparison purposes in an alkaline solution. C-n-EtOH-beta Ala (where n = 10-14) possessed a significant minimum critical micelle concentration (CMC) in surface tension vs. concentration plots, contrary to the conventional surfactant C-12-Me-beta Ala. The area occupied per molecule for C-n-EtOH-beta Ala (0.316 nm(2)) was considerably decreased compared with C-12-Me-beta Ala (0.683 nm(2)); this was attributed to hydrogen bonding between the hydroxy group and water/carboxylate ions in C-n-EtOH-beta Ala, which facilitated the dense adsorption and efficient orientation of the surfactant at the air/water interface. The relationship between the logarithm of the CMC and the n value of C-n-EtOH-beta Ala indicated that the surfactant exhibited nonionic-like behavior because the hydrogen bonding between the hydroxy groups and carboxylate ions suppressed the electrostatic repulsion between hydrophilic groups.