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

Abstract

The interfacial adsorption properties of the novel hydroxy-group-containing amino-acid-type surfactant, N-alkanoyl-N-(2-hydroxyethyl)-β-alanine (Cn-EtOH-βAla, where n is the length of the alkyl chain; n = 8, 10, 12, 14, and 16; EtOH is the hydroxyethyl group; βAla is β-alanine), and the hydroxy-group-free conventional amino-acid-type surfactant, N-dodecanoyl-N-methyl-β-alanine (C12-Me-βAla), were investigated for comparison purposes in an alkaline solution. Cn-EtOH-βAla (where n = 10–14) possessed a significant minimum critical micelle concentration (CMC) in surface tension vs. concentration plots, contrary to the conventional surfactant C12-Me-βAla. The area occupied per molecule for C12-EtOH-βAla (0.316 nm2) was considerably decreased compared with C12-Me-βAla (0.683 nm2); this was attributed to hydrogen bonding between the hydroxy group and water/carboxylate ions in Cn-EtOH-βAla, 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 Cn-EtOH-βAla 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.