Interfacial dilational rheological behaviors of N-acyl amino acid surfactants: effects of amino acid residues

Abstract

Amino acid surfactants have attracted increasing interest in recent years. As biological raw materials, amino acids have diverse molecular structures, which grant the derived surfactants various properties. Herein, the effects of N-methyl and polar, nonpolar, and aromatic α-substituents of amino acids on the interfacial behaviors of amino acid surfactants were systematically studied using interfacial dilational rheological experiments. The amide groups in the glycinate surfactant formed hydrogen bonds, which enhanced the intermolecular interactions. In comparison, an extra N-methyl group in sarcosinate surfactants prevented intermolecular hydrogen bonding and thus decreased the interfacial dilational modulus. The nonpolar α-substituents of alaninate and valinate surfactants enhanced the tendency of surfactant molecules to adsorb to the interface, but caused steric repulsion for hydrogen bond formation, which decreased the dilational modulus but promoted the relaxation processes. The polar α-substituents of serinate and threoninate surfactants enhanced the intermolecular hydrogen bonding and slowed down relaxation processes. The aromatic α-substituent in phenylalanate surfactant improved its interfacial activity. Moreover, the phenyl enhanced the intermolecular interactions through NH-π and π-π interactions. As a result, the interfacial dilational modulus was significantly enhanced. The systematic study of the relationship between amino acid structure and the interfacial behaviors of surfactants provides the research basis for the development and application of N-acyl amino acid surfactants.