Effects of fatty acyl chains on the interfacial rheological behaviors of amino acid surfactants

Abstract

Amino acid surfactants derived from vegetable oils have attracted significant interest in the scientific community due to their excellent biocompatibility and low environmental impact. Because of the wide variety of vegetable oils, derived N-acyl amino acid surfactants may feature various fatty acyl groups and therefore offer diverse properties. According to the composition of commonly used vegetable oils, three main characteristics of fatty acyl structures, namely fatty acyl chain length, the number of CC bonds, and hydroxyl substituent, were summarized in this paper and a series of N-fatty acyl glycinate surfactants were synthesized. The specific effects of these three structural factors on the surface tension and interfacial rheological properties were systematically studied. In doing so, we found that an increase of fatty acyl chain length enhanced the interfacial activity and intermolecular interactions of N-fatty acyl glycinate surfactants. Thus, glycinate surfactants with longer fatty acyls could generate more compact adsorption films with a higher dilational modulus. The cis CC bonds in oleoyl and linoleoyl chains were found to bend the long hydrophobic tails, which might affect the compact arrangement of surfactant molecules and increased the viscoelasticity of interfacial films. In addition, the hydroxyl group on ricinoleoyl was found to inhibit the close packing of the hydrophobic tails and reduced intermolecular interactions. As a result, more viscoelastic films were formed by sodium N-ricinoleoylglycinate. The obtained results gained insights into the relationships between fatty acyl structural characteristics and the interfacial arrangements of N-acyl amino acid surfactants, which present a theoretical foundation for surfactant design and further application.