Synthesis and properties of pH-dependent N-acylglutamate/aspartate surfactants

Amino acid surfactants are a kind of surfactants with ultra-mildness and certain antibacterial ability, which are expected to be used more widely. Two amino acid surfactants containing dicarboxyl group, sodium N-lauroyl glutamate/aspartate (referred as SLG and SLA), were prepared by amidation of the sodium glutamate/aspartate with methyl laurate. The SLG and SLA were characterized by mass spectrometry, 1H NMR and 13C NMR. The purity of products were determined to be 99% by HPLC-ELSD. And the pKa, surface activities, aggregate, foam properties and emulsifying properties of the two surfactants in aqueous solution at different pH values were investigated. The results show that the SLG and SLA exhibit pH-dependent protonation-deprotonation behavior. The surface tension, critical micelle concentration (cmc), foam stability are better at pH = 6 and 7 and the foaming, emulsifying ability are better at pH = 8, 9 and 10 in term of surfactant behavior. Both SLG and SLA can form spherical vesicles at about 5 times cmc at pH= 6–8 and micelle at pH= 9–10. SLA has excellent surface activity, foam and emulsifying properties compared to SLG.