Green Synthesis, Characterization, and Properties of Acyl Lysine, Serine, Threonine, and Methionine Derived from Three Types of Natural Oils

The development of environmentally benign products has been the subject of growing interest in the field of surfactant chemistry. Acyl amino acid surfactants bearing lysine, serine, threonine, and methionine residues were synthesized using natural oils extracted from coconut, palm kernel, and soybean as acyl donors. The chemical structures were confirmed by high-performance liquid chromatography (HPLC/MS) and infrared (IR) spectra. Their surface activities, ion-specific effects, detergency, and foam properties were studied systematically. The critical micelle concentration (CMC) values depend significantly on amino acid and oil types and follow the orders: (i) Lys > Thr approximate to Ser > Met and (ii) Coconut approximate to Palm kernel > Soybean oil. Interestingly, the ion-specific effects showing that the gamma(CMC) value decreases with increasing counterion size and hydrophobicity were observed, and the results were consistent with the famous Hofmeister series. The detergency ability of acyl amino acid surfactants is better than multiple traditional surfactants in distilled water. Although the detergency ability of our products for oil-soiled swatches decreased significantly in hard water, this problem was solved by the C-Lys-Na/AES mixed system showing excellent synergistic effects. Excellent foamability and foam stability were achieved for acyl threonine and serine bearing hydroxyl groups on their headgroups, suggesting that the packing of these surfactants at the air-water interface was assisted by hydrogen bonding.