Abstract:

Amino acid surfactants（AAS）have attracted considerable attention due to their low irritation and excellent foaming properties. However, the poor solubility of AAS in oily systems limits their extensive application in bath oils, so currently conventional sulfate-based surfactants remain dominant in bath oil formulations. In this work, caprylic/capric triglyceride（GTCC）was used as the oil phase and laureth-4（LE-4）was used as the emulsifier. Ten AAS were selected as candidates of the key component in bath oil, which were then screened by transparency test combined with pseudo-ternary phase diagram. It was confirmed that sodium lauroyl methylaminopropionate（SLMA）exhibited the best compatibility with the system, featuring a broad region of transparent liquid in the phase diagram and good formulation applicability. Thus it provided a basis for subsequent formulation optimization. The mass ratio of GTCC∶LE-4∶SLMA was optimized to further improve the overall performance of the bath oil, which was determined to be 4∶3∶3 according to the foaming performance test and protein damage assay. At this ratio, the product achieved the foam volume of 127.3 mL, foam half-life of 677.5 s, and protein damage value as low as 0.729 1 g/L, indicative of excellent foaming performance and mildness. Compared with the traditional sulfate system, the SLMA-based system showed better performance in foaming, mildness, and moisturization while maintaining comparable cleansing ability. These advantages could be clearly perceived in consumer testing. In addition, the system exhibited excellent stability in centrifugation and multi-temperature storage tests. Human skin patch test confirmed no skin irritation, indicative of good safety. This work innovatively applied AAS to transparent oily systems for the first time, which provided a new strategy for the development of high-end bath oils with promising industrialization prospects.

Key words: amino acid surfactant, oil system, bath oil, foaming performance, mildness