Amino-Acids Surfactants and n-Octanol Mixtures─Sustainable, Efficient, and Dynamically Triggered Foaming Systems

Surfactants with amino-acid heads (AASs), namely N-lauroyl-l-alanine (C12-ALA), N-lauroyl-l-leucine (C12-LEU), and N-lauroyl-l phenylalanine (C12-PHE) were mixed with n-octanol (C8OH) and thoroughly investigated via surface tension and foamability experiments. Significant differences between the selected AAS were observed. Moreover, the results obtained under equilibrium and dynamic conditions were remarkably different in terms of surface activity. The surface tension measurements (equilibrium conditions) indicated that the C12-ALA/C8OH mixture exhibits the highest synergistic effect. For the C12-LEU/C8OH system, a moderate synergism was observed, while for C12-PHE/C8OH, the effect was antagonistic. In contrast, in foamability experiments (dynamic conditions), all of the studied AAS/C8OH mixtures exhibited a spectacular synergistic effect at a wide concentration range. The observed effect was referred to as convection-activated surface activity of AAS/C8OH mixtures. The obtained experimental results were interpreted on the molecular level via all-atom detail molecular dynamics simulations (MD). The observed phenomenon was connected with the hydrogen bond-mediated aggregate formation in the bulk solution. Such aggregates act as reservoirs of surfactant molecules for supplementation of the adsorption coverage at the freshly formed liquid/gas interface. Additionally, the differences between the specific AAS were explained via the interplay of AAS–AAS and AAS-C8OH hydrogen bond affinities. The presented results showed an interesting example of a foaming system in which the surface activity can be controlled in situ via convection. This finding also significantly expands the range of potentially interesting molecules that can be used as efficient foaming additives and impacts the current understanding of the role of hydrogen bonding in designing of tuneable surfactant mixtures.

Partial financial support from the Polish National Science Centre grant no. (2020/38/E/ST8/00173 and 2022/45/N/ST8/02307) is acknowledged with gratitude. P.B. and J.Z. acknowledge the partial financial support to the project by the statutory research fund of ICSC PAS. We gratefully acknowledge Poland’s high-performance computing infrastructure PLGrid (HPC Centers: ACK Cyfronet AGH) for providing computer facilities and support within computational grant no. PLG/2023/016229.