Synergistic Effects of Bio-derived Cyclic Lipopeptide Surfactin in Sustainable Surfactant Solutions

Surfactants play an important role in our daily lives, with applications in a wide range of products, from cleaners to pharmaceuticals, food products to cosmetics. However, the main use of petroleum-derived surfactants in these commodities has raised concerns due to their low biodegradability and potential toxicity. Mixing different types of surfactants has long been a subject of interest in both industrial and academic research. The combination of anionic and cationic surfactants, for example, has shown the ability to induce synergistic effects, enhancing the overall properties of the surfactant mixtures. This can result in a reduction in the total surfactant consumption, particularly when it comes to lowering the critical micelle concentration (CMC), a critical parameter in surfactant performance. While such synergies hold promise for sustainability, the negative environmental impacts of petroleum-derived surfactants persist. Surfactin (SF), a cyclic heptapeptide biosurfactant produced by Bacillus subtilis, emerges as a promising alternative to conventional petroleum-derived surfactants. It boasts advantages such as high biodegradability and specific activity under biological conditions. Moreover, SF exhibits unique self-assembly behavior and the ability to act as an ionophore for various cations. However, its higher cost compared to conventional surfactants has led to its blending with them in various formulations to amplify specific functions. Despite significant progress in SF production and surface property analysis, there remains a lack of detailed information on surfactant mixtures comprising SF and petroleum-derived surfactants. Previous studies have hinted at the potential for synergistic effects in such mixtures, but the specific mechanisms and interactions are yet to be fully understood.

In a recent study published in the ACS Sustainable Chemistry & Engineering Journal, led by Dr. Satohiro Yanagisawa, Dr. Tomohiro Imura, and Dr. Toshiaki Taira from the Kaneka Corporation and National Institute of Advanced Industrial Science and Technology in Japan. The authors explored the potential of synergistic effects between and the petroleum-derived anionic surfactant, sodium dodecyl sulfate (SDS), to reduce the CMC and minimize the environmental impact of surfactant usage. The researchers investigated the synergistic effects of SF when added to SDS, a model anionic surfactant. The role of SF’s cyclic peptide moiety in this synergy was examined and compared to a linear form of SF (LSF) with the same amino acid sequence but a different topology. The results showed that the addition of SF to SDS significantly reduced the CMC values, leading to a substantial reduction in total SDS consumption without compromising surface activity. This reduction in CMC is of great importance as it means that lower concentrations of SDS can be used to achieve the same surfactant effect, which has implications for sustainability.

The authors measured the surface tension-concentration plots of mixed SDS/SF solutions. The results revealed that the CMC values of these mixtures were three orders of magnitude lower than that of pure SDS. The addition of SF consistently reduced the CMC values as the mole fraction of SF increased. This phenomenon was confirmed by comparing the results with LSF, which exhibited a weaker CMC-lowering effect, emphasizing the importance of SF’s cyclic peptide moiety in inducing the observed synergy. Furthermore, the researchers calculated molecular interaction parameters, including γ_CMC (surface tension at CMC), A (Gibbs adsorption equation parameter), and (β_σ − β_M) (indicative of synergy), using regular solution theory. These calculations supported the experimental results, demonstrating that the mixed SDS/SF solutions exhibited significant synergistic effects. To gain further insights into the interactions between SDS and SF, the researchers conducted ¹H nuclear magnetic resonance (NMR) titration experiments. These experiments revealed that SDS and SF formed inclusion complexes through hydrogen bonding interactions. The chemical shift changes observed in the NMR spectra provided evidence of intermolecular interactions between the sulfate moiety of SDS and the cyclic peptide moiety of SF.

The study’s findings offer exciting prospects for the use of SF in combination with petroleum-derived anionic surfactants to reduce their environmental impact. By leveraging SF’s ability to screen electrostatic repulsion and forming inclusion complexes with SDS, the researchers achieved a remarkable reduction in the CMC of SDS. This means that less SDS is required to achieve the same surfactant effect, which is both cost-effective and environmentally friendly. With the increasing concern for sustainability, the development of surfactant formulations that minimize environmental impact while maintaining performance is of utmost importance. The combination of SF and SDS showcases the potential for such formulations, not only in cleaning and personal care products but also in various industries, including pharmaceuticals, food, agrochemicals, plastics, and cosmetics. Furthermore, the authors highlighted the importance of understanding the molecular-level interactions between surfactants to design more effective and sustainable solutions.