High efficiency of biosurfactants in stabilizing oil micro-droplets within the aging time scale of milliseconds: a microfluidic study

Abstract

Rapid adsorption of surfactants onto a freshly formed interface is vital for emulsification because emulsification is a competitive process occurring between the very short time span of interface formation and surfactant mass transport. The biosurfactant surfactin has been previously reported to reach adsorption equilibrium at the hydrophobic/hydrophilic interface within hundreds of milliseconds and rapidly reduce the interfacial tension compared to chemically synthesized surfactants. According to a prior study, surfactin is expected to exhibit good performance in stabilizing micro-droplets of oil within the aging time scale of milliseconds. Herein, the stabilities of micro-droplets of n-hexadecane in the presence of a biosurfactant, surfactin (C₁₅-SFT), and a chemically synthesized surfactant, sodium cetyl benzene sulfonate (8-SCBS), were investigated using a microfluidic method. The coalescence frequency of micro-droplets, the evolution of micro-droplet size, and the coalescence time of micro-droplets were evaluated. The results indicated that C₁₅-SFT exhibited superiority over 8-SCBS in stabilizing the micro-droplets of n-hexadecane. Biosurfactant C₁₅-SFT effectively reduced the fusion probability between oil droplets and elongated the coalescence time compared to 8-SCBS, and these phenomena were obvious at a shorter aging time (150 ms) and lower surfactant concentration (0.1 × critical micelle concentration). The stabilities of micro-droplets increased with aging time and the bulk concentration of surfactants. Stable micro-droplets of n-hexadecane were formed in 1 × 10⁻⁴ mol L⁻¹ C₁₅-SFT solution at 600 ms aging time, and the bulk concentration was 1 × 10⁻³ mol L⁻¹ in the case of 8-SCBS. The micro-droplets rarely coalesced in the presence of 1 × 10⁻⁴ mol L⁻¹ C₁₅-SFT after 600 ms aging time, but the micro-droplets in 1 × 10⁻⁴ mol L⁻¹ 8-SCBS coalesced frequently in the midstream and downstream of the coalescence chamber, and big droplets were dominant in the emulsion. The coalescence time of micro-droplets stabilized by C₁₅-SFT was obviously longer than that of those stabilized by 8-SCBS under the same condition, indicating that the interfacial film formed by C₁₅-SFT has much strength to resist coalescence during collisions. This work is helpful for understanding the activity of lipopeptides in the very short early stage of the emulsification process, laying the foundation for biosurfactant research in the fields of enhanced oil recovery, bioremediation of contaminated water or soil, etc.