Synergistic solubilization of phenanthrene using micro-nanobubbles and cationic surfactants: Universal verifying, amplifying, and strengthening the synergy

Micro-nanobubbles (MNBs) with their negatively charged surface and cationic surfactant with a positively charged hydrophilic head were designed to synergistically solubilize polycyclic aromatic hydrocarbons (PAHs). Through response surface method experimentation, the results showed that MNBs had no obvious solubilizing effect for the PAH, phenanthrene. However, MNBs and the cationic surfactant didodecyldimethylammonium bromide (DDAB) had a synergistic solubilization effect on phenanthrene. Through the analysis of physical and chemical factors, it could be inferred that the synergy was caused by a conversion in the structure of the vesicles formed by DDAB after the combination of DDAB and MNBs. This structure conversion improved the phenanthrene solubility. In addition to being amplified, this synergy was verified to be universal and could be strengthened. The synergy, which increased with increasing DDAB and MNB concentrations, could achieve up to about 3 times the solubilizing as that without MNBs. In addition to DDAB, MNBs could also synergistically solubilize phenanthrene with other cationic surfactants. Changing the gas source of MNBs from air to nitrogen or oxygen could increase the negative surface charge, which enhanced the synergistic effect between MNBs and cationic surfactants. These results demonstrate a novel method for synergistic solubilization of PAHs by surfactant and MNBs while it can decrease the excessive use of surfactants causing the pollution issues. The synergistic effect between cationic surfactants and MNBs has the potential to be widely used in environmental and medical fields.