Solubilization of Nitrogen Heterocyclic Compounds Using Different Surfactants

In order to develop surfactant-enhanced remediation for nitrogen heterocyclic compounds (NHCs) (aniline, indole, and quinolone), the solubilization properties of micellar solutions of five surfactants, namely sodium dodecyl sulfate (SDS), rhamnolipid (RL), polysorbate (Tween 80), sorbitan monolaurate (Span 20), and iso-octyl phenoxy polyethoxy ethanol (TX-100) were investigated in this work. The solubilization capacities were quantified using critical micelle concentration (CMC) as well as thermodynamic and kinetic experiments. Besides, nuclear magnetic resonance (1H NMR) spectra were used to infer the locus of NHCs solubilized by SDS and TX-100. The results from the properties of five surfactants indicated that CMC was affected by temperature, while the micellization was spontaneous and could be both endothermic and exothermic based on the type of surfactant and temperature. Furthermore, the difference in compensation temperature was caused by different solubilization mechanism for various surfactants. The solubilization results showed that the solubilization of NHCs in the surfactant solutions followed a pseudo-first-order kinetic model. Meanwhile, the change in proton’s chemical shift depended on the structure of NHCs and the solubilization ability of surfactants. Finally, the orthogonal experiment (L16(43)) was elementarily designed to optimize the solubilization conditions of indole and the results showed that RL could be a better choice for solubilizing NHCs.