An ¹H NMR investigation into the loci of solubilization of 4-nitrotoluene, 2,6-dinitrotoluene, and 2,4,6-trinitrotoluene in nonionic surfactant micelles

In this study, the enhancement in solubility of 4-nitrotoluene (4-NT), 2,6-dinitrotoluene (2,6-DNT), and 2,4,6-trinitrotoluene (TNT) and their loci of solubilization were determined in nonionic surfactant solutions containing linear alcohol ethoxylates (Brij-35, Brij-58, Brij-78, Brij-97 and Brij-98), alkylphenol ethoxylates (Tergitol NP-8 to NP-40), and octylphenol ethoxylates (Triton X-100, X-165, and X-305). In all surfactant solutions, 4-NT had the highest molar solubilization ratio (MSR) and TNT the lowest. For all surfactants, increasing the number of ethoxylate groups (EO) while keeping the hydrocarbon chain length constant decreased or did not significantly change the MSR. To determine the locus of solubilization, ¹H NMR spectroscopy was used to determine how the presence of a nitroaromatic solute changed the chemical shifts of the various hydrogen atoms on the surfactant molecule. For the linear alcohol ethoxylates, the greatest change in chemical shift for all three nitroarenes occurred at hydrogens in the hydrocarbon core of the micelle. At higher solute concentration, the nitroarenes spread to the first EO group near the core/shell interface. For both the Tritons and Tergitols, the chemical shift for a hydrogen on the phenyl ring changed the most upon the addition of the nitroarenes. As the nitroarene concentration increased, the chemical shifts for the hydrogens on the first EO group changed for all Triton and Tergitol surfactants. For all surfactants with shorter EO chains, the shifts for all ethoxylate hydrogens changed at higher nitroarene concentration. These results suggest that the solutes are preferentially solubilized at the core/shell interface but spread further into the shell and the core as solute concentration increases.