FT-IR and nuclear overhauser enhancement study of the state of urea confined in AOT-reversed micelles

The system urea/sodium-bis(2-ethylhexyl)sulfosuccinate (AOT) dispersed in CCl4 and benzene-d6 was investigated by viscosimetry, FT-IR and high-resolution 1H NMR spectroscopy, as a function of urea/AOT molar ratio (Rurea) at fixed AOT molal concentration (0.3467 mol kg−1) and as a function of AOT concentration at fixed Rurea (0.5) at 25 °C. The experimental data are consistent with the hypothesis that urea is encapsulated as small-size hydrogen-bonded cluster in the hydrophilic micellar core of the AOT-reversed micelles and that this structure is maintained well above the volume fraction of the dispersed phase, where a percolative transition occurs. Intermolecular nuclear overhauser enhancements (NOEs) were observed between urea amidic protons and AOT protons located in the proximity of the sulfonate group, thus proving that the solubilizate/surfactant interaction is mainly localized around the polar head of AOT within the micellar core. Long-range NOEs were also detected between urea NH2 and methyl group of the β-alkyl chain of AOT, suggesting that the two alkyl chains of the surfactant play different roles in the interaction with urea and the bulk solvent. Some changes to the structural and dynamical properties were observed during solubilization in AOT-reversed micelles.