NMR study of the influence of pH on phenol sorption in cationic CTAB micellar solutions

Interactions between phenol and cationic cetyltrimethylammonium bromide (CTAB) micelles have been investigated by means of nuclear magnetic resonance spectroscopy. The combined use of 1H and NOESY techniques revealed that phenol has different preferred locations of interaction depending on the pH. At neutral pH (6.70) conditions, phenol molecules are preferentially located in the outer micelle region, at the micelle–water interface, while at more basic pH (9.94), the deprotonated phenol molecules (C6H5O−) are immersed into the palisade layer of the micelle. In addition, quantitative estimates of the solubilized fraction of phenol were obtained by using PFG-NMR. The results indicate that the phenol–CTAB interactions, although already present in neutral pH conditions, are largely favored in basic conditions as a consequence of the strong electrostatic interaction between the negatively charged phenolate ions and the positive charge of the cationic surfactant head group.