Role of Counterions in the Solubilization of Benzene by Cetyltrimethylammonium Aggregates. A Multinuclear NMR Investigation^†

This paper reports about the influence of counterions on the solubilization of benzene in aqueous solutions of cetyltrimethylammonium surfactants (CTAX, X = Br^-, Cl^-, NO₃^-, CH₃SO₃^-, and (SO₄)^2-_1/2, CTAB, CTAC, CTAN, CTAMs, and CTAS, respectively) investigated by ¹H, ¹³C, and ¹⁴N NMR spectroscopy, at [CTAX] = 0.10 M. Benzene solubility and the size of the aggregates upon benzene addition depend strongly on the nature of counterion. Aqueous CTAC, CTAMs, and CTAS solutions reach benzene saturation up to [benzene]/[CTAX] ∼ 3 without showing any aggregate growth; on the other hand aqueous CTAB and CTAN solutions are able to solubilize a smaller amount of benzene up to [benzene]/[CTAX] ∼ 1 and the solute addition promotes a sphere to rod transition. ¹H chemical shifts of the headgroup signals, namely, the trimethylammonium and four methylenes of the aliphatic chain, undergo a more relevant upfield shift upon benzene addition with respect to the hydrophobic chain signals. ¹H NMR headgroup signals correlate to [benzene] through cubic equations, while the other signals do not show any simple correlation to [benzene]. A qualitative model in which benzene displaces water from the clefts which characterize the surface of the micellar aggregates is in agreement with all the experimental data.