NMR investigation of counterion binding to undecyl LL-leucinevalanate micelles

NMR spectroscopy was used to investigate the binding of cationic counterions to anionic micelles formed by undecyl LL-leucinevalaninate (und-LV). Amino acid-based surfactants like und-LV are green alternatives to commercial surfactants. Monomeric and polymeric forms of und-LV micelles have also been used as chiral selectors in capillary electrophoresis (CE) separations. The counterions investigated were Na+, the tetraethylammonium ion, L-Lysine, 1,4-diaminobutane, 1,6-diaminohexane, and cis and trans isomers of 1,2-diaminocyclohexane and 1,4-diaminocyclohexane. NMR measurements of counterion and micelle diffusion coefficients were used to calculate micelle radii and the mole fraction of counterion molecules bound to the micelles. Two-dimensional NMR experiments were used to investigate the structures of the counterion: micelle complexes. The mole fraction of bound counterions did not change with pH in solutions containing Na+ or tetraethylammonium counterions. With all other counterions, however, the mole fraction of micelle-bound counterions was higher below pH 10 when counterions had a + 1 or +2 charge, and lower above pH 10-10.5 when the counterions were neutral or zwitterionic. Changes in micelle radii with pH and two-dimensional NMR experiments suggested that L-Lysine, 1,4-diaminobutane, and 1,6-diaminohexane bound parallel to the micelle surface with their amine functional groups interacting with different surfactant monomers. In contrast, the diaminocyclohexane isomers were found to bind perpendicular to the micelle surface. The mole fractions of micelle-bound cis-1,4-diaminocyclohexane and trans-1,4-diaminocyclohexane were very similar to one another. However, with 1,2-cyclohexanediamine, at low pH the mole fraction of micelle-bound counterions was larger for the cis isomer than for the trans isomer.