NMR Spectroscopy and the Effects of Enantiomerism on the Micelle Formation of Potassium N-n-Dodecanoylalaninate

Abstract

Normal micelle formation has been observed for isotropic solutions of the optically active surfactants, potassium N-n-dodecanoyl- d- and - l-alaninate, and their racemic mixture in deuterium oxide, and in the binary solvent, ca. 11.4 wt% 1,4-dioxane in deuterium oxide. These systems display very low critical micelle concentrations, which are reflected in the concentration dependence of the carbon-13 nuclear magnetic resonance chemical shift and proton magnetic resonance peak position. Critical micelle concentrations are higher for the surfactant–binary solvent systems (cmc ca. 0.019 mat 30–37°C) compared to the similar values of the d-, l-, and d, l-surfactants in deuterium oxide (cmc = 0.011–0.015 mat 30–37°C). The presence of a chiral center does not appreciably alter the cmc value, nor the micellization properties of the optically active surfactants compared to the racemate. The variation of the cmc and micellization behavior among the d-, l-, and racemic surfactants are described in terms of enantiomerism, entropic effects, and the dielectric permittivity of the medium. Information on the structure of the micelles is afforded from difference (δ(micelle) − δ(monomer)) carbon-13 NMR chemical shifts and proton magnetic resonance peak positions as a function of the position of the nucleus in the surfactant ion pair.