NMR Study of Surfactant Micelle Shape Transformation with Concentration

Abstract

The micelle shape transformation of tert-octylpheny phenol polyethylene glycol ether (TX-100), sodium dodecyl benzene sulfonate (SDBS) and tetradecyl trimethyl ammonium bromide (TTAB) in heavy water solutions was studied by H-1 nuclear magnetic resonance (NMR) spectroscopy, including an NMR self-diffusion experiment. These experiments showed that the surfactants formed several shapes of micelles (spherical, ellipsoidal, and rodlike) at the respective concentrations which were far above their critical micelle concentration (cmc). At ambient temperature, normal pressure and when the concentration of the surfactant was above the cmc without other additives, spherical micelles were formed. At higher surfactant concentrations, H-1 NMR spectra and self-diffusion experiments showed that abrupt changes occurred in their chemical shifts (delta) and in their self-diffusion coefficients (D). This indicated that spherical micelles transformed into larger micelles. By careful examination of every proton resonance peak in the H-1 NMR spectra, we found that the protons near the head group experienced larger chemical shift changes than protons near the hydrophobic group. This suggests that the shape of surfactant micelles most likely transforms from spherical to ellipsoidal or rodlike.