Dynamics of Cetyltrimethylammonium Bromide Head Groups in Bulk by Solid-State Deuterium NMR Spectroscopy

Variable temperature, solid-state deuterium (²H) NMR spectroscopy has been used to probe the rather complex head group dynamics of the surfactant cetyltrimethylammonium bromide- d₉ (CTAB- d₉) in bulk. Heating and cooling runs were made as the surfactant underwent supercooling. ²H NMR line shape simulations were used to identify the hierarchy of the molecular motions of CTAB as a function of temperature. Fast continuous methyl rotations about the N-C_methyl axes and 3-fold jumps about the main chain C-N axis were present at all of the temperatures from -40 to 120 °C. With heating, the spectra were consistent with CTAB molecules starting 180° flips about the hydrocarbon chain molecular axis around 0 °C, which continued to flip with increasing flip rates up to 80 °C. At 90 °C, the flips changed to rotation of the CTAB molecules about the hydrocarbon chain axis and that rotation continued to 120 °C. Comparison of spectra of bulk CTAB at different temperatures from heating and cooling runs revealed that the rotation about the long axis of the hydrocarbon chains started at around 90 °C on heating, however, it does not freeze out until between 70 and 80 °C because of supercooling.