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

Abstract

Variable temperature, solid-state deuterium (H) NMR spectroscopy has been used to probe the rather complex head group dynamics of the surfactant cetyltrimethylammonium bromide- d (CTAB- d) in bulk. Heating and cooling runs were made as the surfactant underwent supercooling. H 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 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.

Unravelling Main- and Side-Chain Motions in Polymers with NMR Spectroscopy and Relaxometry: The Case of Polyvinyl Butyral

“…Indeed, at −123 °C, the 2 H NMR spectrum can be satisfactorily reproduced by a combination of two subspectra, one due to CD 3 groups rotating fast about their ternary axes and the other arising from CD and CD 2 moieties undergoing fast two-site jumps ( Figure S4b ). Upon heating up to room temperature, the methyl signal intensity increases in the central part of the spectrum, and there is a partial collapse of the singularities at ±18 kHz ( Figure 2 ); these features are indicative of the onset of a motion of the methyl axis further averaging the residual quadrupolar interaction [ 41 , 42 , 43 ]. All in all, at 20 °C (= T g − 45 °C), the highest temperature we could set in our instrument, the side-chains are still subjected to small amplitude motions.…”

“…Further information on the dynamics of the PVB side-chains arises from the total intensity of the 2 H NMR spectra as a function of temperature, as well as from the relative intensity of the spectral contributions due to the different groups. Indeed, distortions in the line shape and significant loss of signal intensity can be observed in 2 H NMR spectra acquired with the quadrupolar echo pulse sequence when motions of the C-D bonds occur in the so-called intermediate regime, i.e., when the motional rates are of the order of δ 0 [ 31 ], while insignificant and little intensity loss is expected for motions in the slow and fast regime, respectively [ 41 , 42 , 43 , 44 , 45 ]. Moreover, spectral distortions and signal loss depend on the echo delay and on the motion geometry.…”

Solution-Processable Oxidation-Resistant Copper Nanowires Decorated with Alkyl Ligands

“…The decorated alkyl chains cannot maintain an ordered state in such conditions. In fact, the alkyl chains are expected to be in an all-gauche conformation and rotating on the Cu NW surfaces above their order–disorder transition temperatures . The excellent oxidation resistance observed for the ODT-decorated Cu NWs in both the hydrothermal and hygrothermal environments suggests that a tightly packed alkyl layer is the key for minimizing oxidation of the Cu NW surfaces.…”