1H and 13C nmr study of phase transition and molecular motion in ionic liquids forming lyotropic liquid-crystalline ionogels

Abstract

The room temperature ionic liquid (RTIL) 1-decyl-3-methyl-imidazolium bromide [C10mim][Br], dissolved in water, was studied using 1H and 13C NMR spectroscopy. The manifestation of phase transitions and fine features of molecular motion in NMR spectra upon changing temperature and composition have been analyzed. The 1H NMR line shape typical for anisotropic fluids with zero biaxiality (asymmetry) of magnetic shielding and the chemical shift anisotropy (CSA) of ca 0.33 ppm was observed and attributed to water molecules. CSA values for 13C nuclei have been found in the range of 1.2–1.7 ppm. The difference between lyotropic liquid-crystalline (LC) ionogel phase and the solid one has been revealed, where the motions of RTIL and water molecules have been found to be dynamically segregated. The anisotropic 13C NMR signal shape at 16.89 ppm shows the difference between LC ionogel and the lamellar phases, where usually the decreasing order parameter moving along hydrocarbon chain from the polar head is observed. It indicates that the terminal –CH3 groups are more ordered and the supramolecular structures of [C10mim][Br], similar to some higher micellar RTIL aggregates are expected. In order to explain the experimental observations, the quantum chemistry DFT calculations of 1H and 13C magnetic shielding tensors of [C10mim][Br] and various H-bond structures of H2O were performed. Keywords: properties of molecules and molecular ions, line and bands widths, shapes and shifts, NMR and relaxation